A Text-book of Entomology: Including the Anatomy, Physiology, Embryology and Metamorphoses of Insects for Use in Agricultural and Technical Schools and Colleges as Well as by the Working Entomologist

Transcriber’s Note:

The cover image was created by the transcriber and is placed in the public domain.

A TEXT-BOOK OF ENTOMOLOGY

A
TEXT-BOOK OF ENTOMOLOGY
INCLUDING
THE ANATOMY, PHYSIOLOGY, EMBRYOLOGY AND METAMORPHOSES
OF
INSECTS
FOR USE IN AGRICULTURAL AND TECHNICAL SCHOOLS AND COLLEGES
AS WELL AS BY THE WORKING ENTOMOLOGIST

BY
ALPHEUS S. PACKARD, M.D., Ph.D.
PROFESSOR OF ZOÖLOGY AND GEOLOGY, BROWN UNIVERSITY AUTHOR OF “GUIDE TO STUDY OF INSECTS,” “ENTOMOLOGY FOR BEGINNERS,” ETC.
New York
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., Ltd.
1898
All rights reserved
Copyright, 1898,
By THE MACMILLAN COMPANY.
Norwood Press
J. S. Cushing & Co.—Berwick & Smith
Norwood Mass. U.S.A.
v

PREFACE

In preparing this book the author had in mind the wants both of the student and the teacher. For the student’s use the more difficult portions, particularly that on the embryology, may be omitted. The work has grown in part out of the writer’s experience in class work.

In instructing small classes in the anatomy and metamorphoses of insects, it was strongly felt that the mere dissection and drawing of a few types, comprising some of our common insects, were by no means sufficient for broad, thorough work. Plainly enough the laboratory work is all important, being rigidly disciplinary in its methods, and affording the foundation for any farther work. But to this should be added frequent explanations or formal lectures, and the student should be required to do collateral reading in some general work on structural and developmental entomology. With this aim in view, the present work has been prepared.

It might be said in explanation of the plan of this book, that the students having previously taken a lecture course in the zoölogy of the invertebrates, were first instructed in the facts and conclusions bearing on the relations of insects to other Arthropoda, and more especially the anatomy of Peripatus, of the Myriopoda, and of Scolopendrella. Then the structure of Campodea, Machilis, and Lepisma was described, after which a few types of winged insects, beginning with the locust and ending with the bee, were drawn and dissected; the nymph of the locust, and the larva and pupa of a moth and of a wasp and bee being drawn and examined. Had time permitted, an outline of the embryology and of the internal changes in flies during their metamorphoses would have been added.

viThis book gives, of course with much greater fulness and detail for reference and collateral reading, what we roughly outlined in our class work. The aim has been to afford a broad foundation for future more special work by any one who may want to carry on the study of some group of insects, or to extend in any special direction our present knowledge of insect morphology and growth.

Many of our entomologists begin their studies without any previous knowledge of the structure of animals, taking it up as an amusement. They may be mere collectors and satisfied simply to know the name of their captures, but it is hoped that with this book in their hands they may be led to desire farther information regarding what has already been done on the structure and mode of growth of the common insects. For practical details as to how to dissect, to make microscopic slides, and to mount and preserve insects generally, they are referred to the author’s “Entomology for Beginners.”

It may also be acknowledged that even in our best and latest general treatises on zoölogy, or comparative anatomy, or morphology, the portion related to insects is scarcely so thoroughly done as those parts devoted to other phyla, that of Lang, however, his invaluable Comparative Anatomy, being an exception. On this account, therefore, it is hoped that this hiatus in our literature may be in a degree filled.

The author has made free use of the excellent article “Insecta” of Newport, of Lang’s comprehensive summary in his most useful Text-book of Comparative Anatomy, of Graber’s excellent Die Insecten, of Miall and Denny’s The Structure and Life-History of the Cockroach, and of Sharp’s Insecta. Kolbe’s Einführung has been most helpful. But besides these helps, liberal use has been made of the very numerous memoirs and monographic articles which adorn our entomological literature. The account of the embryology of insects is based on Korschelt and Heider’s elaborate work, Lehrbuch der Vergleichenden Entwicklungsgeschichte der Wirbellosen Thiere, the illustrations of this portion being mainly taken from it, through the Messrs. Swan Sonnenschein & Co., London.

viiProfessor H. S. Pratt has kindly read over the manuscript and also the proofs of the portion on embryology and metamorphoses, and the author is happy to acknowledge the essential service he has rendered.

The bibliographical lists are arranged by dates, so as to give an idea of the historical development of each subject. The aim has been to make these lists tolerably complete and to include the earliest, almost forgotten works and articles as well as the most recent.

Much care has been taken to give due credit either to the original sources from which the illustrations are copied, or to the artist; about ninety of the simpler figures were drawn by the author, many of them for this work. For the use of certain figures acknowledgments are due to the Boston Society of Natural History, to the Division of Entomology, U. S. Department of Agriculture, through the kind offices of Mr. L. O. Howard, and to the Illinois State Laboratory of Natural History, through Professor S. A. Forbes and Mr. C. A. Hart. Professor W. M. Wheeler, of the University of Chicago, has kindly loaned for reproduction several of his original drawings published in the Journal of Morphology. A number are reproduced from figures in the reports of the United States Entomological Commission.

Providence, R. I.,
March 4, 1898.
ix

TABLE OF CONTENTS

PART I. MORPHOLOGY AND PHYSIOLOGY
 
    PAGE
Position of Insects in the Animal Kingdom 1
 
Relations of Insects to Other Arthropoda 2
 
  The Crustacea 4
  The Merostomata 5
  The Trilobita 5
  The Arachnida 6
  Relations of Peripatus to insects 9
  Relation of Myriopods to insects 11
  Relations of the Symphyla to insects 18
  Diagnostic or essential characters of Symphyla 22
 
Insecta (Hexapoda) 26
 
  Diagnostic characters of insects 26
 
 
1. EXTERNAL ANATOMY
 
  a. Regions of the body 27
 
  b. The integument (exoskeleton) 28
 
    Chitin 29
 
  c. Mechanical origin and structure of the segments (somites, arthromeres, etc.) 30
 
  d. Mechanical origin of the limbs and of their jointed structure 35
 
The Head and its Appendages 42
 
  a. The head 42
 
    The labrum 42
    The epipharynx and labrum-epipharynx 43
    Attachment of the head to the trunk 46
    The basal or gular region of the head 46
    The occiput 48
    The tentorium 49
    Number of segments in the head 50
    The composition of the head in the Hymenoptera 55
 
x  b. Appendages of the head 57
 
    The antennæ 57
    The mandibles 59
    The first maxillæ 62
    The second maxillæ 68
    The hypopharynx 70
    Does the hypopharynx represent a distinct segment? 82
 
The Thorax and its Appendages 86
 
  a. The thorax: its external anatomy 86
 
    The patagia 89
    The tegulæ 89
    The apodemes 92
    The acetabula 94
 
  b. The legs: their structure and functions 95
 
    Tenent hairs 99
    Why do insects have but six legs? 100
    Loss of limbs by disuse 101
 
  c. Locomotion (walking, climbing, and swimming) 103
 
    Mechanics of walking 103
    Locomotion on smooth surfaces 111
    Climbing 116
    The mode of swimming of insects 116
 
  d. The wings and their structure 120
 
    The veins 121
    The squamæ 123
    The halteres 124
    The thyridium 124
    The tegmina and hemelytra 124
    The elytra 124
 
  e. Development and mode of origin of the wings 126
 
    Embryonic development of the wings 126
    Evagination of the wing outside of the body 132
    Extension of the wing; drawing out of the tracheoles 133
 
  f. The primitive origin of the wings 137
 
    The development and structure of the tracheæ and veins of the wing 144
 
  g. Mechanism of flight 148
 
    Theory of insect flight 150
    Graber’s views as to the mechanism of the wings, flight, etc. 153
 
The Abdomen and its Appendages 162
 
  The median segment 163
  The cercopoda 164
  The ovipositor and sting 167
  The styles and genital claspers (Rhabdopoda) 176
  Velum penis 181
  The suranal plate 181
xi  The podical plates or paranal lobes 182
  The infra-anal lobe 183
  The egg-guide 183
 
The Armature of Insects: Setæ, Hairs, Scales, Tubercles, Etc. 187
 
  The cuticula 187
  Setæ 188
  Glandular hairs and spines 190
  Scales 193
  Development of the scales 195
  Spinules, hair-scales, hair-fields, and androconia 197
 
The Colors of Insects 201
 
  Optical colors 201
  Natural colors 203
  Chemical and physical nature of the pigment 206
  Ontogenetic and phylogenetic development of colors 207
 
 
2. INTERNAL ANATOMY
 
The Muscular System 211
 
  Musculature of a caterpillar 213
  Musculature of a beetle 213
  Minute structure of the muscles 215
  Muscular power of insects 217
 
The Nervous System 222
 
  a. The nervous system as a whole 222
 
  b. The brain 226
 
    The optic or procerebral segment 231
    Procerebral lobes 232
    The mushroom or stalked bodies 233
    Structure of the mushroom bodies 234
    The central body 237
    The antennal or olfactory lobes (Deutocerebrum) 237
    The œsophageal lobes (Tritocerebrum) 237
 
  c. Histological elements of the brain 238
 
  d. The visceral (sympathetic or stomatogastric) system 238
 
  e. The supraspinal cord 240
 
  f. Modifications of the brain in different orders of insects 240
 
  g. Functions of the nerve-centres and nerves 243
 
The Sensory Organs 249
 
  a. The eyes and insect vision 249
 
    The simple or single-lensed eye (ocellus) 249
    The compound or facetted eye (ommateum) 250
    The facet or cornea 250
    The crystalline lens or cone 251
xii    The pigment 253
    The basilar membrane 253
    The optic tract 253
    Origin of the facetted eye 255
    Mode of vision by single eyes or ocelli 255
    Mode of vision by facetted eyes 256
    The principal use of the facetted eye to perceive the movements of animals 259
    How far can insects see? 260
    Relation of sight to the color of eyes 260
    The color sense of insects 260
 
  b. The organs of smell 264
 
    Historical sketch of our knowledge of the organs of smell 264
    Physiological experiments 268
    Relation of insects to smelling substances before and after the loss of their antennæ 269
    Experiments on the use of the antennæ in seeking for food 270
    Experiments testing the influence of the antennæ of the males in seeking the females 270
    Structure of the organs of smell in insects 271
 
  c. The organs of taste 281
 
    Structure of the taste organs 282
    Distribution in different orders of insects 282
    Experimental proof 286
 
  d. The organs of hearing 287
 
    The ears or tympanal and chordotonal sense-organs of Orthoptera and other insects 288
    Antennal auditory hairs 292
    Special sense-organs in the wings and halteres 293
 
  e. The sounds of insects 293
 
The Digestive Canal and its Appendages 297
 
  a. The digestive canal 302
 
    The œsophagus 303
    The crop or ingluvies 303
    The “sucking stomach” or food-reservoir 305
    The fore-stomach or proventriculus 306
    The œsophageal valve 311
    Proventricular valvule 313
    The peritrophic membrane 313
    The mid-intestine 314
    Histology of the mid-intestine 316
    The hind-intestine 316
    Large intestine 316
    The ileum 317
    The gastro-ileal folds 317
    The colon 317
    The rectum 318
xiii    The vent (anus) 319
    Histology of the digestive canal 320
 
  b. Digestion in insects 324
 
    The mechanism of secretion 326
    Absorbent cells 328
 
The Glandular and Excretory Appendages of the Digestive Canal 331
 
  a. The salivary glands 331
 
  b. The silk or spinning glands, and the spinning apparatus 339
 
    The process of spinning 340
    How the thread is drawn out 343
    Appendages of the silk-gland (Filippi’s glands) 345
 
  c. The cæcal appendages 347
 
  d. The excretory system (urinary or Malpighian tubes) 348
 
    Primitive number of tubes 353
 
  e. Poison-glands 357
 
  f. Adhesive or cement-glands 360
 
  g. The wax-glands 361
 
  h. “Honey-dew” or wax-glands of Aphids 364
 
  i. Dermal glands in general 365
 
Defensive or Repugnatorial Scent-Glands 368
 
  Eversible coxal glands 369
  Fœtid glands of Orthoptera 369
  Anal glands of beetles 372
  The blood as a repellent fluid 374
  Eversible glands of caddis-worms and caterpillars 375
  The osmeterium in Papilio larvæ 377
  Dorsal and lateral eversible metameric sacs in other larvæ 377
  Distribution of repugnatorial or alluring scent-glands in insects 382
 
The Alluring or Scent-Glands 391
 
The Organs of Circulation 397
 
  a. The heart 397
 
    The propulsatory apparatus 401
    The supraspinal vessel 403
    The aorta 404
    The pericardial cells 405
    Pulsatile organs of the legs 405
 
  b. The blood 407
 
    The leucocytes 407
 
  c. The circulation of the blood 409
 
    Effects of poisons on the pulsations 412
 
The Blood Tissue 419
 
  a. The fat-body 419
 
  b. The pericardial fat-body or pericardial cells 420
 
    Leucocytes or phagocytes in connection with the pericardial cells 421
 
xiv  c. The œnocytes 423
 
  d. The phosphorescent organs 424
 
    Physiology of the phosphorescence 426
 
The Respiratory System 430
 
  a. The tracheæ 431
 
    Distribution of the tracheæ 432
 
  b. The spiracles or stigmata 437
 
    The position and number of pairs of stigmata 439
    The closing apparatus of the stigma 441
 
  c. Morphology and homologies of the tracheal system 442
 
  d. The spiral threads or tænidia 444
 
  e. Origin of the tracheæ and of the “spiral thread” 447
 
    Internal, hair-like bodies 451
 
  f. The mechanism of respiration and the respiratory movements of insects 451
 
  g. The air-sacs 456
 
    The use of the air-sacs 457
 
  h. The closed or partly closed tracheal system 459
 
  i. The rectal, tracheal gills, and rectal respiration of larval Odonata and other insects 463
 
  j. Tracheal gills of the larvæ of insects 466
 
    Blood-gills 475
 
  k. Tracheal gills of adult insects 476
 
The Organs of Reproduction 485
 
  a. The male organs of reproduction 494
 
    The testes 495
    The seminal ducts 496
    The ejaculatory duct 497
    The accessory glands 497
    The spermatozoa 497
    Formation of the spermatozoön 498
 
  b. The female organs of reproduction 500
 
    The ovaries and the ovarian tubes 500
    Origin of incipient eggs in the germ of the testes 504
    The bursa copulatrix 505
    The spermatheca 506
    The colleterial glands 506
    The vagina or uterus 507
    Signs of copulation in insects 507
 
 
PART II. EMBRYOLOGY OF INSECTS
 
  a. The egg 515
 
    Mode of deposition 518
    Vitality of eggs 520
xv    Appearance and structure of the ripe egg 520
    The egg-shell and yolk-membrane 520
    The micropyle 522
    Internal structure of the egg 524
 
  b. Maturation or ripening of the egg 525
 
  c. Fertilization of the egg 525
 
  d. Division and formation of the blastoderm 526
 
  e. Formation of the first rudiments of the embryo and of the embryonic membranes 531
 
    Formation of the embryonic membranes 532
    The gastrula stage 535
    Division of the embryo or primitive band into body-segments 536
    Differences between the invaginated and overgrown primitive band 538
    Revolution of the embryo where the primitive band is invaginated 540
 
  f. Formation of the external form of the body 542
 
    Origin of the body-segments 542
    The procephalic lobes 544
    Fore-intestine (stomodæum) and hind-intestine (proctodæum), labrum 547
    Completion of the head 548
 
  g. The appendages 548
 
    The cephalic appendages 548
    The thoracic appendages 550
    The abdominal appendages 550
    Appendages of the first abdominal segment (pleuropodia) 551
    Are the abdominal legs of Lepidoptera and phytophagous Hymenoptera true limbs? 552
    The tracheæ 553
 
  h. Nervous system 554
 
    Completion of the definite form of the body 555
 
  i. Dorsal closure and involution of the embryonic membranes 556
 
  j. Formation of the germ-layers 558
 
  k. Farther development of the mesoderm; formation of the body-cavity 563
 
  l. Formation of organs 566
 
    The nervous system 566
    Development of the brain 567
    Development of the eyes 567
    Intestinal canal and glands 569
    The salivary glands 570
    The urinary tubes 572
    The heart 572
    The blood-corpuscles 574
    Musculature; connective tissue; fat-body 574
    The reproductive organs 575
    Development of the male germinal glands 579
 
  m. Length of embryonic life 582
 
  n. The process of hatching 583
 
    The hatching spines 585
 
 
xviPART III. THE METAMORPHOSES OF INSECTS
 
  a. The nymph as distinguished from the larval stage 593
 
  b. Stages or stadia of metamorphosis 594
 
  c. Ametabolous and metabolous stages 594
 
The Larva 599
 
  a. The Campodea-form type of larva 600
 
  b. The eruciform type of larva 602
 
  c. Growth and increase in size of the larva 608
 
  d. The process of moulting 609
 
    The number of moults in insects of different orders 615
    Reproduction of lost limbs 619
    Formation of the cocoon 619
    Sanitary conditions observed by the honey-bee larva, and admission of air within the cocoon 623
 
The Pupa State 625
 
  a. The pupa considered in reference to its adaptation to its surroundings and its relation to phylogeny 631
 
  b. Mode of escape of the pupa from its cocoon 632
 
  c. The cremaster 636
 
    Mode of formation of the cremaster and suspension of the chrysalis in butterflies 637
 
Formation of the Pupa and Imago in the Holometabolous Insects (the Diptera excepted) 640
 
  a. The Lepidoptera 642
 
    The changes in the head and mouth-parts 646
    The change in the internal organs 647
    The wings 654
    Development of the feet and of the cephalic appendages 654
    Embryonic cells and the phagocytes 655
    Formation of the femur and of the tibia; transformation of the tarsus 656
    The antennæ 657
    Maxillæ and labial palpi 658
    Process of pupation 660
 
  b. The Hymenoptera 661
 
    Ocular or oculo-cephalic buds 665
    The antennal buds 665
    The buds of the buccal appendages 665
    The buds of the ovipositor 665
 
Development of the Imago in the Diptera 666
 
  a. Development of the outer body-form 668
 
    Formation of the imago in Corethra 668
    Formation of the imago in Culex 670
xvii    Formation of the imago in Chironomus 671
    Formation of the imago in Muscidæ 673
 
  b. Development of the internal organs of the imago 678
 
    The hypodermis 678
    The muscles 680
    The digestive canal 681
    The tracheal system 683
    The nervous system 684
    The fat-body 685
    Definitive fate of the leucocytes 685
    The post-embryonic changes and imaginal buds in the Pupipara (Melophagus) 686
 
  c. General summary 687
 
Hypermetamorphism 688
 
Summary of the Facts and Suggestions as to the Causes of Metamorphism 705
 
  Theoretical conclusions; causes of metamorphosis 708
TEXT-BOOK OF ENTOMOLOGY
1

PART I.—MORPHOLOGY AND PHYSIOLOGY

POSITION OF INSECTS IN THE ANIMAL KINGDOM

Although the insects form but a single class of the animal kingdom, they are yet so numerous in orders, families, genera, and species, their habits and transformations are so full of instruction to the biologist, and they affect human interests in such a variety of ways, that they have always attracted more attention from students than any other class of animals, the number of entomologists greatly surpassing that of ornithologists, ichthyologists, or the special students of any other class, while the literature has assumed immense proportions.

Insects form about four-fifths of the animal kingdom. There are about 250,000 species already named and contained in our museums, while the number of living and fossil species in all is estimated to amount to between one and two millions.

In their structure insects are perhaps more complicated than any other animals. This is partly due to the serial arrangement of the segments and the consequent segmental repetition of organs, especially of the external appendages, and of the muscles, the tracheæ, and the nerves. The brain is nearly or quite as complicated as that of the higher vertebrates, while the sense-organs, especially those of touch, sight, and smell are, as a rule, far more numerous and only less complex than those of vertebrates. Moreover, in their psychical development, certain insects are equal, or even superior, to any other animals, except birds and mammals.

The animal kingdom is primarily divided into two grand divisions, the one-celled (Protozoa) and many-celled animals (Metazoa). In the latter group the cells and tissues forming the body are arranged in three fundamental cell-layers; viz. the ectoderm or outer layer, the mesoderm, and endoderm. The series of branches, or phyla, comprised 2under the term Metazoa are the Porifera, Cœlenterata, Vermes, Echinodermata, Mollusca, Arthropoda, and Vertebrata. Their approximate relationships may be provisionally expressed by the following

Tabular View of the Eight Branches or Phyla of the Animal Kingdom.

VIII. _Vertebrata._ Ascidians and Fishes to Man. VII. _Arthropoda._ Trilobites, Crustacea, Arachnida, Insects, etc. VI. _Mollusca._ Clams, Snails, Cuttles. V. _Echinodermata._ Crinoids, Starfish, Seaurchins, etc. IV. _Vermes._ Flat and Round Worms, Polyzoa, Brachiopods, Annelids. III. _Cœlenterata._ Hydra, Jellyfishes.                     II. _Porifera._ Sponges. METAZOA. Manycelled animals with 3 celllayers. I. PROTOZOA. Singlecelled animals.

RELATIONS OF INSECTS TO OTHER ARTHROPODA

The insects by general consent stand at the head of the Arthropoda. Their bodies are quite as much complicated or specialized, and indeed, when we consider the winged forms, more so, than any other class of the branch, and besides this they have wings, fitting them for an aërial life. It is with little doubt that to their power of flight, and thus of escaping the attacks of their creeping arthropod enemies, insects owe, so to speak, their success in life; i.e. their numerical superiority in individuals, species, and genera. It is also apparently their power of moving or swimming swiftly from one place to another which has led to the numerical superiority in species of fishes to other Vertebrata. Among terrestrial vertebrates, the birds, by virtue of their ability to fly, greatly surpass in number of species the reptiles and mammals.

The Arthropoda are in general characterized by having the body 3composed of segments (somites or arthromeres) bearing jointed appendages. They differ from the worms in having segmented appendages, i.e. antennæ, jaws, and legs, instead of the soft unjointed outgrowths of the annelid worms. Moreover, their bodies are composed of a more or less definite number of segments or rings, grouped either into a head-thorax (cephalothorax) and hind-body, as in Crustacea, or into a head differentiated from the rest of the body (trunk), the latter not being divided into a distinct thorax and abdomen, as in Myriopoda; or into three usually quite distinct regions—the head, thorax, and hind-body or abdomen, as in insects. In certain aberrant, modified forms, as the Tardigrada, or the Pantopoda, and the mites, the body is not differentiated into such definite regions.

In their internal organs arthropods agree in their general relations with the higher worms, hence most zoölogists agree that they have directly originated from the annelid worms.

The position and general shape of the digestive canal, of the nervous and circulatory systems, are the same in Arthropoda as in annelid (oligochete) worms, so much so that it is generally thought that the Arthropoda are the direct descendants of the worms. It is becoming evident, however, that there was no common ancestor of the Arthropoda as a whole, and that the group is a polyphyletic one. Hence, though a convenient group, it is a somewhat artificial one, and may eventually be dismembered into at least three or four phyla or branches.

The following diagram may serve to show in a tentative way the relations of the classes of Arthropoda to each other, and also may be regarded as a provisional genealogical tree of the branch.

9. _Insecta._ 7. _Chilopoda._ 4. _Arachnida._ 6. _Diplopoda._ 3. _Merostomata._ 8. _Symphyla._ 1. _Crustacea._ 6_a_. _Pauropoda._ 2. _Trilobita._ 5. _Peripatus._ 4_a_. _Pantopoda._ 4_b_. _Tardigrada._ _Different Annelida._ Trochosphæra.

4We will now rapidly review the leading features of the classes of Arthropoda.

The Crustacea.—These Arthropoda are in many most important characteristics unlike the insects; they have two pairs of antennæ, five pairs of buccal appendages, and they are branchiate Arthropoda. They have evidently originated entirely independently, and by a direct line of descent from some unknown annelid ancestor which was either a many-segmented worm, with parapodia, or the two groups together with the Rotifera may have originated from a common appendigerous Trochosphæra. Their segments in the higher forms are definite in number (23 or 24) and arranged into two regions, a head-thorax (cephalothorax) and hind-body (abdomen). Nearly all the segments, both of the cephalothorax and abdomen, bear a pair of jointed limbs, and to them at their base are, in the higher forms, appended the gills (branchiæ). The limbs are in the more specialized forms (shrimps and crabs) differentiated into eye-stalks, two pairs of antennæ, a pair of palpus-bearing jaws (mandibles), two pairs of maxillæ and three pairs of maxillipeds; these appendages being biramose, and the latter bearing gills attached to their basal joints. The legs are further differentiated into ambulatory thoracic legs and into swimming or abdominal legs, and in the latter the first pair of the male is modified into copulatory organs (gonopoda). The male and female reproductive organs as a rule are in separate individuals, hermaphrodites being very unusual, and the glands may be paired or single. The sexual outlets are generally paired, and, as in the male lobster and other Macrura, open in the basal joint of the last pair of legs, and in the female in the third from the last; while originally in all Crustacea the sexual organs were most probably paired (Fig. 3, B).

They are, except a few land Isopoda, aquatic, mostly marine, and when they have a metamorphosis, pass through a six-legged larval stage, called the Nauplius, the shrimps and crabs passing through an additional stage, the Zoëa. Crustacea also differ much from insects in the highly modified nature of the nephridia, which are usually represented by the green gland of the lobster, or the shell-glands of the Phyllopoda, which open out in one of the head-segments; also in the possession of a pair of large digestive glands, the so-called liver.

Intermediate in some respects between the Crustacea and insects, but more primitive, in respect to what are perhaps the most weighty characters, than the Crustacea, are the Trilobita, the Merostomata (Limulus), and, finally, the Arachnida, these being allied groups. In the Trilobita and Merostomata (Limulus), the head-appendages are 5more like feet than jaws, while they have in most respects a similar mode of embryonic development, the larval forms being also similar.

Fig. 1.—Restoration of under side of a trilobite (Triarthrus becki), the trunk limbs bearing small triangular respiratory lobes or gills.—After Beecher.

The Merostomata.—The only living form, Limulus, is undoubtedly a very primitive type, as the genital glands and ducts are double, opening wide apart on the basal pair of abdominal legs (Fig. 3). Moreover, their head-appendages, which are single, with spines on the basal joint, are very primitive and morphologically nearer in shape to those of the worms (Syllidæ, etc.) than even those of the Crustacea. Besides, their four pairs of coxal glands, with an external opening at the base of the fifth pair of head-appendages, and which probably are modified nephridia (Crustacea having but a single pair in any one form, either opening out on the second antennal, green gland, or second maxillary, shell-gland, segment), indicate a closer approximation to the polynephrous worms. Limulus has other archaic features, especially as regards the structure of the simple and compound eyes and the simple nature of the brain.

The Trilobita.—These archaic forms are still more generalized and primitive than the Merostomata and Crustacea, and probably were the first Arthropoda to be evolved from some unknown annelid worm. They had jointed biramose limbs of nearly uniform shape and size on each segment of the body, which were not, as in Crustacea, differentiated into antennæ, jaws (mandibles), maxillæ, maxillipeds, and two kinds of legs (thoracic and 6abdominal), showing that they are a much more primitive type, and nearer to the annelids than any other Arthropoda. Their gills, as shown by the researches of Walcott and of Beecher, were attached to nearly if not every pair of limbs behind the antennæ (Figs. 1, 2). The fact that in Trilobita the first pair of limbs is antenniform does not prove that they are Crustacea, since Eurypterus has a similar pair of appendages.

Fig. 2.—Restored section of Calymene: C, carapace; en, endopodite; en′, exopodite; with the gills on the epipodal or respiratory part of the appendage.—After Walcott.

The limbs in trilobites, as well as the abdominal ones of merostomes, and all those of Crustacea, except the first antennæ, are biramose, consisting of an outer (exopodite) and an inner division (endopodite). In this respect the terrestrial air-breathing tracheate forms, Arachnida, Myriopoda, and Insecta, differ from the branchiate forms, as their legs are single or undivided, being adapted for supporting the body during locomotion upon the solid earth. It is to be observed that when, as in Limulus, the body is supported by cephalic ambulatory limbs, they are single, while the abdominal limbs, used as they are in swimming, are biramose, much as in Crustacea.

The Arachnida.—The scorpions and spiders are much less closely allied to the myriopods and insects than formerly supposed. Their embryology shows that they have descended from forms related to Limulus, possibly having had an origin in common with that animal, or having, as some authors claim, directly diverged from some primitive eurypteroid merostome. But they differ in essential respects, and not only in the nature and grouping of their appendages; the first pair instead of antenniform being like mandibles, and the second pair like the maxillæ, with the palps, of insects, the four succeeding segments (thoracic) bearing each a pair of legs. They also have a brain quite unlike that of Limulus, the nervous cord behind the brain, however, being somewhat similar, though that of Limulus differs in being enveloped by an arterial coat. Arachnida respire by tracheæ, besides book-lungs, which, however, are possibly derivatives of the book-gills of Limulus, while they perform the office of excretion by means of the malpighian tubes, and like Limulus possess two large digestive glands (“liver”). Their embryos have, on at least six abdominal segments, rudiments of limbs, three pairs of which form the spinnerets, showing their origin from Limulus-like or eurypteroid forms; their coxal glands are retained from their eurypteroid ancestors. The Arachnida probably descended from marine merostomes, and not from an independent annelid ancestry, hence we have represented them in the diagram on p. 3 as branching off from the merostomatous phylum, rather than from an independent one.

7

Fig. 3.—Paired genital openings of different classes of arthropods. A, the most primitive, of Limulus polyphemus: gen. p, generative papillæ; d, duct; vd, vas deferens; t, tendinous stigmata; stig, stigmata; e, external branchial muscle; ant, anterior lamellar muscle.—After Benham, with a few changes. B, lobster (Homarus vulgaris), ♀: oe, genital aperture on 3d pair of legs; ov, ovary; u, unpaired portion of the same; od, oviduct. C, ♀, scorpion: ov, ovary, with a single external opening. D, ♂: t, testis; vd, vasa deferentia; sb, seminal vesicle; a, glandular appendage; p, penis.—After Blanchard. E, a myriopod (Glomeris marginata, ♀): os, ovarian sac, laid open; od, paired oviducts. F, ♂: t, testis; gvd, common vas deferens; pa, paired ducts.—After Favre, from Lang. G, Lepisma saccharina, young ♂: vd, vas deferens, ed, ejaculatory duct; ga, external appendages.—After Nassonow. H, Ephemera, ♂, showing the double outlets.—After Palmén.

8The characters in which arachnids approach insects, such as tracheæ and malpighian tubes (none occur, as a rule, in marine or branchiate arthropods), may be comparatively recent structures acquired during a change from a marine to a terrestrial life, and not primitive heirlooms.

Arachnida also show their later origin than merostomes by the fact that their sexual glands are in most cases single, and though with rare exceptions the ducts are paired, these finally unite and open externally by a common single genital aperture in the median line of the body, at the base of the abdomen (Fig. 3, C, D). In this respect Limulus, with its pair of genital male or female openings, situated each at the end of a papilla, placed widely apart at the base of the first abdominal limbs, is decidedly more archaic. Unlike Crustacea and insects, Arachnida do not, except in the mites (Acarina), which is a very much modified group, undergo a metamorphosis.

We see, then, that the insects, with the Myriopoda, are somewhat isolated from the other Arthropoda. The Myriopoda have a single pair of antennæ, and as they have other characters in common with insects, Lang has united the two groups in a single class Antennata; but, as we shall see, this seems somewhat premature and unnecessary. Yet the two groups have perhaps had a common parentage, and may prove to belong to a distinct, common phylum.

Not only by their structure and embryology, as well as their metamorphosis, do the myriopods and insects stand apart from the Arachnida and other arthropods, but it seems probable that they have had a different ancestry, the arthropods being apparently polyphyletic.

There are two animals which appear to connect the insects with the worms, and which indicate a separate line of descent from the worms independent of that of the other classes. These are the singular Peripatus, which serves as a connecting link between arthropods and worms, and Scolopendrella (Symphyla). These two animals are guide-posts, pointing out, though vaguely to be sure, the way probably trod by the forms, now extinct, which led up to the insects.

9Relations of Peripatus to Insects.—We will first recount the characteristics of this monotypic class. Peripatus (Fig. 4) stands alone, with no forms intermediate between itself and the worms on the one hand, and the true Arthropoda on the other. Originally supposed to be a worm, it is now referred to a class by itself, the Malacopoda of Blainville, or Protracheata of Haeckel. It lives in the tropics, in damp places under decaying wood. In general appearance it somewhat resembles a caterpillar, but the head is soft and worm-like, though it bears a pair of antenna-like tentacles. It may be said rather to superficially resemble a leech with clawed legs, the skin and its wrinkles being like those of a leech. There is a pair of horny jaws in the mouth, but these are more like the pharyngeal teeth of worms than the jaws of arthropods. The numerous legs end each in a pair of claws. The ladder-like nervous system is unlike that of annelid worms or arthropods, but rather recalls that of certain molluscs (Chiton, etc.), as well as that of certain flat and nemertine worms. Its annelid features are the large number of segmentally arranged true nephridia, and the nature of the integument. Its arthropodan features, which appear to take it out of the group of worms, are the presence of tracheæ, of true salivary and slime glands, of a pair of coxal glands (Fig. 4, C, cd) as well as the claws at the end of the legs. The tracheæ, which are by no means the only arthropodan features, are evidently modified dermal glands. The heart is arthropodan, being a dorsal tube lying in a pericardial sinus, with many openings. This assemblage of characters is not to be found in any marine or terrestrial worm.

The tracheæ (Fig. 4, D, tr) are unbranched fine tubes, without a “spiral thread,” and are arranged in tufts, in P. edwardsii opening by simple orifices or pores (“stigmata”) scattered irregularly over the surface of the body; but in another species (P. capensis) some of the stigmata are arranged more definitely in longitudinal rows,—on each side two, one dorsally and one ventrally. “The stigmata in a longitudinal row are, however, more numerous than the pairs of legs.” (Lang.)

The salivary glands, opening by a short common duct into the under side of the mouth, in the same general position as in insects, are evidently, as the embryology of the animal proves, transformed nephridia, and being of the arthropodan type explain the origin and morphology of those of insects. It is so with the slime glands; these, with the coxal glands, being transformed and very large dermal glands. Those of insects arose in the same manner, and are evidently their homologues, while those of Peripatus were probably originally derived from the setiparous glands in the appendages (parapodia) of annelid worms.

10

Fig. 4.A, Peripatus novæ zealandiæ.—After Sedgwick, from Lang. B, Peripatus capensis, side view, enlarged about twice the natural size.—After Moseley, from Balfour. C, Anatomy of Peripatus capensis. The enteric canal behind the pharynx has been removed. g, brain; a, antenna; op, oral or slime papillæ; sd, slime gland; sr, slime reservoir, which at the same time acts as a duct to the gland; so4, so5, so6, so9, nephridia of the 4th, 5th, 6th, and 9th pairs of limbs; cd, elongated coxal gland of the last pair of feet; go, genital aperture; an, anus; ph, pharynx; n, longitudinal trunk of the nervous system.—After Balfour, from Lang. D, Portion of the body of Peripatus capensis opened to show the scattered tufts of tracheæ (tr); v, v, ventral nerve cords.—After Moseley.

11The genital glands and ducts are paired, but it is to be observed that the outlets are single and situated at the end of the body. In the male the ejaculatory duct is single; in its base a spermatophore is formed. It will be seen, then, that Peripatus is not only a composite type, and a connecting link between worms and tracheate arthropods, but that it may reasonably be regarded, if not itself the ancestor, as resembling the probable progenitor of myriopods and insects, though of course there is a very wide gap between Peripatus and the other antennate, air-breathing Arthropoda.

Fig. 4.E, Peripatus edwardsii, head from the under side: a, base of antenna; op, oral papilla; the figure also shows the papillæ around the mouth, and the four jaws.—After Balfour, from Lang. F, Anterior end of Peripatus capensis, ventral side, laid open: a, antenna; z, tongue; k, jaw; sd, salivary gland; gs, union of the two salivary glands; ph, pharynx; œ, œsophagus; l, lip papillæ around the mouth; op, oral or slime papilla; sld, duct or reservoir of the slime gland.—After Balfour, from Lang.

Relation of Myriopods to Insects.—The Myriopoda are the nearest allies of the insects. They have a distinct head, with one pair of antennæ. The eyes are simple, with the exception of a single genus (Cermatia), in which they are aggregated or compound. The trunk or body behind the head is, as a rule, long and slender, and composed of a large but variable number of segments, of equal size and shape, bearing jointed legs, which invariably end in a single claw.

The mouth-parts of the myriopods are so different in shape and general function from those of insects, that this character, together with the equally segmented nature of the portion of the body behind the head (the trunk), forbids our merging them, as some have been 12inclined to do, with the insects. There are two sub-classes of myriopods, differing in such important respects that by Pocock[1] and by Kingsley they are regarded as independent classes, each equivalent to the insects.

Of these the most primitive are the Diplopoda (Chilognatha), represented by the galley-worms (Julus, etc.).

Fig. 5.—Mandible of Julus: l, lacinia; g, galea; p, dens mandibularis; ma, “mala”; lt, lamina tritoria; st, stipes; c, cardo; m, muscle.—After Latzel.

In the typical Diplopoda the head consists of three segments, a preoral or antennal, and two postoral, there being two pairs of jaw-like appendages, which, though in a broad morphological sense homologues of the mandibles and first maxillæ of insects, are quite unlike them in details.

Fig. 6.—Under lip or deutomala of Scoterpes copei: hyp, hypostoma or mentum; lam. lab, lamina labialis; stip. e, stipes exterior; with the malella exterior (mal. e) and malella interior (mal. i); the stipes interior, with the malulella; and the labiella (hypopharynx of Vom Rath) with its stilus (stil.).

As we have previously stated,[2] the so-called “mandibles” of diplopods are entirely different from those of insects, since they appear to be 2– or 3–jointed, the terminal joint being 2–lobed, thus resembling the maxillæ rather than the mandibles of insects, which consist of but a single piece or joint, probably the homologue of the galea or molar joint of the diplopod protomala. The mandible of the Julidæ (Fig. 5, Julus molybdinus), Lysiopetalidæ, and Polydesmidæ consists of three joints; viz. a basal piece or cardo, a stipes, and the mala mandibularis, which supports two lobes analogous to the galea and lacinia of the maxilla of an insect. There is an approach, as we shall see, in the mandible of Copris, to that of the Julidæ, but in insects in general the lacinia is wanting, and the jaw consists of but a single piece.

The deutomalæ (gnathochilarium), or second pair of diplopod jaws, are analogous to the labium or second maxillæ of insects, forming a flattened, plate-like under-lip, constituting the floor of the mouth (Fig. 6). This pair of appendages needs farther study, especially in the late embryo, before it can be fully understood. So far as 13known, judging by Metschnikoff’s work on the embryology of the diplopods, these myriopods seem to have in the embryo but two pairs of post-antennal mouth-parts, which he designated as the “mandibles” and “labium.” Meinert, however, regards as a third pair of mouth-parts or “labium” what in our Fig. 7 is called the internal stipes (stip. i.), behind which is a triangular plate, lamina labialis (lam. lab), which he regards as the sternite of the same segment.

Fig. 7.—Deutomala of Julus, the lettering as in Fig. 6.

Fig. 8.—Head of Scolopendra, seen from beneath, showing the “mandible” (protomala) with its cardo (card.) and stipes (st.), also the labrum and epilabrum.

The hypopharynx, our “labiella,” (Fig. 6), with the supporting rods or stili linguales (sti. l), of Meinert, are of nearly the same shape as in some insects.

Of the clypeus of insects there is apparently no homologue in myriopods, though in certain diplopods there is an interantennal clypeal region. The labium of insects is represented by a short, broad piece, which, however, unlike that of insects, is immovable, and is flanked by a separate piece called the epilabrum (Fig. 8). Vom Rath has observed an epipharynx, which has the same general relations as in insects.

Fig. 9.—Larva of Julus: a, the 3d abdominal segment, with the new limbs just budding out; b, new segments arising between the penultimate and the last segment.—After Newport.

The embryology of myriopods is in many respects like that of insects. The larva of diplopods hatches with but few segments, and with but three pairs of limbs; but these are not, as in insects, appended to consecutive segments, but in one species the third, and in another, Julus multistriatus? (Fig. 10), the second, segment from the head is footless, while Vom Rath represents the first segment of an European Blaniulus as footless, the feet being situated consecutively on segments 2 to 4. The new segments arise at “the growing point” situated between the last and penultimate segment, growing out in groups of sixes (Newport) or in our Julus multistriatus? in fives (Fig. 10). In 14adult life diplopods (Julus) have a single pair of limbs on the three first segments, or those corresponding to the thoracic segments of insects, the succeeding segments having two pairs to each segment.

Fig. 10.—Freshly hatched larva of Julus multistriatus? 3 mm. long: a, 5 pairs of rudimentary legs, one pair to a segment.

Sinclair (Heathcote) regards each double segment in the diplopods as not two original segments fused together, nor a single segment bearing two pairs of legs, but as “two complete segments perfect in all particulars, but united by a large dorsal plate which was originally two plates which have been fused together.” (Myriopods, 1895, p. 71.) That the segments were primitively separate is shown, he adds, by the double nature of the circulatory system, the nerve cord, and the first traces of segmentation in the mesoblast. Kenyon believes that from the conditions in pauropods, Lithobius, etc., there are indications of alternate plates (not segments) having disappeared, and of the remaining plates overgrowing the segments behind them, so as to give rise to the anomalous double segments.[3]

Fig. 11.—Sixth pair of legs of Polyzonium germanicum, ♀: cs, ventral sacs; cox, coxa; st, sternal plate; sp, spiracle.—After Haase.

Diplopods are also provided with eversible coxal sacs, in position like those of Symphyla and Synaptera; Meinert, Latzel, and also Haase having detected them in several species of Chordeumidæ, Lysiopetalidæ, and Polyzonidæ (Fig. 11). In Lysiopetalum anceps these blood-gills occur in both sexes between the coxæ of the third to sixteenth pair of limbs. In the Diplopods the blood-gills appear to be more or less permanently everted, while in Scolopendrella they are usually retracted within the body (Fig. 15, cg).

Diplopods also differ externally from insects in the genital armature, a complicated apparatus of male claspers and hooks apparently arising from the sternum of the sixth segment and being the modified seventh pair of legs. In myriopods 15there are no pleural pieces or “pleurites,” so characteristic of winged insects.

Perhaps the most fundamental difference between diplopods and insects is the fact that the paired genital openings of the former are situated not far behind the head between the second and third pair of legs. Both the oviducts and male ejaculatory ducts are paired, with separate openings. The genital glands lie beneath, while in chilopods they lie above the intestine; this, as Korschelt and Heider state, being a more primitive relation, since in Peripatus they also lie above the digestive canal.

The nervous system of diplopods is not only remarkable for the lack of the tendency towards a fusion of the ganglia observable in insects, but for the fact that the double segments are each provided with two ganglia. The brain also is very small in proportion to the ventral cord, the nervous system being in its general appearance somewhat as in caterpillars.

The arrangement of the tracheæ and stigmata is much as in insects, but in the Diplopoda the tracheary system is more primitive than in chilopods, a pair of stigmata and a pair of tracheal bundles occurring in each segment, while the bundles are not connected by anastomosing branches, branched tracheæ only occurring in the Glomeridæ. The tracheæ themselves are without spiral threads (tænidia). It is noteworthy that the tracheæ arise much later than in insects, not appearing until the animal is hatched; in this respect the myriopods approximate Peripatus.

In the Chilopoda also the parts of the head, except the epicranium, are not homologous with those of insects, neither are the mouth-parts, of which there are five pairs.

The structure of the head of centipedes is shown in part in Fig. 12, compare also Fig. 8. It will be seen that it differs much from that of the diplopods, though the mandibles (protomalæ) are homologous; they are divided into a cardo and stipes, thus being at least two-jointed.

The second pair of postoral appendages is in centipedes very different from the gnathochilarium of diplopods. As seen in Fig. 12 2, they are separate, cylindrical, fleshy, five-jointed appendages, the maxillary appendages of Newport, which are “connected transversely at their base with a pair of soft appendages” (c), the lingua of Newport. The third and fourth pair are foot-jaws, and we have called them malipedes, as they have of course no homology with the maxillipedes of Crustacea. The second pair of these malipedes, forming the last pair of mouth-appendages, is the poison-fangs (4), 16which are intermediate between the malipedes and the feet; Meinert does not allow that these are mouth-appendages.

Fig. 12.—Structure of a chilopod. A, Lithobius americanus, natural size. B, under side of head and first two body-segments and legs, enlarged: ant, antenna; 1, jaws; 2, first accessory jaw; c, lingua; 3, second accessory jaw and palpus; 4, poison-jaw. (Kingsley del.) C, side view of head (after Newport): ep, epicranium; l, frontal plate; sc, scute; 1, first leg; sp, spiracle.

The embryology of Geophilus by Metschnikoff shows plainly the four pairs of post-antennal appendages. The embryo Geophilus is hatched in the form of the adult, having, unlike the diplopods, no metamorphosis, its embryological history being condensed or abbreviated. But in examining Metschnikoff’s figures certain primitive diplopod features are revealed. The body of the embryo shortly before hatching is cylindrical; the sternal region is much narrower than in the adult, hence the insertions of the feet are nearer together, while the first six pairs of appendages begin to grow out before the hinder ones. Thus the first six pairs of appendages of the embryo Geophilus correspond to the antennæ, two pairs of jaws, and three pairs of legs of the larval Julus. These features appear to indicate that the chilopods may be an offshoot from the diplopod stem. The acquisition of a second pair of legs to a segment in diplopods, as in the phyllopod Crustacea, is clearly enough a secondary character, as shown by the figures of Newport in his memoir on the development of the Myriopoda (Pl. IV.). Thus the tendency in the Myriopoda, both diplopods and chilopods, is towards the multiplication of segments and the elongation of the body, while in insects the polypodous embryo has the three terminal segments of the 17abdomen well formed, these being, however, before hatching, partly atrophied, so that the body of insects after birth tends to become shortened or condensed. This indicates the descent of insects from ancestors with elongated polypodous hind-bodies like Scolopendrella. Korschelt and Heider suggest that the stem-form of myriopods was a homonomously jointed form like Peripatus, consisting of a rather large number of segments, but we might, with Haase, consider that the great number of segments which we now find indicates a late acquisition of this form.

The genital opening in chilopods is single, and situated in the penultimate segment of the body, as in insects. While recognizing the close relationship of the Myriopoda with the insects, it still seems advisable not to unite them into a single group (as Oudemans, Lang, and others would do), but to regard them as forming an equivalent class. On the other hand, when we take into account the form and structure of the head, antennæ, and especially the shape of the first pair of mouth-appendages, being at least two-jointed in both groups, we think these characters, with the homonomously segmented body behind the head, outweigh the difference in the position of the genital outlet, important as that may seem. It should also be taken into account that while insects are derived from polypodous ancestors, no one supposes, with the exception of one or two authors, that these ancestors are the Myriopoda, the latter having evidently descended from a six-legged ancestor, quite different from that of the Campodea ancestor of insects.[4]

In regard to the sexual openings of worms, though their position is in general in the anterior part of the body, it is still very variable, though, in general, paired. In the oligochete worms the genital zone, with the external openings, is formed by the segments lying between the 9th and 14th rings, though in some the genital organs are situated still nearer the head. The myriopods, which evolved from the worms earlier than insects, appear to have in their most primitive forms (the Diplopoda) retained this vermian position of the genital outlets. In the later forms, the chilopods, the genital openings have been carried back to near the end of the body, as in insects. From observations made by three different observers on the freshly hatched larva of the Julidæ, it appears that the ancestral diplopods were six-footed, or oligopod, the larva of Pauropus 18(Fig. 13) approaching nearest to our idea of the ancestral myriopod, which might provisionally be named Protopauropus.

Relations of the Symphyla to Insects.—Opinions respecting the position of the Symphyla, represented by Scolopendrella (Fig. 14), are very discordant. By most writers since Newport, Scolopendrella has been placed among the myriopods. The first author, however, to examine its internal anatomy was Menge (1851), who discovered among other structures (tracheæ, etc.) the silk-glands situated in the last two segments, and which open at the end of each cercus. He regarded the form as “the type of a genus or family intermediate between the hexapod Lepismidæ and the Scolopendridæ.”

Fig. 13.Pauropus huxleyi, much enlarged. A, enlarged view of head, antennæ, and first pair of legs (original). B, young.—After Lubbock. C, longitudinal section of Pauropus huxleyi, ♂: a, brain; b, salivary gland; k, mid-intestine; g, rectum; h, ventral nerve-cord; c, bud-like remnants of coxæ; d, penis; e, vesicula seminalis; f, ductus glandularis; i1, divisions of testes.—After Kenyon.

In 1873[5] the writer referred to this form as follows: “It may be regarded as a connecting link between the Thysanura and Myriopoda, 19and shows the intimate relation of the myriopods and the hexapods, perhaps not sufficiently appreciated by many zoölogists.”

In 1880 Ryder regarded it as “the last survival of the form from which insects may be supposed to have descended,” and referred it to “the new ordinal group Symphyla, in reference to the singular combination of myriopodous, insectean, and thysanurous characters which it presents.[6]

Fig. 14.Scolopendrella immaculata, from above,—after Lang; also from beneath, the genital opening on the 4th trunk-segment: sac, eversible or coxal sac; an, anus; c, cereopod; v, vestigial leg.—After Haase, from Peytoureau. A B C, head and buccal appendages of Scolopendrella immaculata: A, head seen from above; cl, clypeus. B, head from beneath; l, first pair of legs; mx, 1st maxilla; mx1, 2d maxilla; t, “labial plates” of Latzel, labium of Muhr. C, 1st maxilla; l, lacinia; g, galea; p, rudiment of the palpus.—After Latzel. D, end of the body: p11, eleventh, p12, twelfth undeveloped pair of legs; p13, modified, vestigial legs, bearing tactile organs (so); sg, cercopod, with duct of spinning gland, dg; cd, eversible or coxal gland; h8s, coxal spur of the 11th pair of legs.—After Latzel from Lang.

Wood-Mason considered it to be a myriopod, and “the 20descendant of a group of myriopods from which the Campodeæ, Thysanura, and Collembola may have sprung.” We are indebted to Grassi for the first extended work on the morphology of Scolopendrella (1885). In 1886 he added to our knowledge facts regarding the internal anatomy, and gives a detailed comparison with the Thysanura, besides pointing out the resemblances of Scolopendrella to Pauropus, diplopods, chilopods, as well as Peripatus.

Fig. 15.—Section of Scolopendrella immaculata: œ, œsophagus; oe. v, œsophageal valve entering the mid-intestine (“stomach”); i, intestine; r, rectum; br, brain; ns, abdominal chain of ganglia; ovd, oviduct; ov, ovary; s. gl, silk-gland, and op, its outer opening in cercus, ur. t, urinary tube; cg, coxal glands or blood-gills.—Author del.

In 1888 Grassi expressed his view as to the position of the Symphyla, stating that it should not be included in the Thysanura, since it evidently has myriopod characters; these being the supraspinal vessel, the ventral position of the genital glands; the situation of the genital opening in the fourth segment of the trunk, its ganglionic chain being like that of diplopods, its having limbs on all the segments, etc. On the other hand, Grassi has with much detail indicated the points of resemblance to the Thysanura. The principal ones are the thin integument, the want of sympathetic ganglia, the presence of a pair of cephalic stigmata, like that said to occur in certain Collembola, and in the embryo of Apis; two endoskeletal processes situated near the ventral fascia of the head; the epicranial suture also occurring in Thysanura, Collembola, Orthoptera, and other winged insects, and being absent in diplopods and chilopods. He also adds that the digestive canal both in Symphyla and Thysanura is divided into three portions; the malpighian tubes in Thysanura present very different conditions (there being none in Japyx), among which may be comprised those of Scolopendrella. In both groups there is a single pair of salivary glands. The cellular epithelium of the mid-intestine of Scolopendrella is of a single form as in Campodea and Japyx. The fat-body, dorsal vessel, with its valves and ostia, are alike in the two groups, as are the appendages of the end of the abdomen, the anal cerci (cercopoda) of Scolopendrella being the homologues of the 21multiarticulate appendages of Lepisma, etc., and of the forceps of Japyx. In those of Scolopendrella, we have found the large duct leading from the voluminous silk-gland, a single large sac extending forwards into the third segment from the end of the body (Fig. 15, s. gl). Other points of resemblance, all of which he enumerates, are the slight differences in the number of trunk-segments, the presence in the two groups of the abdominal “false-legs” (parapodia), the dorsal plate, and the mouth-parts. As regards the latter, Grassi affirms that there is a perfect parallelism between those of Scolopendrella and Thysanura. To this point we will return again in treating more especially of those of the Symphyla. Finally, Grassi concludes that there is “a great resemblance between the Thysanura and Scolopendrella.” He, however, believed that the Symphyla are the forerunners of the myriopods, and not of the insects, his genealogical tree representing the symphylan and thysanuran phyla as originating from the same point, this point also being, rather strangely, the point of origin of the arachnidan phylum.

Haase (1889) regarded Scolopendrella as a myriopod, and Pocock (1893) assigned the Symphyla to an independent class, regarding Scolopendrella as “the living form that comes nearest to the hypothetical ancestor of the two great divisions of tracheates.” Schmidt’s work (1895) on the morphology of this genus is more extended and richly illustrated than Grassi’s, his method of research being more modern. He also regards this form as one of the lower myriopods.

In conclusion, it appears to us that, on the whole, if we throw out the single characteristic of the anteriorly situated genital opening, the ovarian tubes being directed toward the end of the body (Fig. 15, ovd, ov), there is not sufficient reason for placing the Symphyla among the Myriopoda, either below or near the diplopods. This is the only valid reason for not regarding Scolopendrella as the representative of a group from which the insects have descended, and which partly fills the wide abyss between Peripatus and insects. With the view of Pocock, that both insects and myriopods have descended from Scolopendrella, we do not agree, because this form has so many insectean features, and a single unpaired genital opening. For the same reason we should not agree with Schmidt in interpolating the Symphyla between the Pauropoda and Diplopoda. In these last two progoneate groups the genital openings are paired, hence they are much more primitive types than Scolopendrella, in which there is but a single opening. It seems most probable that the Symphyla, though progoneate, are more recent forms than the 22progoneate myriopods, which have retained the primitive feature of double sexual outlets. It is more probable that the Symphyla were the descendants of these polypodous forms. Certainly Scolopendrella is the only extant arthropod which, with the sole exception of the anteriorly situated genital opening, fulfils the conditions required of an ancestor of Thysanura, and through them of the winged insects. No one has been so bold as to suggest the derivation of insects from either diplopods or chilopods, while their origin from a form similar to Scolopendrella seems not improbable. Yet Uzel has very recently discovered that Campodea develops in some respects like Geophilus, the primitive band sinking in its middle into the yolk, with other features as in chilopods.[7] The retention of a double sexual opening in the diplopods is paralleled by the case of Limulus with its double or paired sexual outlets, opening in a pair of papillæ, as compared with what are regarded as the generalized or more primitive Crustacea, which have an unpaired sexual opening.

The following summary of the structural features of the Symphyla, as represented by Scolopendrella, is based mainly on the works of Grassi, Haase, and Schmidt, with observations of my own.

Diagnostic or essential characters of Symphyla.Head shaped as in Thysanura (Cinura), with the Y-shaped tergal suture, which occurs commonly in insects (Thysanura, Collembola, Dermaptera, Orthoptera, Platyptera, Neuroptera, etc.), but is wanting in Myriopoda (Diplopoda and Chilopoda); antennæ[8] unlike those of Myriopoda in being very long, slender, and moniliform. Clypeus distinct. Labrum emarginate, with six converging teeth. Mandibles 2–jointed, consisting of a vestigial stipes and distal or molar joint, the latter with eight teeth. First maxillæ with an outer and inner mala situated on a well-developed stipes; with a minute, 1–jointed palpus. Second pair of maxillæ: each forming two oblong flat pieces, median sutures distinct, with no palpi; these pieces are toothed in front, and appear to be homologous with the two 23median pieces of the gnathochilarium of Diplopoda. Hypopharynx? Epipharynx?

Trunk with from fifteen to sixteen dorsal, more or less free subequal scutes, the first the smallest. Pedigerous segments twelve; also twelve pairs of 5–jointed legs, which are of nearly equal length, the first pair 4–, the others 5–jointed, all ending in two claws, as in Synaptera and winged insects. A pair of 1–jointed anal cerci homologous with those of Thysanura and Orthoptera, into each of which opens a large abdominal silk-gland. Abdominal segments with movable styles or “pseudopods” (“Parapodia” of Latzel and of Schmidt), like those of Campodea and Machilis, and situated on the base of the coxal joint in front of the ventral sac. Within the body near the base of each abdominal style is an eversible coxal sac or blood-gill (Fig. 15, cg). The single genital opening is on the fourth trunk-segment in both sexes (Fig. 15, indicated by the arrow). The malpighian tubes (ur. t) are two in number, opening into the digestive canal at the anterior end of the hind intestine; they extend in front to the third or second segment from the head. They are broad and straight at their origin, becoming towards the end very slender and convoluted.

The three divisions of the digestive tract are as in insects, the epithelium of the mid-gut being histologically as in Campodea and Japyx; rectal glands are present. A pair of very large salivary glands are situated in the first to the fourth trunk-segments, consisting of a glandular portion with its duct, which unite into a common duct opening on the under side of the head, probably in the labium.

But a single pair of stigmata is present, and these are situated in the front of the head, beneath the insertion of the antennæ and within the stipes of the mandibles; the tracheæ are very fine, without spiral threads (tænidia), and mostly contained within the head, two fine branches extending on each side into the second trunk-segment.

After birth the body increases in length by the addition of new segments at the growing point.

In respect to the nervous system, there are no diagnostic characters; there are, however, not as many as two pairs of ganglia to a segment. The brain is well developed, sending a pair of slender nerves to the small eyes. The ganglia of the segment bearing the first pair of legs is fused with the subœsophageal ganglion. Grassi was unable to detect a true sympathetic system, but he suspects the existence of a very small frontal ganglion.

The slender dorsal vessel, provided with ostia and valvules, pulsates along the entire length of the trunk; an aorta passes into the head.

24The internal genital organs of both sexes are paired, and extend along the greater part of the trunk; in either sex they may be compared to two long, slender, straight cords extending from the fourth to the tenth pair of legs. The two oviducts do not unite before reaching the sexual opening (Fig. 15, ovd).

The male sexual organs are more complicated than the feminine. The paired testicular tubes lie in trunk-segments 6 to 12, on each side, and partly under the intestinal canal, communicating with each other by a cross-anastomosis situated under the intestine, and which, like the testes, is filled with sperm. Of the paired seminal ducts (vas deferens) in trunk-segment 4, each unites again into a thick tube, sending a blind tube forward into the third segment. Under the place of union of the two vasa deferentia arise the paired ductus ejaculatorii, which open beneath in the uterus masculinus. The anterior blind ends of the vasa deferentia form a sort of small paired vesiculæ seminales in which a great quantity of ripe sperm is stored. The uterus masculinus is in its structure homologous with the evaginable penis of Pauropus, Polyxenus, and some diplopods, and the sexual opening has without doubt become secondarily unpaired. The sexual opening is rather long and is closed by two longitudinal folds. “In several respects the male sexual organs of Scolopendrella are like those of Pauropus; in the last-named form we have indeed an unpaired testis, but also in Scolopendrella we see the beginning of such a singleness; namely, the presence of an anastomosis uniting the two tubes, their communication by means of a transverse connecting canal and a glandular structure in the epithelium forming them. The male sexual organs of Pauropus differ only through a still greater complication.” (Schmidt.)

Scolopendrella in habits resembles chilopods, being found in company with Geophilus burrowing deep in light sand under leaves, or living at the surface of the ground under sticks or stones. It is very agile in its movements, and is probably carnivorous. It was considered by Haase to be eyeless, but the presence of two ocelli has been demonstrated both by Grassi and by Schmidt. Whether the pigment and corneous facet are present is not certain. The embryology is entirely unknown (although Henshaw reports finding a hexapodous young one), and it need not be said that a knowledge of it is a very great desideratum. It is most probable that the young is hexapodous, since the first pair of limbs are 4–jointed, all the rest 5–jointed; while Newport, and also Ryder, observed specimens with nine, ten, eleven, and twelve pairs, and Wood-Mason confirms their observations, “which prove that a pair of legs 25is added at each moult,” and he concludes that the addition of new segments “therefore takes place in this animal by the intercalation of two at each moult between the antepenultimate and penultimate sterna, as in the Chilognatha, and as also in some of the Chilopoda.”

There is but one family, Scolopendrellidæ, and a single genus, Scolopendrella, which seems to be, like other archaic types, cosmopolitan in its distribution.

Our commonest species is S. immaculata Newport, which occurs from Massachusetts to Cordova, Mexico, and in Europe from England to the Mediterranean and Russia; Mr. O. F. Cook tells me he has found a species in Liberia, West Africa. The other species are S. notacantha Gervais, Europe and Eastern United States; S. nivea Scopoli (S. gratiæ Ryder), Europe and United States; S. latipes Scudder, Massachusetts.

LITERATURE ON SCOLOPENDRELLA

Newport, George. Monograph of the class Myriopoda, order Chilopoda. (Trans. Linn. Soc. xix, pp. 349–439, 1 Pl., 1845.)

Menge, A. Myriapoden der Umgegend von Danzig. (Neuste Schriften der naturforsch. Gesell. Danzig. iv, 1851.)

Ryder, John H. Scolopendrella as the type of a new order of articulates (Symphyla). (Amer. Nat., May, 1880, xiv, pp. 375, 376.)

—— The structure, affinities, and species of Scolopendrella. (Proc. Acad. Nat. Soc. Phil., pp. 79–86, 1881, 2 Figs.)

Packard, A. S. Scolopendrella and its position in nature. (Amer. Nat., 1881, pp. 698–704, Fig.)

Muhr, Jos. Die Mundtheile von Scolopendrella und Polyzonium. Prag, 1882, 1 Pl.

Mason, J. Wood. Morphological notes bearing on the origin of insects. (Trans. Ent. Soc. London, 1879, pp. 145–167, Figs.)

—— Notes on the structure, post-embryonic development, and systematic position of Scolopendrella. (Ann. and Mag. Nat. Hist., July, 1883, pp. 53–63.)

Latzel, Robert. Die Myriapoden der osterreichisch-ungarischen Monarchie, ii, Wien, 1884, pp. 1–39, Pls.

Haase, Erich. Die Abdominalanhänge der Insekten mit Berücksichtigung der Myriapoden. (Morph. Jahrbuch, xv, pp. 331–435, 2 Pls., 1889.)

Schmidt, Peter. Beiträge zur Kenntnis der niederen Myriapoden. (Zeits. f. wissen. Zool., lix, pp. 436–510, 2 Pls., 1895.)

26

INSECTA (HEXAPODA)

We are now prepared to discuss the fundamental or essential characters of the insects, including the wingless subclass (Synaptera), and the winged (Pterygota).

Diagnostic characters of insects.Body consisting of not more than twenty-one segments, which are usually heteronomous or of unequal size and shape, arranged in three usually well-defined regions; i.e. a head, thorax, and hind-body or abdomen. Head small and flattened or rounded, composed of not less than six segments, and bearing, besides the eyes, at least four pairs of jointed appendages; i.e. one pair of antennæ, and three pairs of masticatory appendages, the distal or molar portion of which is primarily divided into three divisions, supported on a stipes and cardo, and in certain orders modified into piercing or sucking structures. The head is composed of an epicranium, bearing a distinct clypeus and labrum, with the epipharynx. Mandibles 1–jointed, without a palpus and very generally with no, or uncertain, traces of a lacinia and a stipes. Two pairs of maxillæ; the first pair separate, usually 3–lobed, comprising a lacinia, galea, and palpifer, with a palpus which is never more than 6–jointed. The second pair united to form the labium or under lip, composed of two laciniæ fused together; in the generalized forms with a rudimentary galea; bearing a pair of palpi, never more than 4–jointed; with paraglossæ sometimes present.

(A third pair of mouth-appendages situated between the antennæ and mandibles in the embryo of Anurida, and Apis, and adult Campodea.)

The epipharynx forming the roof of the mouth, and bearing gustatory organs. Hypopharynx usually well developed, lying on the under side of the mouth, just above the labium, and receiving the end of the salivary duct.

Eyes of two kinds: a pair of compound, and from two to three simple eyes (ocelli).

The thorax consisting of three segments, the two latter segments in the winged orders highly differentiated into numerous tergal and lateral pieces and a single sternum; in the Synaptera the segments are undivided. (In the higher Hymenoptera the basal abdominal segment coalesced with the thorax.) Three pairs of legs, each foot ending in a pair of claws. Two pairs of wings (except in the Synaptera), a pair to each of the two hinder thoracic segments; the wings occasionally reduced or wanting in certain adaptive forms, which, however, had winged ancestors.

Abdomen consisting at the most of from ten to twelve segments. No 27functional abdominal legs except in the Thysanura, and in the larvæ of Lepidoptera. A pair of 1– or many-jointed cercopods on the tenth segment; and in certain forms a pair of styles on the ninth segment. In certain orders an ovipositor or sting formed of three pairs of styliform processes; in Collembola a single pair of processes forming the elater.

The genital openings opisthogoneate, usually single, but paired in Thysanura (Lepisma), Dermaptera, and Plectoptera (Ephemeridæ).

The digestive canal in the winged orders is highly differentiated, the fore-intestine being divided into an œsophagus and proventriculus, the hind-intestine into an ileum, colon, and rectum, with rectal glands.

The nervous system consists of a well-developed brain, in the more specialized orders highly complicated; no more than thirteen pairs of ganglia, which may be more or less fused in the more specialized orders. Three frontal ganglia, and a well-developed, sympathetic system present.

Stigmata confined (except possibly in Sminthurus) to the thorax and abdomen, not more than ten pairs in all, and usually but nine pairs. Tracheal system as a rule highly differentiated; invariably with tænidia.

Dorsal vessel with ostia and valvules; no arteries except the cephalic aorta; no veins. After birth there is in the more specialized pterygote orders a reduction in the number of terminal segments of the abdomen.

Development either direct (Synaptera), or with an incomplete (with nymph and winged or imaginal stages), or complete metamorphosis; in the latter case with a larval, pupal, and imago stage.

The insects may be divided into two sub-classes,—the Synaptera, and the winged orders, Pterygota, of Gegenbaur (1877), since the differences between the two groups appear on the whole to be of more than ordinal rank.

1. EXTERNAL ANATOMY

a. The regions of the body

The insects differ from other arthropods in that the body is divided into three distinct regions,—the head, thorax, and abdomen, the latter regions in certain generalized forms not always very distinctly differentiated. The body behind the head may also conveniently be called the trunk, and the segments composing it the trunk-segments.

In insects the head is larger in proportion to the trunk than in other classes, notably the Crustacea; the thorax is usually slightly or somewhat larger than the head, while the hind-body or abdomen 28is much the larger region, as it consists of ten to eleven, and perhaps in the Dermaptera and Orthoptera twelve, segments, and contains the mid- and hind-intestine, as well as the reproductive organs.

When we compare the body of an insect with that of a worm, in which the rings are distinctly developed, we see that in insects ring distinctions have given way to regional distinctions. The segments lose their individuality. It is comparatively easy to trace the segments in the hind-body of an insect, as in this region they are least modified; so with the thorax; but in the head of the adult insect it is impossible to discover the primitive segments, as they are fused together into a sort of capsule, and have almost entirely lost their individuality.

In general it may be said that the head contains or bears the organs of sense and of prehension and mastication of the food; the thorax the organs of locomotion; and the abdomen those of reproduction.

When we compare the body of a wasp or bee with that of a worm, we see that there is a decided transfer of parts headward; this process of cephalization so marked in the Crustacea likewise obtains in insects. Also the two hinder regions of the body are, in a much greater degree than in worms, governed by the brain, the principal seat of the intelligence, which, so to speak, dominates and unifies the functions of the body, both digestive, locomotive, and reproductive, as also those of the muscles moving the different segments and regions of the body. To a large extent arthropodan morphology and class distinctions are based on the regional arrangement of the somites themselves. Thus in the process of grouping of the segments into the three regions, some increase in size, while others undergo a greater or less degree of reduction; one segment being developed at the expense of one or more adjoining ones. This principle was first pointed out by Audouin, and is called Audouin’s law. It is owing to the greater development of certain segments and the reduction of others, both of the body-segments and of the segments of the limbs, that we have the wonderful diversity of form in the species and genera, and higher groups of insects, as well as those of other arthropods.

b. The integument (exoskeleton)

The skin or integument of insects consists, primarily, as in worms and all arthropods, of an epithelial layer of cells called the hypodermis. This layer secretes the cuticle, which is of varying thickness 29and flexibility, and is usually very dense, impermeable, and light, compared with the crust of the Crustacea, where the cuticle becomes heavy and solid by the deposition of the carbonate and phosphate of lime. This is due to the presence of a substance called by Odier chitin.[9] The cuticle is thin, delicate, and flexible between the joints; it is likewise so in such diaphanous aquatic larvæ as that of Corethra, and in the gills of aquatic insects, also in the walls of the tracheæ and of the salivary ducts. The cuticle thus forms a more or less solid crust which is broken into joints and pieces (sclerites), forming supports for the attachments of the muscles and serving to protect the soft parts within.

Chitin.—If we allow an insect to soak for a long time in acids, or boil it in liquid potassa or caustic potash, the integument is not affected. The muscles and the other soft parts are dissolved, leaving the cuticle clear and transparent. This insolubility of the cuticle is due to the presence of chitin, the insoluble residue left after such treatment. It also resists boiling in acids, in any alkalies, alcohol or ether. The chemical formula is C15H26N2O10.[10]

“Chitin forms less than one-half by weight of the integument, but it is so coherent and uniformly distributed that when isolated by chemical reagents, and even when cautiously calcined, it retains its original organized form. The color which it frequently exhibits is not due to any essential ingredient; it may be diminished or even destroyed by various bleaching processes.” (Miall and Denny.)

“The chemical stability of chitin is so remarkable that we might expect it to accumulate like the inorganic constituents of animal skeletons, and form permanent deposits. Schlossberger (Ann. d. chem. u. pharm., bd. 98) has, however, shown that it changes slowly under the action of water. Chitin kept for a year under water partially dissolved, turned into a slimy mass, and gave off a peculiar smell. This looks as if it were liable to putrefaction. The minute proportion of nitrogen in its composition may explain the complete disappearance of chitin in nature.” (Miall and Denny, The Cockroach, p. 29.)

Chitin, or a substance closely similar to it, occurs in worms and in their tubes, especially in the pharyngeal teeth of annelids and in their setæ. The shell of Lingula and the pen of cuttle-fish contain true chitin (Krukenberg). The integument of Limulus, of trilobites, and of Arachnida, as well as Myriopoda, appears to consist of chitin.[11]

The chitin is rapidly deposited at the end of embryonic life, also during the larval and pupal stages. As is well known, insects after 30moulting are white, but in a few hours turn dark, and those which live in total darkness are white, showing that light has a direct effect in causing the dark color of the integument.

Moseley analyzed one pound weight of Blatta, and found plenty of iron with a remarkable quantity of manganese.

Schneider regarded chitin as a hardening of the protoplasm rather than a secretion, and the cuticle is looked upon as an exudation. It is structureless, not consisting of cells, and consists of fine irregular laminæ. “A cross-section of the chitinous layer or ‘cuticle’ examined with a high power shows extremely close and fine lines perpendicular to the laminæ.” In the cockroach the free surface of the cuticle is divided into polygonal, raised spaces or areas which correspond each to a chitinous cell of the hypodermis. (Miall and Denny.)

Numerous pore-canals pass through the cuticle of all the external parts of the body. The larger canals nearly always form the way for the passage of secretions from dermal cells, or connect with the cavities of hairs or setæ; when very fine and not connected with hairs or scales, they are either empty or filled with air, and may possibly serve for respiration.

Vosseler distinguishes in the cuticle two layers of different physical and chemical characters. Besides the external chitinous layer there is an inner layer which entirely agrees with cellulose. (Zool. Centralblatt, ii, 1895, p. 117.)

The reparative nature of chitin is seen in the fact that Verhoeff finds that a wound on an adult Carabus, and presumably on other insects, is speedily closed, not merely by a clot of blood, but by a new growth of chitin.

c. Mechanical origin and structure of the segments (somites, arthromeres, metameres, zonites)

The segments are merely thickenings of the skin connected by folds or duplications of the integument, and not actually separate or individual rings or segments. This is shown by longitudinal (sagittal) sections through the body, and also by soaking or boiling the entire insect in caustic potash, when it is seen that the integument is continuous and not actually subdivided into separate somites or arthromeres, since they are seen to be connected by a thin intersegmental membrane (Fig. 16). But this segmentation or metamerism of the integument is, however, the external indication of the segmentation of the arthropodan body most probably inherited 31from the worms, being a disposition of the soft parts which is characteristic of the vermian type. This segmentation of the integument is correlated with the serial repetition of the ganglia of the nervous system, of the ostia of the dorsal vessel, the primitive disposition of the segmental and reproductive organs, of the soft, muscular dissepiments which correspond to the suture between the segments, and with the metameric arrangement of the muscles controlling the movements of the segments on each other, and which internal segmentation or metamerism is indicated very early in embryonic life by the mesoblastic somites.

Fig. 16.—Diagram of the anterior part of an insect, showing the membranous intersegmental folds, g.—After Graber.

In the unjointed worms, as Graber states, the body forms a single but flexible lever. In the earthworm the muscular tube or body-wall is enclosed by a stiffer cuticle, divided into segments; hence the worm can move in all required directions, but only by sections, as seen in Fig. 16, which represents the thickened integument divided into segments, and folded inward between each segment, this thin portion of the skin being the intersegmental fold. Each segment corresponds to a special zone of the subdivided muscular tube (m), the fascia extending longitudinally. The figure shows the mode of attachment of the fascia of the muscle-tube to the segment. The anterior edge is inserted on the stiff, unyielding, inner surface of each segment: the hinder edge of the muscle is attached to the thin, flexible, intersegmental fold, which thus acts as a tendon on which the muscle can exert its force. (Graber.)

Fig. 17.—Diagram of the integument and arrangement of the segmental muscles: A, relaxed; m, muscle; g, membranous articulation; r, chitinous ring. B, the same contracted on both sides. C, on one side.—After Graber.

“Fig. 17 makes this still clearer. The muscles (m) extend between two segments immediately succeeding each other. Supposing the anterior one (A) to be stationary, what do we then see when the muscle contracts? Does it also become shorter? The intersegmental 32fold is drawn forwards, and hence the entire hinder segment moves forward and is shoved into the front one, and so on with the others, as at B. Afterwards, if the strain of the muscle is relieved by the diminishing action of the tensely stretched, intersegmental membrane, it again returns to a state of rest.” (Graber.)

Fig. 18.—Diagrams to demonstrate the mechanism of the motion of the segmented body in the Arthropoda: One larger segment (cf) and 4 smaller. The exoskeleton is indicated by black lines, the interarticular membranes by dotted lines. The hinges between consecutive segments are marked at, tergal (dorsal) skeleton; s, sternal (ventral) skeleton; d, dorsal longitudinal muscles = extensors (and flexors in an upward direction); v, ventral longitudinal muscles = flexors. In B, the row of segments is stretched; in A, by the contraction of the muscles (d) bent upward; in C, downward; tg, tergal; sg, sternal interarticular membranes.—After Lang.

While we look upon the dermal tube of worms as a single but flexible lever, the body of the arthropods, as Graber states, is a linear system of stiff levers. We have here a series of stiff, solid rings, or hooks, united by the intersegmental membrane into a whole. When the muscles, extending from one ring to the next behind contract, and so on through the entire series, the rings approximate each other.

The ectoskeletal segments bend to one side by the contraction of the muscles on one side, the point of the outer segmental fold opposite the fixed point becoming converted into the turning-point (C).

The usual result of the arrangement of the locomotive system is the simple curving of the body (C), and then the alternate bending of the body to right and left, which produces the serpentine movements characteristic of the earthworms, the centipede, and many insect larvæ. The most striking example of the wonderful variety of movements which can be made by an insect are those of the Syrphus larva. When feeding amid a herd of aphides, it is seen to now raise the front part of the 33body erect and stiff, then to bend it down, or rapidly turn it to either side, or move it in a complete circle. (Graber, pp. 23–26.)

The arrangement and mode of working of the muscles, says Lang, is illustrated by Fig. 18, which shows us five segments, one larger (ct) and four smaller, in vertical projection. The thicker portion of the integument is marked by strong outlines, the delicate and flexible interarticular membranes (tg, sg) in dotted lines. The hinges between two consecutive segments are marked a. A dorsal muscle (d) is attached to the larger segment (ct), and runs through the smaller segments, being inserted in the dorsal portion of the crust (t) of each by means of a bundle of fibres. A ventral muscle (v) does the same on the sternal side (s).

“The skeletal segments,” adds Lang, “may be compared to a double-armed lever, whose fulcrum lies in the hinges. If the dorsal muscle contracts, it draws the dorsal arm of the lever (the tergal portion of the skeleton) in the direction of the pull towards the larger segments; the tergal interarticular membranes become folded, the ventral stretched, and the four segments bend upward (Fig. 18, A). If the ventral muscle contracts, while at the same time the dorsal slackens, the row of segments will be bent downwards (Fig. 18, C).”

L. B. Sharp suggests, that in the Crustacea the rings formed by “the regularity and stress of muscular action” would be hardened by the deposition of lime at the most prominent portion, i.e. between what we have called the intersegmental folds. (American Naturalist, 1893, p. 89.) Cope also states that “with the beginning of induration of the integument, segmentation would immediately appear, for the movements of the body and limbs would interrupt the deposit at such points as would experience the greatest flexure. The muscular system would initiate the process, since flexure depends on its contractions, and its presence in animals prior to the induration of the integuments in the order of phylogeny, furnishes the conditions required.” (The Primary Factors of Organic Evolution, p. 268, 1895.)

It is apparent that the jointed or metameric structure of the bodies of insects and other arthropods is an inheritance from the segmented worms. In the worms the body is a continuous dermo-muscular tube, while in arthropods this tube is divided into regions, and the cuticle is thicker and more resistant. To go back to the incipient stages in the process of segmentation of the body, we conceive that the worms probably arose from a creeping gastrula-like form, the gastræa. The act of creeping gradually induced an elongated shape of the body. The movement of such an organism in a 34forward direction would gradually evolve a fore and aft, dorsal and ventral, and bilateral symmetry. As soon as this was attained, as the effect of creeping over rough irregular surfaces there would result mechanical lateral strains intermittently acting during the serpentine movements of the worm. The integument would, we can readily suppose, tend to bend or yield, or become permanently wrinkled, at more or less regular intervals. The arrangement of the muscles would gradually conform to this habit of creeping, and finally the nervous system and other organs more directly connected with the creeping movements of the organism would tend to be correlated in their arrangement with that of the segments. In this way the homonomous segments of the annelid body probably became developed, and their relations and shapes were eventually fixed by inheritance. After this stage was reached, and limbs began to appear, the segments would tend to become heteronomous, and to be grouped into regions.

Fig. 19.Dujardinia rotifera, with jointed tentacles and caudal appendages.—With some changes, after Quatrefages.

The origin of the joints or segments in the limbs of arthropods was probably due to the mechanical strains to which what were at first soft fleshy outgrowths along the sides of the body became subjected. Indeed, certain annelid worms of the family Syllidæ have segmented tentacles and parapodia, as in Dujardinia (Fig. 19). We do not know enough about the habits of these worms to understand how this metamerism may have arisen, but it is possibly due to the act of pushing or repeated efforts to support the body while creeping over the bottom among broken shells, over coarse gravel, or among seaweeds.

It is obvious, however, that the jointed structure of the limbs of arthropods, if we are to attempt any explanation at all of the origin of such structure, was primarily due mainly to lateral strains and 35impacts resulting from the primitive endeavors of the ancestral arthropods to raise and to support the body while thus raised, and then to push or drag it forward by means of the soft, partially jointed, lateral limbs which were armed with bristles, hooks, or finally claws.

On the other hand, by adaptation, or as the result of parasitism and consequent lack of active motion, the original number of segments may by disuse be diminished. Thus in adult wasps and bees, the last three or four abdominal segments may be nearly lost, though the larval number is ten. During metamorphosis the body is made over, and the number, shape, and structure of the segments greatly modified. In the female of the Stylopidæ the thorax loses all traces of segments, and is fused with the head, and the abdominal segments are faintly marked, losing their chitin.

While the maxillæ have several joints, the mandibles are 1–jointed, but there are traces of two joints in Campodea, certain beetles, etc. In the antenna there is a great elasticity in respect to the number of joints, which vary from one or two to a hundred or more. It is likewise so in the thoracic legs, where the number of tarsal joints varies from one to five; also in the cercopoda, the number of joints varying from one or two to twelve or more.

d. Mechanical origin of the limbs and of their jointed structure

We have already hinted at the mode of origin of the limbs of arthropods. Like the body or trunk, the limbs are chitinous dermo-muscular tubes, with a dense solid cuticle, and internal muscles, and were it not for their division at more or less regular intervals into segments, forming distinct sets of levers, set up by the strains in these tubular supports, there would be no power of varied motion.

Even certain worms, as already stated, have their tentacles and parapodia, or certain appendages of their parapodia, more or less jointed, but there are no indications of claws or of any other hard chitinous armature at the extremity, and the skin is thin and soft.

In the most simple though not the most primitive arthropods, such as the Tardigrades, whose body is not segmented, there are four pairs of short unjointed legs, ending each in two claws, which have probably arisen in response to the stimulus of pushing or dragging efforts.

The legs of Peripatus are unjointed, and have a thin cuticle, but end in a pair of claws, which have evidently arisen as a supporting 36armature, the result of the act of moving or pulling the body over the uneven surface of the ground.

Fig. 20.—A prothoracic leg of Chironomus larva; and pupa.

Fig. 21.A, larva of Ephydra californica: a, b, c, pupa.

There is good reason to suppose that such limbs arose from dynamical causes, similar to those exciting the formation of secondary adaptations such as are to be seen in the prop or supporting legs of certain dipterous larvæ, as the single pair of Chironomus (Fig. 20) and Simulium, or the series of unjointed soft tubercles of Ephydra (Fig. 21), etc., which are armed with hooks and claws, and are thus adapted for dragging the insect through or over vegetation or along the ground.

Now by frequent continuous use of such unjointed structures, the cuticle would tend to become hard, owing to the deposit of a greater amount of chitin between the folds of the skin, until finally the body being elongated and homonomously segmented, the movements of walking or running would be regular and even, and we would have homonomously jointed legs like those of the trilobites, or of the most generalized Crustacea and of Myriopoda.

In the most primitive arthropods,—and such we take it were on the whole the trilobites, rather than the Crustacea,—the limbs were of nearly the same shape, being long and slender and evenly jointed from and including the antennæ, to the last pair of limbs of the abdominal region. In these forms there appear to be, so far as we now know, no differentiation into mandibles, maxillæ, maxillipedes, and thoracic legs, or into gonopoda. The same lack of diversity of structure and function of the head-appendages has survived, with little change, in Limulus. In the trilobites (Fig. 1) none of the limbs have yet been found to end in claws or forceps; being in this respect nearly as primitive as in the worms. Secondary adaptations have arisen in Limulus, the cephalic appendages being forcipated, adapted as supports to the body and for pushing it onward through the sand or mud, while the abdominal legs are broad and flat, adapted for swimming and bearing the broad gill-leaves.

It is thus quite evident that we have three stages in the evolution of the arthropodan limb; i.e. 1, the syllid stage, of simple, jointed, soft, yielding appendages not used as true supports (Fig. 19); 2, the 37trilobite stage, where they are more solid, evenly jointed, but not ending in claws; and by their comparatively great numbers (as in the trilobite, Triarthrus) fully supporting the body on the bottom of the sea. In Limulus they are much fewer in number, thicker, and acting as firm supports, the cephalic limbs of use in creeping, and ending in solid claws. 3, The third stage is the long slender swimming head-appendages of the nauplius stage of Crustacea.

As regards the evolution of limbs of terrestrial arthropods, we have the following stages: 1, the soft unjointed limbs of Tardigrades, ending in two claws, and those of Peripatus, and the pseudo- or prop-legs of certain dipterous larvæ; 2, finally the evolution of the long, solid, jointed limbs of Pauropus and other primitive myriopods, the legs forming solid, firm supports elevating the body, and enabling the insect to drag itself over the ground or to walk or run. When the body is elongated and many-segmented, the legs are necessarily numerous; but when it is short, the legs become few in number, i.e. six, in the hexapodous young of myriopods and in insects, or eight in Arachnida. Whenever the legs are used for walking, i.e. to raise and support the body, they end in a solid point or in a pair of forceps or claws. On the other hand, as in phyllopods, where the legs are used mainly for swimming, they are unarmed and are soft and membranous, or, as in the limbs of the nauplius or zoëa stage of crustaceans, end in a simple soft point, which often bears tactile setæ.

The tarsal joints are more numerous in order to give greater flexibility to the limb in seizing and grasping objects, both to drag the body forwards and to support it.

Unlike those of the Crustacea, the limbs of insects are not primitively biramose, but single, the three-lobed first maxillæ, and secondarily bilobed second maxillæ being the result of adaptation. Embryology on the whole proves the truth of this assumption; the maxillæ of both pairs are at first single buds, afterwards becoming lobed. All the appendages of the body, including the ovipositor or sting, are modified limbs, as shown by their embryological development.

It is noticeable that in the crab, where the body is raised by the limbs above the bottom, it is much shorter and more cephalized than in the shrimps. Also in the simply walking and running spiders, the hind-body is shorter than in scorpions, while in the running and flying insects, such as the Cicindelidæ, and in the swiftly flying flies and bees, there is a tendency to a shortening of the body, especially of the abdomen. The long body of the dragon-fly is an 38impediment to flight, but compensated for by the action of the large wings.

The arthropodan limb is a compound leverage system. It is, says Graber, a lateral outgrowth of the trunk, which repeats in miniature that of the main trunk, its single series of joints or segments forming a jointed dermo-muscular tube. Yet the lateral appendages of an insect differ from the main trunk in two ways: (1) they taper to the end which bears the two claws, and (2) their segments are in the living animal arranged not in a straight line, but at different angles to each other. The basal joint turning on the trunk acts as the first of a whole series of levers. The second joint, however, is connected with the musculature of the first or basal joint, and thus each succeeding joint is moved on the one preceding. Each lever, from the first to the last, is both an active and a passive instrument. (Graber.)

While, however, as Graber states, the limbs possess their own sets of muscles and can move by the turning of the basal joint, the labor is very much facilitated, as is readily seen, by the trunk, though the latter has to a great extent delegated its locomotive function to the appendages, which again divide its labor among the separate joints.

Graber then calls attention to the analogy of the mechanics of locomotion of insects to those of vertebrates. An insect’s and a vertebrate’s legs are constructed on the same general mechanical principles, the limbs of each forming a series of levers.

Fig. 22.—Diagram of the knee-joint of a vertebrate (A) and an insect’s limb (B): a, upper; b, lower, shank, united at A by a capsular joint, at B by a folding joint; d, extensor or lifting muscle; d1, flexor or lowering muscle of the lower joint. The dotted line indicates in A the contour of the leg.—After Graber.

Fig. 22, A, represents diagrammatically the knee joint of a vertebrate, and B that of an insect; a, the femur or thigh, and b, the tibia or shank. In the vertebrate the internally situated bones are brought into close union and bend by means of a hinge-joint; so also in the chitinous-skinned insect.

The stiff dermal tube of the insect acts as a lever by means of the thin intersegmental membrane (c) pushed in or telescoped in to the thigh joint, a special joint-capsule being superfluous. The muscles are in general the same in both types; they form a circle. In both the shank is extended by the contraction of the upper muscles (d) and is bent by the contraction of the lower 39(d1). The intersegmental membrane of the insect’s limb is in a degree a two-armed lever, whose pivot (f) lies in the middle. The internal invagination of the intersegmental fold (B, g-h) affords the necessary support to the muscles acting like the tendon in the vertebrate. (Graber.)

Fig. 23.—Primitive band or germ of a Sphinx moth, with the segments indicated, and their rudimentary appendages: c, upper lip; at, antennæ; md, mandibles; mx, mx′, first and second maxillæ; l, l′, l″, legs; al, abdominal legs.—After Kowalevsky.

Graber also calls attention to the fact that this insect limb differs in one important respect from that of land vertebrates. The leverage system in the last is divided at the end into five parallel divisions or digits. In arthropods, on the contrary, all the joints succeed one another in a linear series.

In insects, as well as in other arthropods, modifications of the limbs usually take the form of a simple reduction in the number of segments. Thus while the normal number of tarsal joints is five, we have trimerous and dimerous Coleoptera, and in certain Scarabæidæ the anterior tarsi are lost.

Savigny was the first, in 1816, in his great work, “Théorie des organes de la bouche des Crustacés et des Insectes,” to demonstrate that not only were the buccal appendages of biting insects homologous with those of bugs, moths, flies, etc., but that they were homologous with the thoracic legs, and that thus a unity of structure prevails throughout the appendages of the body of all arthropods. Oken also observed that “the maxillæ are only repeated feet.”

What was modestly put forth as a theory by the French morphologist has been abundantly proved by the embryology of insects of different orders to be a fact. As shown in Fig. 23 the antennæ and buccal appendages arise as paired tubercles exactly as the thoracic legs. The abdominal region also bears similar embryonic or temporary limbs, all of which in those insects without an ovipositor disappear, except the cercopoda, after birth.

40

LITERATURE ON THE EXTERNAL ANATOMY

General

Swammerdam, Johann. Biblia naturæ. (In Dutch, German, and English.) 1737–1738, fol., London, 1758, Pls.

Réaumur, Réné Antoine Ferchault, de. Mémoires pour servir à l’histoire des insectes. i-iv, 4º, Paris, 1734–1742.

Lyonet, Pieter. Traité anatomique de la chenille, qui ronge le bois de saule, etc. 4º, pp. xxii, 616. À la Haye, 1732. Tab. 18.

—— Recherches sur l’anatomie et les metamorphoses de differentes espèces d’insectes. Ouvrage posthume, publié par M. W. de Haan. pp. 580, tab. 54, 1832.

Latreille, Pierre André. Des rapports généraux de l’organization extérieure des animaux invertébres articulés, et comparaison des Annelides avec les Myriapodes. (Mémoires du Mus. d’Hist. Nat., 1820, vi, pp. 116–144.)

—— De quelques appendices particuliers du thorax de divers insectes. (Mémoires du Mus. d’Hist. Nat., 1821, vii, pp. 1–21).

—— Observations nouvelles sur l’organization extérieure et générale des animaux articulés et à pieds articulés, et application de ces connoissances à la nomenclature des principales parties des mêmes animaux. (Mèmoires du Mus. d’Hist. Nat., viii, 1822, pp. 169–202.)

Cuvier, George Leopold Christian Dagobert. Rapport sur les recherches anatomiques sur le thorax des animaux articulés et celui des insectes en particulier par M. V. Audouin. 4º, pp. 15, tab. 1, Paris, 1823.

Audouin, Jean Victor. Recherches anatomiques sur le thorax des animaux articulés et celui des insectes hexapodes en particulier. (Annales des Sciences naturelles, i, pp. 97–135, 416–432, 1824.)

Kirby, William, and William Spence. Introduction to entomology. i-iv, 1816–1828, London.

Straus-Durckheim, Hercule. Considérations générales sur l’anatomie comparée des animaux articulés. Paris, 1828, atlas of 19 plates.

MacLeay, William Sharp. Explanation of the comparative anatomy of the thorax in winged insects, with a review of the present state of the nomenclature of its parts. (Zoöl. Journal, v, pp. 145–179, 1830, 2 Pls.)

Burmeister, Hermann. A manual of entomology. Trans. by W. E. Shuckard, 8º, pp. 654, London, 1836, 32 Pl.

Westwood, John Obadiah. An introduction to the modern classification of insects, i, ii, 8º, pp. 462, 587, 158, 1 Pl. and 133 blocks of figs., 1839–1840.

Newport, George. Art. Insecta in Todd’s Cyclopædia of Anatomy and Phys. ii, pp. 853–994, 1839, Figs. 329–439.

Erichson, Wilhelm Ferdinand. Entomographien. Berlin, 1840.

Brullé, Auguste. Recherches sur les transformations des appendices dans les articulés. (Annales des Sciences nat. Sér. 3, ii, pp. 271–374, tab. 1, 1844.)

Winslow, A. P.: son. Om byggnaden af thorax hos Insekterna. Helsingborg, 1862, 1 Pl, pp. 24.

Packard, Alpheus Spring. Guide to the study of insects. 1869.

—— Systematic position of the Orthoptera in relation to other insects. (Third report U. S. Ent. Commission, pp. 286–345, 1883, Pls. xxiii-lxi.)

Graber, Vitus. Die Insekten. 12º, pp. 403, 603, München, 1877, many Figs.

Huxley, Thomas Henry. A manual of the anatomy of invertebrated animals. 12º, pp. 397–451, Figs., London, 1877.

41Hammond, Arthur. Thorax of the blow-fly. (Journ. Linn. Soc., London, xv. Zoöl., 1880, pp. 31.)

Brauer, Friedrich. Ueber das Segment médiaire Latreille’s. (Sitzb. d. k. Akad. d. Wissensch. Wien, 1882, pp. 218–241, 3 tab.)

—— Systematisch-zoologische Studien. (Ibid., 1885, pp. 237–413.)

Gosch, C. C. A. On Latreille’s theory of “Le Segment médiaire.” (Nat. Tidsskrift (3), xiii, pp. 475–531, 1883.)

Miall, L. C., and Denny, Alfred. The structure and life-history of the cockroach (Periplaneta orientalis). An introduction to the study of insects. 8º, pp. 224, London, 1886.

Cheshire, Frank R. Bees and bee-keeping. i, Scientific, London, 1886, Pls. and Figs.

Lang, Arnold. Text-book of comparative anatomy. i, pp. 426–508, 1891, many Figs.

Kolbe, H. J. Einführung in die Kenntniss der Insekten. 8º, pp. 709, 324 figs., Berlin, 1893.

Sharp, David. The Cambridge natural history. Insecta, i, 8º, pp. 83–584, 1895, Figs. 47–371.

Also the works of Bos, Chabrier, Cholodkowsky, Comstock, Dewitz, Eaton, Erichson, Gerstaecker, Girard, Grassi, Hagen, Haase, Kellogg, Knoch, Lacordaire, Latreille, Leuckart, Lendenfeld, Lowne, Lubbock, Mayer, Meinert, F. Müller, Osten-Sacken, Pagenstecher, Reinhard, Schaum, Schiödte, Scudder, J. B. Smith, Spinola, Stein, Weismann, Wood-Mason.

42

THE HEAD AND ITS APPENDAGES

a. The head

Fig. 24.—Presumed larva of Nemoptera (Necrophilus arenarius), Pyramids of Egypt.—After Roux, from Sharp.

While the head is originally composed of probably not less than six segments, these are in the adult insect fused together into a capsule or hard chitinous box, the epicranium, with no distinct traces of the primitive segments. The head contains the brain and accessory ganglia, the mouth or buccal cavity, also the air-sacs in many winged forms, and gives support to the external organs of sense, the antennæ, and to the buccal appendages, the larger part of the interior being filled with the muscles moving these structures. The solid walls of the head serve as a lever or support for the attachment of these muscles, especially those of the mandibles. Thus there is a correlation between the large size of the mandibles of the soldier white ants and ants, the head being correspondingly large to accommodate the great mandibular muscles. The other extreme is seen in the larva of Necrophilus (Fig. 24), with its long slender neck and diminutive head.

The clypeus.—This is that part of the head situated in front of the epicranium, and anterior to the eyes, forming the roof of the posterior part of the mouth, and is, as embryology shows, probably a tergal sclerite. It varies greatly in shape and size in the different orders of insects. It is often divided into two parts, the clypeus posterior and clypeus anterior, or which may be designated as the post- and ante-clypeus (Figs. 29, B).

The labrum.—The “upper lip” or labrum is an unpaired flap-like piece hinged to the front edge of the clypeus, and may be seen to move up and down when the insect moves its mandibles. It forms the roof of the anterior part of the mouth (Figs. 69, 74), and its inner side is lined with a soft membrane, usually provided with hairs and sense-papillæ or cups, forming the epipharynx.

The labrum is more or less deeply bilobed, especially in caterpillars and in adult Staphylinidæ, and has been thought by some 43writers (Kowalevsky, Carrière, and also Chatin) to represent a pair of appendages, but Heymons (1895) refutes this view, stating as his reason that the labrum arises between the two halves of the nervous system (protocerebrum), while all the true appendages arise on each side of the nervous system. (See also Fig. 34.)

Fig. 25.—Front view of the head of C. spretus: E, epicranium; C, clypeus; L, labrum; O, o, ocelli; e, eye; a, antenna; md, mandible; mx, portion of maxilla uncovered by the labrum; p, maxillary palpus; p′, labial palpus.

In the fleas (Siphonaptera) both the clypeus and labrum are wanting.

While it apparently forms an anterior specialized portion of the procephalic lobes, Viallanes regarded it as belonging to the third, or his tritocerebral, segment, since the labral nerves arise from the tritocerebral ganglia. But since in all the early as well as late stages of embryonic life it appears to be situated in front of the mouth, it would seem to belong to the first segment.

In the embryo of Blatta it first appears as a thick crescentic fold being slightly divided anterior to the mouth, and in Doryphora it appears as a heart-shaped or deeply bilobed prominence situated in front of the mouth (Wheeler).

The epipharynx and labrum-epipharynx.—The epipharynx is the under surface or pharyngeal lining of the clypeus and labrum, forming the membranous roof of the mouth. As it contains the organs of taste and has been generally overlooked by entomologists, we may dwell at some length on its structure in different orders.

Réaumur was, so far as we have been able to ascertain, the first author to describe and figure the epipharynx, which he observed in the honey bee and bumble bee, and called la langue, remarking that it closes the opening into the œsophagus, and that it is applied against the palate. According to Kirby and Spence, De Geer described the epipharynx of the wasp; and Latreille referred to it, calling it the sous labre.

The name epipharynx was bestowed upon this organ by Savigny, who thus speaks of that of the bees: “Ce pharynx est, à la vérité, non seulement caché par la lèvre supérieure, mais encore exactement recouvert par un organe particulier que Réaumur a déjà décrit. C’est une sorte d’appendice membraneux qui est reçu entre les deux branches des mâchoires. Cette partie ayant pour base le bord supérieur du pharynx, peut prendre le nom d’épipharynx ou d’épiglosse.

44He also describes that of Diptera. What Walter has lately proved to be the epipharynx of Lepidoptera was regarded by Savigny and all subsequent writers as the labrum.

The latest account of the function of this organ is that by Cheshire, who states that the tube made by the maxillæ and labial palpi cannot act as a suction pipe, because it is open above. “This opening is closed by the front extension of the epipharynx, which closes down to the maxillæ, fitting exactly into the space they leave uncovered, and thus the tube is completed from their termination to the œsophagus.”

Fig. 26.—Epipharynx of Phaneroptera angustifolia: cl, clypeus; lbr. e, labrum-epipharynx; t c, taste cups, both on the clypeal and on the labral regions.

Fig. 27.—Epipharynx of Hadenœcus subterraneus, cave cricket.

It is singular that this organ is not mentioned in Burmeister’s Manual of entomology, in Lacordaire’s Introduction à l’entomologie, or by Newport in his admirable article Insecta in Todd’s Cyclopedia of anatomy. Neither has Straus-Durckheim referred to or figured it in his great work on the anatomy of Melolontha vulgaris.

In their excellent work on the cockroach, Miall and Denny state that “The epipharynx, which is a prominent part in Coleoptera and Diptera, is not recognizable in Orthoptera” (p. 45). We have, however, found it to be always present in this order (Figs. 26, 27).

We are not aware that any modern writers have described or referred to the epipharynx of the mandibulate orders of insects. Although Dr. G. Joseph speaks of finding taste-organs on the palate of almost every order of insects, especially plant-feeding forms, we are unable to find any specific references, his detailed observations being apparently unpublished.

The epipharynx is so intimately associated with the elongated labium of certain Diptera, that, with Dr. Dimmock, we may refer to the double organ as the labrum-epipharynx; and where, as in the lepidopterous Micropteryx semipurpurella, described and figured by Walter, and the Panorpidæ (Panorpa and Boreus), the labrum seems pieced out with a thin, pale membranous fold which appears to be 45an extension of the epipharynx, building up the dorsal end of the labrum, this term is a convenient one to use.

In the lower orders of truly mandibulate insects, from the Thysanura to the Coleoptera, excluding those which suck in liquid food, such as the Diptera, Lepidoptera, and Hymenoptera, and the Mecoptera (Panorpidæ) with their elongated head and feeble, small mandibles, the epipharynx forms a simple membranous palatal lining of the clypeus and labrum. In such insects there is no soft projecting or pendant portion, fitted to close the throat or to complete a partially tubular arrangement of the first and second maxillæ.

In all the mandibulate insects, then, the epipharynx forms simply the under surface or pharyngeal lining of the clypeus and labrum, the surface being uniformly moderately convex, and corresponding in extent to that of the clypeus and labrum, posteriorly merging into the palatal wall of the pharynx; the armature of peculiar gathering-hairs sometimes spreading over its base, being continuous with those lining the mouth and beginning of the œsophagus. The suture separating the labrum from the clypeus does not involve the epipharynx, though since certain gustatory fields lie under the front edge of the clypeus, as well as labrum, one may in describing them refer to certain fields or groups of cups or pits as occupying a labral or clypeal region or position.

The lack of traces of a suture in the epipharynx corresponding to the labral suture above, suggests that the labrum does not represent a pair of coalesced appendages, and that it, with the clypeus, simply forms the solid cuticular roof of the mouth.

The only soft structures seen between the epipharynx and labrum, besides the nerves of special sense, are the elevator muscles of the labrum, and two tracheæ, one on each side.

The structure and armature of the epipharyngeal surface even besides the taste-pits, taste-cups and rods, is very varied, the setæ assuming very different shapes. There seem to be two primary forms of setæ, (1) the normal forms which arise from a definite cell; and (2) soft, flattened, often hooked hairs which are cylindrical towards the end, but arise from a broad triangular base, without any cell-wall. These are like the “gathering hairs” of Cheshire, situated on the bees’ and wasps’ tongue; they also line the walls of the pharynx and extend toward the œsophagus. They are also similar to the “hooked hairs” of Will. The first kind, or normal setæ, are either simply defensive, often guarding the sense-cups or sensory fields on which the sense-cups are situated, or they have a nerve extending to them and are simply tactile in function.

46The surface of the epipharynx, then, appears to be highly sensitive, and to afford the principal seat of the gustatory organs, which are described under the head of organs of taste.

LITERATURE ON THE EPIPHARYNX

Réaumur. Mémoires pour servir à l’histoire des insectes, v, 1740, p. 318, Pl. 28, Figs. 4, 7, 8, 9, 10, 11 l.

Kirby and Spence. Intr. to entomology, iii, 1828, p. 457.

De Geer. ii, 1778; v, 26, Fig. 11, M.

Kirby and Spence. Pl. xii, Fig. 2 K.

Latreille. Organisation extérieure des insectes, p. 184. (Quoted from Kirby and Spence.)

Savigny. Mémoires sur les animaux sans vertèbres. Partie Ire, 1816, p. 12.

Walter, Alfred. Beiträge zur Morphologie der Schmetterlinge. Erster Theil. Zur Morphologie des Schmetterlingsmundtheile. (Jena. Zeits., xviii, 1885, p. 752.)

Cheshire, F. R. Bees and bee-keeping, i, London, 1886, p. 93.

Joseph, Gustav. Zur Morphologie des Geschmacksorganes bei Inseckten. (Amtlicher Bericht der 50 Versammlung deutscher Naturforscher u. Artzte in München. 1877, pp. 227, 228.)

Dimmock, George. The anatomy of the mouth-parts and of the sucking apparatus of some Diptera, 1881. (Also in Psyche, iii, pp. 231–241, Pl. 1, 1882.)

Packard, A. S. On the epipharynx of the Panorpidæ. (Psyche, 1889, v, pp. 159–164.)

—— Notes on the epipharynx and the epipharyngeal organs of taste in mandibulate insects. (Psyche, v, pp. 193–199, 222–228, 1889.)

Attachment of the head to the trunk.—The head is either firmly supported by the broad prothoracic segment in Orthoptera, many beetles, etc., into which it is more or less retracted, or it is free and attached by a slender neck, easily turning on the trunk, as in dragon-flies, flies, etc. In some insects there are several chitinous plates, situated on an island in the membrane on the under side of the neck; these are the “cervical sclerites” of Sharp, occurring “in Hymenoptera, in many Coleoptera, and in Blattidæ.”

The basal or gular region of the head.—At the hinder part of the head is the opening (occipital foramen) into the trunk. The cheek (gena) is the side of the head, and to its inner wall is attached the mandibular muscle; it thus forms the region behind the eye and over the base of the mandibles. In the Termitidæ, where the head is broad and flat, it forms a distinct piece on the under side of the head bounding the gulo-mental region (Fig. 28). In the Neuroptera (Corydalus, Fig. 29, and Mantispa, Fig. 30) it is less definitely outlined.

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Fig. 29.—Head of Corydalus cornutus, ♂: A, from above. B, from beneath. C, from the side. a. cly, clypeus anterior; p. cly, clypeus posterior; lbr, labrum; md, mandible; mx, base of first maxilla; mp, its palpus; m, mentum; sm, submentum; plpr, palpifer; lig, fused second maxillæ; ant, antenna; occ, occiput.

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Fig. 28.—Head of Termopsis angusticollis, seen from beneath, showing the gena and gula: m, mentum; sm, submentum; labr, under side of the labrum; x, hypopharyngeal chitinous support.

All the gulo-mental region of the head appears to represent the base of the second maxillæ, and the question hence arises whether the submentum is not the homologue of the cardines of the first maxillæ fused, and the mentum that of the stipites of the latter also fused together. If this should prove to be the case, the homologies between the two pairs of maxillæ will be still closer than before supposed. Where the gula is differentiated, this represents the basal piece of the second maxillæ. In Figs. 28, 29, 30, and 31, these three pieces are clearly shown to belong to the second maxillary segment. It is evident that these pieces or sclerites belong to the second maxillary or labial segment of the head, as does the occiput, which may represent the tergo-pleural portion of the segment. Miall and Denny also regarded the submentum as the basal piece of the second maxillæ.

Fig. 30.—Head of Mantispa brunnea, under side: e, eye; other lettering as in Fig. 29.

Fig. 31.—Head of Limnephilus pudicus, under side: e, eye; l, ligula; p, palpifer; lp, labial palpi.

The occiput (Fig. 29, B, C), as stated beyond, is very rarely present as a separate piece; in the adult insect we have only observed it in Corydalus. The occipital region may be designated as that part of the head adjoining and containing the occipital foramen. Newport considers the occiput as that portion of the base of the head “which is articulated with the anterior margin of the prothorax. It is perforated by a large foramen, through which the organs of the head are connected 49with those of the trunk. It is very distinct in Hydroüs and most Coleoptera, and in some, the Staphylinidæ, Carabidæ, and Silphidæ is constricted and extended backwards so as to form a complete neck.” (See also p. 51.)

Fig. 32.—Interior and upper and under surface of the head of Hydroüs piceus: d, clypeus; e, labrum; g, maxilla; h, its palpus; i, labium; k, labial palpus; p, sutura epicranii; q, cotyloid cavity; r, torulus; s, v, laminæ squamosa; t, laminaæ posteriores; u, tentorium; w, laminæ orbitales; x, os transversum; y, articulating cavity for the mandible; z, os hypopharyngeum.—After Newport.

The tentorium.—The walls of the head are supported or braced within by two beams or endosternites passing inwards, and forming a solid chitinous process or loop which extends in the cockroach downwards and forwards from the lower edge of the occipital foramen. “In front it gives off two long crura or props, which pass to the ginglymus, and are reflected thence upon the inner surface of the clypeus, ascending as high as the antennary socket, round which they form a kind of rim.” (Miall and Denny.) The œsophagus passes upwards between its anterior crura, the long flexor of the mandible lies on each side of the central plate; the supraœsophageal ganglion rests on the plate above, and the subœsophageal ganglion lies below it, the nerve cords which unite the two passing through the circular aperture. (Miall and Denny.) In Coleoptera (Hydroüs) it protects the nervous cord which passes under it. (Newport, Fig. 32, u.)

Fig. 33.—Posterior view of head of Anabrus; t, tentorium. Joutel del.

In Anabrus the tentorium is V-shaped, the two arms originating on each side of the base of the clypeus next to the base of each mandible the origin being indicated by two small foramina partly concealed externally and passing inwards and backwards and uniting just before reaching the posterior edge of the large occipital foramen (Fig. 33).

50Palmén regards the tentorium as representing a pair of tracheæ (with the cephalic spiracles) which have become modified for supports or for muscular attachment, since he finds that in Ephemera the tentorium breaks across the middle during exuviation, each half being drawn out of the head like the chitinous lining of a tracheal tube. This view is supported by Wheeler, who has shown that the tentorium of Doryphora originates from five pairs of invaginations of the longitudinal commissures, and which are anterior to those of the second maxillary segment. “These invaginations grow inwards as slender tubes, which anastomose in some places. Their lumina are ultimately filled with chitin.” (Jour. Morph., iii, p. 368.)

This view has also been held by Carrière and Cholodkowsky, but Heymons concludes from his embryological studies on Forficula and Blattidæ (1895) that it is unfounded. That this is probably the case is proved by the fact that the apodemes of the thoracic region are evidently not modified tracheæ, since the stigmata and tracheæ are present.

Number of segments in the head.—While it is taken for granted by many entomologists that the head of insects represents a single segment, despite the circumstance that it bears four pairs of appendages, the more careful, philosophical observers have recognized the fact that it is composed of more than a single segment. Burmeister recognized only two segments in the head; Carus and Audouin recognized three; Macleay and Newman four; Straus-Durckheim even so many as seven. Huxley supposed that there are five segments bearing appendages, remarking, “if the eyes be taken to represent the appendages of another somite, the insect head will contain six somites.” (Manual of Anat. Invert. Animals, p. 398.)

These discordant views were based on the examination of the head in adult insects; but if we confine ourselves to the imago alone, it is impossible to arrive at a solution of the problem.

Newport took a step in the right direction when he wrote: “It is only by comparing the distinctly indicated parts of the head in the perfect insect with similar ones in the larva that we can hope to ascertain the exact number of segments of which it is composed.” He then states that in the head of Hydroüs piceus are the remains of four segments, though still in the next paragraph, when speaking of the head as a whole, he considers it as the first segment, “while,” he adds, “the aggregation of segments of which it is composed we shall designate individually subsegments.”

That the head of insects is composed of four segments was shown on embryological grounds by the writer (1871) and afterwards by Graber (1879). The antennæ and mouth-parts are outgrowths budding out from the four primitive segments of the head; the antennæ grow out from the under side of the procephalic lobes, and these should therefore receive the name of antennal lobes. In like manner 51the mandibles and first and second maxillæ arise respectively from the three succeeding segments.

Fig. 34.—Embryo of Anurida maritima: tc. ap, minute temporary appendage of the tritocerebral segment, the premandibular appendage; at, antenna; md, mandible; mx1, first maxilla; mx2, second maxilla; p1p3, thoracic; ap1, ap2, abdominal appendages; an, anus—After Wheeler.

While the postoral segments and their appendages are readily seen to be four in number, the question arises as to whether the eyes represent the appendages of one or more preoral segments. In this case embryology thus far has not afforded clear, indubitable evidence. We are therefore obliged to rely on the number of neuromeres, or primitive ganglia. In the postoral region of the head, as also in the trunk, a pair of neuromeres correspond to each segment. (See also under Nervous System, and under Embryology.) We therefore turn to the primitive number of neuromeres constituting the procephalic lobes or brain.

From the researches of Patten, Viallanes, and of Wheeler, especially of Viallanes, it appears that the brain or supraœsophageal ganglion is divided into three primitive segments. (See Nervous System, Brain.) The antennæ are innervated from the middle division or deutocerebrum. Hence the ocular segment, i.e. that bearing the compound and simple eyes, is supposed to represent the first segment of the head. This, however, does not involve the conclusion that the eyes are the homologues of the limbs, however it may be in the Crustacea.

The second head-segment is the antennal, the antennæ being the first pair of true jointed appendages.

The third segment of the head is very obscurely indicated, and the facts in proof of its existence are scanty and need farther elucidation.

Viallanes’ tritocerebral lobes or division of the brain is situated in a segment found by Wheeler to be intercalated between the antennal and mandibular segments. He also detected in Anurida maritima, the rudiments of a pair of appendages, smaller than those next to it, and which soon disappear (Fig. 34, tc. ap). He calls this segment the intercalary.[12] Heymons (1895) designates it as the 52“Vorkiefersegment,” and it may thus be termed the premandibular segment.

Fig. 35.—Head of embryo of honey bee: B, a little later stage than A. pr.m, premandibular segment; cl, clypeus; ant, antenna; md, mandible; mx, first maxilla; mx′, second maxilla; sp, spiracle.—After Bütschli.

As early as 1870 Bütschli observed in the embryo of the honey bee the rudiments of what appeared to be a pair of appendages between the antennæ and mandibles, but, judging by his figures, nearer to and more like the mandibles than the rudimentary antennæ (Fig. 35); they seemed to him “almost like a pair of inner antennæ.”

“I find,” he says, “in no other insects any indication of this peculiar appendage, which at the time of its greatest development attains a larger size than the antennæ, and which, afterwards becoming less distinct, forms by fusion with that on the other side a sort of larval lower lip. That this appendage does not belong to the category of segmental appendages is indicated by the site of its origin on the upper side of the primitive band.” (Zeitschr. wissen. Zool., xx, p. 538.)

Grassi has also observed it in Apis, and regards it as the germ of a first, but deciduous, pair of jaws. In the embryo of Hylotoma Graber (Figs. 134, 135) found what he calls three pairs of “preantennal projections,” one of which he thinks corresponds to the “inner antennæ” of Bütschli. This subject needs further investigation.

It thus appears that the procephalic lobes of the embryo of insects, with the rudiments of the antennæ, constitute the primitive head, and perhaps correspond to the annelidan head, while gradually the antennal appendages were in the phylogenetic development of the class fused with the two segments of the primary head. That the second maxillary segment, the occiput, was the last to be added, and at first somewhat corresponded in position to the poison-fangs 53of centipedes (Chilopods), is shown by our observations on the embryology of Æschna (Fig. 36).

Fig. 36.—Æschna nearly ready to hatch: 4, labium, between T and e the occipital tergite; 5–7, legs.

Fig. 37.—Head of embryo Nematus, showing the labial segment: occ, forming the occiput; cl, clypeus; lb, labrum; md, mandible; mdm, muscle of same; mx, maxilla; mx′, second maxilla (labium); oe, œsophagus.

The mandibular segment appears to form a large part of the post-antennal region of the epicranium on account of the great mandibular muscle which arises from so large an area of the anterior region of the head (Fig. 37).

Judging from the embryo of Nematus (Fig. 37), the first maxillary segment is tergally aborted, there being no tergo-pleural portion left.[13]

The second maxillary segment tergally appears to be represented by the occipital region of the head.

All the gular region, including the submentum and mentum, probably represents the base of the labium or second maxillæ.[14] The so-called “occiput” forms the base of the head of Corydalus, a neuropterous insect, which, however, is more distinct in the larva. In most other adult insects the occiput is either obsolete or fused with the hinder part of the epicranium. We have traced the history of this piece (sclerite) in the embryo of Æschna, a dragon-fly, and have found that it represents the tergal portion of the sixth or labial segment. In our memoir on the development of this dragon-fly, Pl. 2, Fig. 9, the head of the embryo is seen to be divided into two regions, the anterior, formed of the antennal, mandibular, and first maxillary segments, and the posterior, formed of the sixth or labial segment. 54This postoral segment at first appears to be one of the thoracic segments, but is afterwards added to the head, though not until after birth, as it is still separate in the freshly hatched nymph (Fig. 4; see also Kolbe, p. 132, Fig. 59, sq. 5). A. Brandt’s figure of Calopteryx virgo (Pl. 2, Fig. 19) represents an embryo of a stage similar to ours, in which the postoral or sixth (labial) segment is quite separate from the rest of the head. The accompanying figure, copied from our memoir, also shows in a saw-fly larva (Nematus ventricosus) the relations of the labial or sixth segment to the rest of the head. The suture between the labial segment and the preoral part of the head disappears in adult life. From this sketch it would seem that the back part of the head, i.e. of the epicranium, may be made up in part of the tergite or pleurites of the mandibular segment, since the mandibular muscles are inserted on the roof of the head behind the eyes. It is this labial segment which in Corydalus evidently forms the occiput, and of which in most other insects there is no trace in larval or adult life, unless we except certain Orthoptera (Locusta), and the larva of the Dyticidæ.

The following table is designed to show the number and succession of the segments of the head, with their respective segments.

Tabular View of the Segments, Pieces (Sclerites), and Appendages of the Head
 
Name of Segment Pieces or Regions of the Head-capsule Appendages, etc.
Preoral, in early embryo. 1. Ocellar (Protocerebral). Epicranium, anterior region with the clypeus labrum, and epipharynx. Compound and simple eyes (Ocelli).
     
Postoral, in early embryo. 2. Antennal (Deutocerebral). Epicranium, including the antennal sockets. Antennæ.
     
3. Premandibular, or intercalary (Tritocerebral). Wanting in postembryonic life, except in Campodea. Premandibular appendages (in Campodea).
     
4. Mandibular. Epicranium behind the antennæ, genæ. Mandibles.
     
5. 1st Maxillary. Epicranium, hinder edge? Tentorium. 1st Maxillæ.
     
6. 2d Maxillary, or labial. Occiput. 2d Maxillæ or Labium. Post-gula, gula, submentum, mentum, hypopharynx (lingua, ligula), paraglossæ, spinneret.
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Fig. 38.—Larva (a) of a chalcid, about to pupate, with the head, including the eyes and three ocelli, in the prothoracic segment: b, c, pupa.

The composition of the head in the Hymenoptera.—Ratzeburg stated in 1832 that the head in the adult Hymenoptera (Cynips, Hemiteles, and Formica) does not correspond to that of the larva, but is derived from the head and the first thoracic segment of the larva. Westwood and also Goureau made less complete but similar observations, though Westwood afterwards changed his opinion, and the same view was maintained by Reinhard. Our own observations (as seen in Fig. 38) led us to suppose that this was a mistaken view; that the larval head, being too small to contain that of the semipupa, was simply pushed forward, as in caterpillars. Bugnion, however, reaffirms it in such a detailed way that we reproduce his account. He maintains that the views of Ratzeburg are exact and easy to verify in the chalcid genus Encyrtus, except, however, that which concerns the ventral part and the posterior border of the prothoracic segment.

As the time of transformation approaches, the head of the larva, he says, is depressed and soon concealed under the edge of the prothoracic segment; the latter elongates, becomes thicker and more convex, and within can be seen the two oculo-cephalic imaginal buds. The head of the perfect insect is derived not only from the head of the larva, but also from the portion of the prothoracic segment which is occupied by the buds, i.e. almost its entire dorsolateral face. But the hinder and ventral part of this segment (which contains the imaginal buds of the first pair of legs) takes no part in the formation of the head; these parts, according to Bugnion, towards the end of the larval period detaching themselves so as to become fused with the thorax and constitute the pronotum and the prosternum.

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Fig. 39.—Anterior half of larva of Encyrtus, ventral face, showing the upper (wing) and lower (leg) thoracic imaginal buds: b, mouth; ch, chitinous arch; gl, silk gland; g, brain; n, nervous cord; a1, bud of fore, a2, bud of hind, wing; p1p3, buds of legs; st1st3, stigmata.

Fig. 40.—Anterior part of Encyrtus larva, 1.2 mm. in length; dorsal face; the cellular masses beginning to form the buds of the wings, eyes, and antennæ: o, eye bud; e, stomach.

Fig. 41.—Older Encyrtus larva, lateral view, showing the buds of the antennæ (f), legs, and wings; oe, œsophagus; q1, q2, q3, buds of the genital armature; x, rudiment of the sexual gland (ovary or testis); u, urinary tube; i, intestine (rectum); a, anus.

Fig. 42.—A still older larva, ready to transform. The imaginal buds of the antennæ, eyes, wings, and legs have become elongated; lettering as in Fig. 41.—This and Figs. 39–41 after Bugnion.

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This mode of formation of the head may be observed still more easily in Rhodites, Hemiteles, and Microgaster, from the fact that their oculo-cephalic buds are much more precocious, and that the eyes are charged with pigment at a period when the insect still preserves its larval form.

“... I believe that this mode of formation of the head occurs in all Hymenoptera with apodous larvæ, in this sense; that a more or less considerable part of the first thoracic segment is always soldered to the head of the larva to constitute the head of the perfect insect. The arrangement of the nervous system is naturally in accord with this peculiarity of development, and the cephalic ganglia of the larva to which the ocular blastems later adapt themselves, are found not in the head, but in the succeeding segment (Figs. 39, 40, 41).

“Relying on these facts, I maintain that the encroachment of the head on the prothorax is a consequence of the preponderance in size of the brain, and indicates the superiority of the Hymenoptera over other insects....”

That the pronotum is derived from the larval prothoracic segment is proved by the fact that the first pair of stigmata becomes what authors call the “prothoracic” stigmata of the perfect insect. But Bugnion thinks that the projection which carries it, and which he calls the shoulder (Figs. 41 and 42), belongs to the mesonotum.

b. Appendages of the head

The antennæ.—These are organs of tactile sense, but also bear olfactory, and in some cases auditory organs; they are usually inserted between or in front of the eyes, and moved by two small muscles at the base, within the head. In the more generalized insects the antennæ are simple, many-jointed appendages, the joints being equal in size and shape. The antennæ articulate with the head by a ball and socket joint, the part on which it moves being called the torulus (Fig. 32, r). In the more specialized forms it is divided into the scape, the pedicel, and a flagellum (or clavola); but usually, as in ants, wasps, and bees, there are two parts, the basal three-jointed one being the scape, and the distal one, the usually long filiform flagellum. The antennæ, especially the flagellum, vary greatly in form in insects of different families and orders, this variation being the result of adaptation to their peculiar surroundings and habits. The number of antennal joints may be one (Articerus, a clavigerid beetle), or two in Paussus and in Adranes cœcus (Fig. 4312), where they are short and club-shaped; in flies (Muscidæ, etc.), they are very short and with few joints, and when at rest lying in a cavity adapted for their reception. In the lamellicorn beetles the flagellum is divided into several leaves, and this condition may be approached in the serrate or flabellicorn antennæ of other beetles. In Lepidoptera, and in certain saw-flies and beetles, they are either pectinate or bipectinate, being in one case at least, that of the Australian Hepialid (Abantiades argenteus), tripectinate (Fig. 44), and in the dipterous (Tachinid) genus Talarocera the third joint is bipectinate (Fig. 45). In Xenos and in Parnus they may be deeply forked, 58while in Otiocerus, two long processes arise from the base, giving it a trifid shape. In dragon-flies and cicadæ, they are minute and hair-like, though jointed, while in the larvæ of many metabolous insects they are reduced to minute three-jointed tubercles. In aquatic beetles, bugs, etc., the antennæ are short, and often, when at rest, bent close to the body, as long antennæ would impede their progress.

Fig. 43.—Different forms of antennæ of beetles: 1, serrate; 2, pectinate; 3, capitate (and also geniculate); 4–7, clavate; 8, 9, lamellate; 10, serrate (Dorcatoma); 11, irregular (Gyrinus); 12, two-jointed antenna of Adranes cæcus.—After LeConte. a, first joint of flagellum of antenna of Troctes silvarum; b, of T. divinatorius.—After Kolbe.

Fig. 44.—Tripectinate antenna of an Australian moth.

While usually more or less sensorial in function, Graber states that the longicorn beetles in walking along a slender twig use their antennæ as a rope-dancer does his balancing pole.

Fig. 45.—Antenna of Talarocera nigripennis, ♂.—After Williston.

Recent examination of the sense-organs in the antennæ of an ant, wasp, or bee enables us, he says, to realize what wonderful organs the antennæ are. In such insects we have a rod-like tube which can be folded up or extended out into space, containing the antennal nerve, which arises directly from the brain and sends a branch to each of the thousands of olfactory pits or pegs which stud its surface. The antenna is thus a wonderfully complex organ, and the insect must 59be far more sensitive to movements of the air, to odors, wave-sounds, and light-waves, than any of the vertebrate animals.

That ants appear to communicate with each other, apparently talking with their antennæ, shows the highly sensitive nature of these appendages. “The honey-bee when constructing its cells ascertains their proper direction and size by means of the extremities of these organs.” (Newport.)

How dependent insects are upon their antennæ is seen when we cut them off. The insect is at once seriously affected, its central nervous system receiving a great shock, while it gives no such sign of distress and loss of mental power when we remove the palpi or legs. On depriving a bee of its antennæ, it falls helpless and partially paralyzed to the earth, is unable at first to walk, but on partly recovering the use of its limbs, it still has lost the power of coördinating its movements, nor can it sting; in a few minutes, however, it becomes able to feebly walk a few steps, but it remains over an hour nearly motionless. Other insects after similar treatment are not so deeply affected, though bees, wasps, ants, moths, certain beetles, and dragon-flies are at first more or less stunned and confused.

The antennæ afford salient secondary sexual differences, as seen in the broadly pectinated antennæ of male bombycine moths, certain saw-flies (Lophyrus), and many other insects.

The mouth-parts, buccal appendages, or trophi, comprise, besides the labrum, the mandibles and maxillæ.

The mandibles.—These are true jaws, adapted for cutting, tearing, or crushing the food, or for defence, while in the bees they are used as tools for modelling in wax, and in Cetonia, etc., as a brush for collecting pollen. They are usually opposed to each other at the tips, but in many carnivorous forms their tips cross each other like shears. They are situated below the clypeus on each side, and are hinged to the head by a true ginglymus articulation, consisting of two condyles or tubercles to which muscles are attached, the principal ones being the flexor and great extensor (Fig. 48). They are solid, chitinous, of varied shapes, and in the form of the teeth those of the same pair differ somewhat from each other (Fig. 46 A). In the pollen-eating beetles (Cetoniæ) and in the dung-beetles (Aphodius, etc.) the edge is soft and flexible. In the males of Lucanus, etc. (Fig. 47), and of Corydalus (Fig. 29), they are of colossal size, and are large and sabre-shaped in the larvæ of water-beetles, ant-lions, Chrysopa, etc. where they are perforated at the tips, through which the blood of their prey is sucked.

While the mandibles are generally regarded as composed of a 60single piece, in Campodea and Machilis there appears to be an additional basal piece apparently corresponding to the stipes of the first maxilla, and separated by a faint suture from the molar or distal joint. In Campodea there is a minute movable appendage figured both by Meinert and by Nassonow, which appears to represent the lacinia of the maxilla (Fig. 48). Wood-Mason has observed in the mandibles of the embryo of a Javanese cockroach, Blatta (Panesthia) javanica, indications of “the same number of joints as in that of chilognathous myriopods, or one less than in that of Machilis.” Also he adds: “In both ‘larvæ’ and adults of Panesthia javanica a faint groove crosses the ‘back’ of the mandible at the base. This groove appears to be the remains of the joint between the third and apical segments of the formerly 4–segmented mandibles.”

Fig. 46.—Various forms of mandibles. A, right and left of Termopsis. A′, showing at the shaded portion the “molar” of Smith. B, Termes flavipes, soldier; md, its mandible. C, Panorpa.

Fig. 47.Chiasognathus grantii, reduced. Male.—After Darwin.

Fig. 48.—Mandible of Campodea: l, prostheca or lacinia; g, galea; f, f, flexor muscles; e, extensor; r, r, retractor; rt, muscle retaining the mandible in its place.—After Meinert. A, extremity of the same.—After Nassonow.

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Fig. 49.—Mandible of Passalus cornutus with the prostheca (l): A, that of a Nicaraguan species; a, inside, b, outside view, with the muscle.

He also refers to the prostheca of Kirby and Spence (Fig. 49), which he thinks appears to be a mandibular lacinia homologous with it in Staphylinidæ and other beetles (J. B. Smith also considers it as “homologous to the lacinia of the maxilla”), and on examining it in P. cornutus and a Nicaragua species (Fig. 49), we adopt his view, since we have found that it is freely movable and attached by a tendon and muscle to the galea. In the rove beetles (Goërius, Staphylinus, etc.) and in the subaquatic Heteroceridæ, instead of a molar process, is a membranous setose appendage not unlike the coxal appendages of Scolopendrella, movably articulated to the jaw, which he thinks answers to the molar branch of the jaws in Blatta and Machilis. “It has its homologue in the diminutive Trichopterygidæ in the firmly chitinized quadrant-shaped second mandibular joint, which is used in a peculiar manner in crushing the food”; also in the movable tooth of the Passalidæ, and in the membranous inner lobe of the mandibles of the goliath-beetles, etc.

J. B. Smith has clearly shown that the mandibles are compound in certain of the lamellicorns. In Copris carolina (Fig. 50), he says, the small membranous mandibles are divided into a basal piece (basalis), the homologue of the stipes in the maxilla; another of the basal pieces he calls the molar, and this is the equivalent of the subgalea, while a third sclerite, only observed in Copris, is the conjunctivus, the lacinia (prostheca) being well developed. Smith therefore concludes “that the structure of the mandible is fundamentally the same as that of the labium and maxilla, and that we have an equally complex organ in point of origin. Its usual function, however, demands a powerful and solid structure, and the sclerites are in most instances as thoroughly chitinized and so closely united to the others that practically there is only a single piece, in which the homology is obscured.” (Trans. Amer. Ent. Soc., xix, pp. 84, 85. 1892.) From the studies of Smith and our observations on Staphylinus, Passalus, Phanæus, etc. (Fig. 50, A, B) we fully agree with the view that the mandibles are primarily 3–lobed appendages like the maxillæ. Nymphal Ephemerids have a lacinia-like process. (Heymons.)

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Fig. 50.—Mandible of Copris carolina.—After Smith. AC. anaglypticus. A (figure to right), do. of Leistotrophus cingulatus; B, of Phanæus carnifex; g′, end of galea,—g, enlarged; c, conjunctivus. C, of Meloë angusticollis: l, lacinia; a, lacinia enlarged.

Mandibles are wanting in the adults of the more specialized Lepidoptera, being vestigial in the most generalized forms (certain Tineina and Crambus), but well developed in that very primitive moth, Eriocephala (Fig. 51). They are also completely atrophied in the adult Trichoptera, though very large and functional in the pupa of these insects (Fig. 52), as also in the pupa of Micropteryx (Fig. 53). They are also wanting in the imago of male Diptera and in the females of all flies except Culicidæ and Tabanidæ.

They are said by Dr. Horn to be absent in the adult Platypsyllus castoris, though well developed in the larva; and functional mandibles are lacking in the Hemiptera.

The first maxillæ.—These highly differentiated appendages are inserted on the sides of the head just behind the mandibles and the mouth, and are divided into three lobes, or divisions, which are supported upon two, and sometimes three basal pieces, i.e. the basal 63joint or cardo, the second joint or stipes, with the palpifer, the latter present in Termitidæ (Fig. 54, plpgr), but not always separately developed (Fig. 55). The cardo varies in shape, but is more or less triangular and is usually wedged in between the submentum and mandible. It is succeeded by the stipes, which usually forms the support for the three lobes of the maxilla, and is more or less square in shape.

Fig. 51.—Mandible of Eriocephala calthella: a, a′, inner and outer articulation; s, cavity of the joint (acetabulum); A, end seen from one side of the cutting edge.—After Walter.

Fig. 52.A, Pupa of Phryganea pilosa.—After Pictet. B, mandibles of pupa of Molanna angustata.—After Sharp.

Fig. 53.—Pupa of Micropteryx purpuriella, front view: md, mandibles; mx.p, maxillary palpus, end drawn separately; mx.’p, labial palpi; lb, labrum; A, another view from a cast skin.

The three distal divisions of the maxilla are called, respectively, beginning with the innermost, the lacinia, galea, and palpifer, the latter being a lobe or segment bearing the palpus. The lacinia is more or less jaw-like and armed on the inner edge with either flexible or stiff bristles, spines, or teeth, which are very variable in shape and are of use 64as stiff brushes in pollen-eating beetles, etc. The galea is either single-jointed and helmet-shaped or subspatulate, as in most Orthoptera, or 2–jointed in Gryllotalpa, or lacinia-like in Myrmeleon (Fig. 55, C); or, in the Carabidæ (Fig. 56) and Cicindelidæ, it is 2–jointed and in form and function like a palpus.

Fig. 54.A, maxilla of Termopsis angusticollis. B, Termes flavipes: c, cardo; sti, stipes; plpgr, palpiger; palp, palpus; lac, lacinia; g, gal, galea.

Fig. 55.A, maxilla of Mantispa brunnea. B, Ascalaphus longicornis. C, Myrmeleon diversum. Lettering as in Fig. 54.

65

Fig. 56.—Maxilla of a carabid, Anophthalmus tellkampfii: l, lacinia; g, 2–jointed galea; p, palpus; st, stipes; c, cardo.

Fig. 57.—Maxilla of Nemognatha, ♀, from Montana. A, base of maxilla enlarged to show the taste-papillæ (tp) and cups (tc), on the galea (ga). B, part of end of galea to show the imperfect segments and taste-organs: n, nerve; a ganglionated nerve supplies each taste-papilla or cup; l, lacinia; p, palpifer; s, subgalea.

Fig. 58.—Maxilla of Panorpa.

Fig. 59.—Maxilla of Limnephilus pudicus: mx, stipes; lac, galea.

The palpus is in general antenniform and is composed of from 1 to 6 joints, being usually 4– or 5–jointed, and is much longer than the galea. In the maxilla of the beetle Nemognatha (Fig. 57), the galea is greatly elongated, the two together forming an imperfect tube or proboscis and reminding one of the tongue of a moth, while the lacinia is reduced. In the Mecoptera the lacinia and galea are closely similar (Fig. 58); in the Trichoptera only one of the lobes is present (Fig. 59), while in the Lepidoptera the galea unites with its mate to form the so-called tongue (Fig. 60). The maxilla of the male of Tegeticula yuccasella is normal, though the galeæ are separate; but in the female, what Smith regards as the palpifer (the “tentacle” of Riley) is 66remarkably developed, being nearly as long as the galea (Fig. 61) and armed with stout setæ, the pair of processes being adapted for holding a large mass of pollen under the head.

Fig. 60.—Tongue of Aletia xylina, with the end magnified.—Pergande del., from Riley. A, much reduced maxilla (mx) of Paleacrita vernata; mx.p, palpus.

Fig. 61.A, maxilla of Tegeticula yuccasella, ♂: g, galea. B, ♀: pl, enormously developed palpifer; mx.p, palpus; c, cardo; st, stipes; sty, stylus.

In coleopterous larvæ the maxillæ are 2–lobed 67(Fig. 62), the galea being undifferentiated, but in those of saw-flies the galea is present (Fig. 63, gal).

Fig. 62.—Larva of Rhagium lineatum: lat, lateral view of head and thoracic segments; mx, first maxilla; ml, undifferentiated lacinia and galea; v, under side of head and pro- and meso- thoracic segments; v.m.s., one of the middle ventral segments, magnified six times; mx′, 2d maxilla.

It now seems most probable that in the first maxillæ we have the primary form of buccal appendage of insects, the appendage being composed of three basal pieces with three variously modified distal lobes or divisions; and that the mandibles and second maxillæ are modifications of this type.

How wonderfully the maxillæ of the Lepidoptera are modified, and the peculiar shapes assumed in the Diptera, Hymenoptera, and other groups, will be stated in the accounts of those orders, but it is well to recall the fact that in the most primitive and generalized moth, Eriocephala, the lacinia is well developed (Fig. 64).

As Newport remarks, the office of the maxillæ in the mandibulate insects is of a twofold kind; since they are adapted not only for seizing and retaining the food in the mouth, but also as accessory jaws, since they aid the mandibles in comminuting it before it is passed on to the pharynx and swallowed. Hence, as the food varies so much in nature and situation, it will be readily seen that the maxillæ, especially their distal parts, vary correspondingly. Thus far no close observations on the exact use of the first and second maxillæ have been published.

The palpi also are not only organs of touch, but in some cases act as hands and also bear minute sense-organs, the function 68of which is unknown, but would appear to be usually that of smell.

Fig. 63.Selandria larva, common on Carya porcina, with details of mouth-parts: leg, leg; mx, maxilla; gal, galea; lac, lacinia.

The second maxillæ.—The “under-lip” or labium of insects is formed by the fusion at the basal portion of what in the embryo are separate appendages, and which arise in the same manner as the first maxillæ. They are invariably solidly united, no cases of partial or incomplete fusion being known. The so-called labium is situated in front of the gula or gular region, and is bounded on each side by the gena, or cheek. As already observed, the second maxillæ appear to be the appendages of the last or occipital segment of the head.

Fig. 64.—Maxilla of Eriocephala calthella: l, lacinia; g, galea; mx.p, maxillary palpus; st, stipes; c, cardo.—After Walter.

The second maxillæ are very much differentiated and vary greatly in the different orders, being especially modified in the haustellate or suctorial orders, notably the Hymenoptera and Diptera. In the mandibulate orders, particularly the Orthoptera, where they are most generalized and primitive in shape and structure, they consist of the following parts: the gula (a postgula is present in Dermaptera), submentum (lora of Cheshire, i, p. 91), mentum, palpifer, the latter bearing the palpi; the lingua (ligula) and paraglossæ, while the hypopharynx or lingua is situated on the upper side. The labial palpi are of the same general shape as those of the first maxillæ, but shorter, with very rarely more than three joints, though in Pteronarcys there are four. Leon has detected vestigial labial palpi in several Hemiptera (Fig. 73). As to the exact nature and limits of the gula, we are not certain; it is not always present, and may be only a differentiation of the submentum, or the latter piece may be regarded as a part of the gula.

We are disposed to consider the second maxillæ as morphologically nearly the exact equivalents of the first pair of maxillæ, and if we 69adopt this view it will greatly simplify our conception of the real nature of this complicated organ. The object of the fusion of the basal portion appears to be to form an under-lip, in order both to prevent the food from falling backwards out of the mouth, and, with the aid of the first pair of maxillæ, to pass it forward to be crushed between the mandibles, the two sets of appendages acting somewhat as the tongue of vertebrates to carry and arrange or press the morsels of food between the teeth or cutting edges of the mandibles.

The spines often present on the free inner edges of the first and second maxillæ (Figs. 54, 62) form rude combs which seem to clean the antennæ, etc., often aiding the tibial combs in this operation.

The submentum and mentum, or the mentum when no submentum is differentiated (with the gula, when present), appear to be collectively homologous with the cardines of the first pair of maxillæ, together with the palpifers and the stipites.[15] These pieces are more or less square, and have a slightly marked median suture in Termitidæ, the sign of primitive fusion or coalescence.

The most primitive form of the second maxillæ occurs in the Orthoptera and in the Termitidæ. The palpifer is either single (Periplaneta, Diapheromera, Gryllidæ) or double (Blatta orientalis, Locustidæ). In Prisopus the single piece in front of the palpifer is in other forms divided, each half (Blatta, Locustidæ, Acrydidæ) bearing the two “paraglossæ,” which appendages in reality are the homologues of the lacinia and galea of the first maxillæ.[16] In the Termitidæ (Fig. 65) the lingua is not differentiated from the palpifer, and the two paraglossæ (or the lamina externa and interna of some authors) with the palpus are easily seen to be the homologues of the three lobes of the first maxillæ. In the Perlidæ (Pteronarcys, Fig. 66) the palpifer is divided, while the four paraglossæ arise, as in Prisopus and Anisomorpha, from an undivided piece, the lingua not being visible from without. In the Neuroptera the lingua or ligula is a large, broad, single lobe, without “paraglossæ,” and the palpifer is either single (Myrmeleon, Fig. 67), or divided (Mantispa, Fig. 68). 70In Corydalus (Fig. 29) the palpifer forms a single piece, and the lingua is undivided, though lobed on the free edge.

Fig. 65.—Second maxillæ of Termopsis angusticollis: li, the homologue of the lacinia; le, galea.

In the metabolic orders above the Neuroptera the lingua is variously modified, or specialized, with no vestiges of the lacinia or galea, except in that very primitive moth, Eriocephala, in which Walter found a minute free galea, me, and an inner lobe (Figs. 76, 77), the lacinia.

Fig. 66.—Second maxillæ of Pteronarcys californica.

Fig. 67.—Second maxillæ of Myrmeleon diversum.

Fig. 68.—Second maxillæ of Mantispa brunnea.

The hypopharynx.—While in its most generalized condition, as in Synaptera, Dermaptera, Orthoptera, and Neuroptera, this anterior median fold or outgrowth of the labium forming the floor of the mouth may retain the designation of “tongue,” lingua, or ligula; in its more specialized form, particularly when used as a piercing or lapping organ, the use of the name hypopharynx seems most desirable. And this is especially the case since, like the epipharynx, it is morphologically a median structure, and while the epipharynx forms the soft, sensitive roof of the mouth, or pharynx; its opposite, the hypopharynx, rises as a fold from the floor of the mouth, forming in its most generalized condition a specialized fold of the buccal integument. In certain cases, as in the honey-bee, the very long slender “tongue” or hypopharynx is evidently, as in the case of the epipharynx, a highly sensitive armature of the mouth.

71In all insects this organ—whether forming a soft, tongue-like, anterior portion or fold of the labium, and “continuous with the lower wall of the pharynx,” or a hard, piercing, awl-like appendage (fleas and flies), or a long, slender, hairy or setose, trough-like structure like the “tongue” of the honey-bee—has a definite location at the end and on the upper side of the labium, and serves to receive at its base the external opening of the salivary duct.

The hypopharynx, as well shown in its lingua condition in Orthoptera, is continuous with and forms the anterior part or fold of the base of the coalesced second maxillæ. It does not seem to be paired, or to represent a pair of appendages.

Opinion regarding the homology of this unpaired piercing organ is by no means settled, and while there is a general agreement as to the nature of the paired mouth-parts, recent observers differ very much as to the morphology of the organ in question.

It is the langue or lingua of Savigny (1816), the ligula of Kirby and Spence (1828), the langue ou languette (lancette médiane du suçoir) of Dugès (1832), the lingua of Westwood (Class, ins., ii, p. 489, 1840), “the unpaired median piercing organ” (“the analogon of the epipharynx of Diptera”) of Karsten (1864), the “tongue” of Taschenberg (1880).

The name hypopharynx was first proposed by Savigny in 1816, who, after naming the membranous plate which has for its base the upper side of the pharynx, the epipharynx, remarks: “Dans quelques genres, notamment dans les Eucères, le bord inférieur de ce même pharynx donne naissance à un autre appendice plus solide que le précédent, et qui s’emboîte avec lui. Je donnerai à ce dernier le nom de langue ou d’hypopharynx. Voilà donc la bouche des Hyménoptères composée de quatre organes impaires, sans y comprendre la ganache ou le menton; savoir, la lèvre supérieure, l’épipharynx, l’hypopharynx, et la lèvre inférieure, et de deux organes paires, les mandibules et les mâchoires.

As stated by Dimmock: “The hypopharynx is usually present in Diptera (according to Menzbier absent in Sargus), and contains a tube, opening by a channel on its upper surface; this channel extends back, more or less, from the tip, and is the outlet for the salivary secretion. The tip of the hypopharynx may be naked and used as a lance (Hæmatopota, according to Menzbier), or may be hairy (Musca). The upper side of the base of the hypopharynx is continuous with the lower wall of the pharynx; its under surface may entirely coalesce with the labium (Culex, male), may join the labium more or less, anterior to the month (Musca), or, if 72either mandibles or maxillæ are present, its base may join them (Culex, female).” (p. 43.)

Fig. 69.—Section of head of Machilis maritima: hyp, hypopharynx; lbr, labrum; t, tentorium; ph, room in which the mandibles move on each other; p, paraglossa; mx, labium; sd, salivary duct; s.gl, salivary gland. oe, œsophagus.—After Oudemans.

We will now briefly describe the lingua, first of the mandibulate or biting insects, and then its specialized form, the hypopharynx of the haustellate and lapping insects.

The lingua (hypopharynx) exists in perhaps its most generalized condition in the Thysanura (Fig. 69), where it forms a soft projection, having the same relations as in Anabrus and other Orthoptera.[17]

In the cockroach (Fig. 70), as stated by Miall and Denny, the lingua is a chitinous fold of the oral integument situated in front of the labium, and lying in the cavity of the mouth. The common duct of the salivary glands enters the lingua, and opens on its hinder surface. The lingua is supported by a chitinous skeleton (Figs. 70, B; 82, shp). “The thin chitinous surface of the lingua is hairy, like other parts of the mouth, and stiffened by special chitinous rods or bands.” (Miall and Denny.)

Fig. 70.—Hypopharynx of Periplaneta orientalis; the arrow points out of the opening of the salivary duct: A, origin of salivary duct. B, side view. C, front view.—After Miall and Denny.

In the Acrydiidæ (Melanoplus femur-rubrum) the tongue is a large, membranous, partly hollow expansion of the base of the labium. It may be exposed by depressing the end of the labium, when the opening of the salivary duct may be seen at the bottom or end of the space or gap between the hinder base of the tongue, and the inner anterior base of the labium, as shown by the arrows in Fig. 70. It is somewhat pyriform, slightly keeled above, and bearing fine stiff bristles, which, as they point more or less inwards, probably aid in retaining the food within the mouth. The 73base of the tongue is narrow, and extends back to near the pharynx, there being on the floor of the mouth, behind the tongue, two oblique, slight ridges, covered with stiff, golden-yellow hairs, like those on the tongue. The opening of the salivary duct is situated on the under or hinder side of the hypopharynx, between it and the base of the labium, the base of the former being cleft; the hollow thus formed is situated over the opening, and forms the salivary receptacle.

Fig. 71.—Section through the anterior part of the head of Anabrus (the mandibles removed), showing the relations of the hypopharynx (hyp) to the opening of the salivary duct (sd): g, galea; l, lacinia; mt, mentum; oe, œsophagus; lbr, labrum; cl, clypeus.

In the Locustidæ (Anabrus, Fig. 71) the tongue (hypopharynx) is a broad, somewhat flattened lobe arising from the upper part of the base of the mentum and behind the palpifer. This lobe is cavernous underneath, the hollow being the salivary receptacle (sr); the latter is situated over the opening of the salivary duct, which is placed between the base of both the hypopharynx and the labium. The salivary fluid apparently has to pass up and around on each side of the hypopharynx in order to mix with the food.

These relations in the Orthoptera are also the same in the Perlidæ, where the hypopharynx is well developed, forming an unusually large tongue-like mass, nearly filling the buccal cavity.

Fig. 72.—Lingua of a May-fly, Heptagenia longicauda, ×16: m, central; l, lateral pieces.—After Vayssière from Sharp.

In the Odonata the lingua is a small, rounded lobe, as also in the Ephemeridæ; in the nymph, however, of Heptagenia (Fig. 72) it is highly developed, according to Vayssière, who seems inclined to regard it as representing a pair of appendages. The tongue in Hemiptera is said by Léon to be present in Benacus griseus (Say) and to correspond to the subgalea of Brullé or hypodactyle of Audouin (Fig. 73), but this appears to correspond to the labium proper, rather than a true lingua, the latter not being differentiated in this order. In the Coleoptera the lingua is rather small. In beetles, as Anopthalmus (Fig. 74), it forms a setose lobe; and a well-developed nerve, the lingual nerve, passes to it, dividing at the end into several branches (n-l). In Sialis the lingua is short, much less developed than usual, being rounded, 74and bears on the edge what appear to be numerous taste-hairs, like those on the ends of the maxillary and labial palpi.

Fig. 73.A, labium of Zaitha anura. B, of Z. margineguttata. C, of Gerris najas: mt, mentum; lp, labial palpi; sg, subgalea; l, lacinia (= intermaxillare and præmaxillare of Brullé); g, galea.—After Léon.

In the adult Panorpidæ the lingua is a minute, simple lobe.

Fig. 74.—Section through head of a carabid, Anopthalmus telkampfii: br, brain; f. g, frontal ganglion; soe, subœsophageal ganglion; co, commissure; n. l, nerve sending branches to the lingua (l); mn, maxillary nerve; mx, 1st maxilla; mm, maxillary muscle; mx′, 2d maxilla; mt, muscle of mentum; le, elevator muscle of the œsophagus; l of the clypeus, and a third beyond raising the labrum (lbr); eph, epipharynx; g, g, salivary glands above; g2, lingual gland below the œsophagus (oe); m, mouth; pv, proventriculus; md, mandible. A, section passing through lingual gland (g2).

In the larval Trichoptera the spinneret is well developed, and in structure substantially like that of caterpillars, and it is plainly the homologue of the hypopharynx, receiving as it does the end of the silk-duct.

In the adult Trichoptera the hypopharynx is a very large, tongue-like, fleshy outgrowth, and is, both in situation and structure, since it contains the opening of the silk-duct, exactly homologous with the hypopharynx of insects of other orders, being somewhat intermediate between the fleshy tongue or lingua of the mandibulate insects, especially the 75Neuroptera, and the hypopharynx of the bees (Fig. 86). Lucas describes and figures it under the name of “haustellum,” but does not homologize it with the hypopharynx. The caddis-flies have been observed to drink water and take in both fluid and fine particles of solid food, and to use the haustellum for this purpose, the end being provided with minute sense-organs like those on the first maxillary lacinia, and possibly of a gustatory nature.

Fig. 75.—Head of Anabolia furcata: A, front view, showing the labrum removed. B, side view; ant, antenna; oc, ocellus; ol, labrum; gh, articulatory process; cmx1, cardo; stmx1, stipes; lemx1, outer lobe (galea); ptmx1, palpus of 1st maxilla; pl, palpus of 2d maxilla; ha, haustellum; so, gustatory pits; spr, opening of salivary duct; chsp, chitinous hook of the clasp; spr, furrow or gutter of the haustellum.—After Lucas.

Fig. 76.—Hypopharynx of Eriocephala calthella: lig, ligula, its membranous hinder edge; lig′, anterior horny edge of the ligula-tube opening outwards; hp, contour of the hypopharynx; mi, mala interior (lacinia); me, mala exterior (galea), of second maxilla; mx′ p, labial palpus.—After Walter.

The spinneret of the larvæ of Lepidoptera is evidently the homologue of the hypopharynx of insects of other orders. It will be seen that the homology of the different parts is identical, the common duct of the silk-glands opening at the end of the hypopharynx, which here forms a complete tube or proboscis extending beyond the end of the labium, in adaptation to its use as a spinning organ.

Walter refers to Burgess’s discovery of a hypopharynx in Danais archippus, 76remarking that this organ in the adult Eriocephalidæ (Fig. 76) exhibits a great similarity to the relations observable in the lower insects, adding:—

Fig. 77.— Labium of Micropteryx anderschella seen from within (the labial palpi (mx.′ p) removed to their basal joint). Lettering as in Fig. 76.—After Walter.

“The furrow is here within coalesced with the inner side of the labium, and though I see in the entire structure of the head the inner edge of the ligula tube extended under the epipharynx as far as the mandible, I must also accept the fact that here also the hypopharynx extends to the mouth-opening as in all other sucking insects with a well-developed under-lip, viz. the Diptera and Hymenoptera.”

He has also discovered in Micropteryx a paired structure which he regards as the hypopharynx (Fig. 77). As he states:

Fig. 78.—Hypopharynx (hph) of Danais: cl, clypeus; sd, salivary duct; m, labial palp muscles; fm, frontal muscle; ph, pharynx; cor, cornea.—After Burgess.

“A portion of the inner surface of the tube-like ligula is covered by a furrow-like band which, close to the inner side, is coalesced with it, and in position, shape, as well as its appendages or teeth on the edge, may be regarded as nothing else than the hypopharynx.”

A hypopharynx is also present in the highest Lepidoptera, Burgess having detected it in Danais archippus. He states that the hypopharynx forms the floor of the pharyngeal cavity; “it is convex on each side of a median furrow (Fig. 78, hph) and somewhat resembles in shape the human breast. The convex areas are dotted over with little papillæ, which possibly may be taste-organs.”

As a piercing organ the hypopharynx reaches its greatest development in the Siphonaptera and Diptera, where the chitinous parts are greatly hypertrophied, the fleshy tongue-like portion so developed in the mandibulate orders being greatly reduced. The chitinous parts are alike on each side of the median organ, being bilaterally symmetrical.

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Fig. 79.A, hypopharynx of Pulex canis: x, basal portion situated within the head; s. d, common duct of the four bladder-shaped salivary glands; s. d′, opening of the tubular salivary glands into the throat. B, end of the hypopharynx, showing the gutter-like structure and teeth at the end.—After Landois.

Fig. 80.—Beak of Vermipsylla: hyp, hypopharynx.—After Wagner.

In the fleas the hypopharynx is a large, slender, unpaired, long, chitinous trough, as long as the mandibles, and toothed at the end. Figures 79 and 80 show its relations to the other parts of the mouth; in Fig. 79, x, is seen where the salivary duct opens into the pharynx. Although this organ is not unanimously referred to the hypopharynx, yet from the description of Landois and others, it is evident that this structure does not correspond to the labrum or epipharynx, but belongs to or arises from the floor of the mouth, and, being in close relation to the labium, and also receiving the salivary duct, must be a true hypopharynx.

78In the Diptera the hypopharynx reaches its highest development as a large, stout, awl-like structure.

Meinert, in his detailed and elaborately illustrated work, Trophi Dipterorum (1881), has made an advance on our knowledge of the hypopharynx and its homologies, both by his evidently faithful descriptions and dissections, and by his admirably clear figures.

Fig. 81.Culex pipiens, section of head: oe œsophagus; sm, upper muscle, lm, lower muscle of the œsophagus; ph, pharynx; rm, retractor muscle of the receptacle (r) of the salivary duct (s.d); lbr, labrum; ep, left style of the epipharynx; f, part of front of head.—After Meinert.

Fig. 82.—Pharynx and hypopharynx of Simulium fuscipes: lph, lower lamina of the pharynx; p, the salivary duct (s.d) perforating the pharynx; o, orifice of the duct; shp, styles of the hypopharynx; mph, membranous edge of the hypopharynx; m, protractor muscle of the pharynx; gp, gustatory papillæ.—After Meinert.

“The hypopharynx, a continuation of the lower edge (lamina) of the pharynx, most generally free, more or less produced, acute anteriorly, forms with the labrum the tube of the pump (antliæ). (The hypopharynx when obsolete, or coalesced with the canal of the proboscis, is the theca; in such a case the siphon or tube is formed by the theca and labrum.) Meanwhile the hypopharynx, the largest of all the trophi (omnium trophorum maximus), constitutes the chief piercing organ (telum) of Diptera. The hypopharynx is moved by protractor, most generally quite or very powerful, and by retractor muscles.

“The efferent duct of the thoracic salivary glands (ductus salivalis) perforates the hypopharynx, more or less near the base, that the saliva may be ejected through the canal into the wound, or that it may be conducted along the labella. Very rarely the salivary duct, perforating the hypopharynx, is continued in the shape of a free, very slender tube.

“The salivary duct behind the base of the hypopharynx forms the receptacle or receptaculum, provided with retractor and levator muscles.”

79

Fig. 83.—Labrum-epipharynx (lbr and eph) and hypopharynx (hyp) of Tabanus brominus: oe, posterior cylindrical portion of the œsophagus; a, anterior swollen portion of the same; ph, pharynx; ph.m, pharyngeal muscle; p.ph, protractor muscle of the pharynx; r.oe, retractor muscle of the œsophagus; r.ph, retractor muscle of the pharynx; f.oe, flexor muscle of the pharynx; t.oe, twisting muscle of the œsophagus; s.r, receptacle of the salivary duct; l, its elevator muscle; s, its retractor muscle; cl, clypeus.—After Meinert.

Fig. 84.—Œsophagus (oe), pharynx (ph) with epipharynx and labrum (lbr) of Asilus atricapillus: m, ph, pharyngeal muscle; sr, salivary receptacle; t, twisting; r, l′r, retractor muscles; other lettering as in Fig. 83.—After Meinert.

It has been carefully studied by Meinert in a species of Culex (Fig. 81), Simulium (Fig. 82), Tabanus (Fig. 83), and in Asilus (Fig. 84), where it is seen to attain enormous proportions. In the Hymenoptera, this organ in its most specialized condition is a trough-like rod, adapted for lapping nectar (Fig. 85, 86, hyp). The tongue or hypopharynx of the honey-bee has been elaborately described by Cheshire in his Bees and Bee Keeping.[18] He calls it the tongue or ligula. It is situated in a tube formed by the maxillæ and labial palpi, and can be partially retracted into the mentum. He states that it can move up and down in the tube thus formed, and then describes it as covered by a hairy sheath, its great elasticity being due to a rod running through its centre enabling it to be used as a lapping tongue. The sheath

80“passes round the tongue to the back, where its edges do not meet, but are continuous with a very thin plaited membrane (G, pm) covered with minute hairs. This membrane, after passing towards the sides of the tongue, returns to the angle of the nucleus, or rod, over the under surface of which it is probably continued. The rod passes through the tongue from end to end, gradually tapering towards its extremity, and is best studied in the queen, where I trace many nerve threads and cells. It is undoubtedly endowed with voluntary movement, and must be partly muscular, although I have failed completely in getting any evidence of striation. The rod on the underside has a gutter, or trough-like hollow (cd, the central duct) which is formed into a pseudotube (false tube) by intercrossing of black hairs. It will also be seen that, by the posterior meeting of the sheath, the space between the folded membrane (G, sd) becomes two pseudotubes of larger size, which I shall call the side ducts.

Fig. 85.—Head of honey bee, worker: a, antenna; g, epipharynx; m, mandible; mx, maxilla; mxp, maxillary palpus; pg, paraglossa; lp, labial palpus; l, hypopharynx; b, its spoon.—After Cheshire; from Bull. Div. Ent. U. S. Dept. Agr.

“These central and side ducts run down to that part of the tongue where the spoon, or bouton (K, Fig. 86) is placed. This is provided with very delicate split hairs (b, Fig. 86) capable of brushing up the most minute quantity of nectar, which by capillarity is at once transferred by the gathering hairs (which are here numerous, long, and thin) to two side groove-like forms at the back of the bouton, and which are really the opened-out extremity of the centre and side ducts, assuming, immediately above the bouton, the form seen in F, Fig. 86. The central duct, which is only from 1
600
inch to 1
1000
inch in diameter, because of its smaller size, and so greater capillary attraction, receives the nectar, if insufficient in quantity to fill the side ducts. But good honey-yielding plants would bring both centre and side ducts into requisition. The nectar is sucked up until it reaches the paraglossæ (pa, B, Fig. 86), which are plate-like in front, but membranous extensions, like small aprons, behind; and by these the nectar reaches the front of the tongue, to be swallowed as before described.”

81

Fig. 86.—Tongue or ligula of the honey bee: A, under side of the tongue; lp, labial palpi; r, r, rod; p, pouch; sh, sheath; gh, gathering hairs; b, bouton or spoon. B, under lip or labium, with appendages, partly dissected; l, lora or submentum; a, a, retractor linguæ longus; sd, salivary duct; rb and b, retractor linguæ biceps; mx, maxillæ; lp, labial palpi; pa, paraglossa; gr, feeding groove; sh, sheath of ligula. C, D, E, sections of ligula; hp, hyaline plate of maxilla; h, hairs acting as stops; mx, maxilla; lp, labial palpi; sd, side duct. F, cross-section of extremity of tongue near the “spoon”; th, tactile hairs; r, rod; n, nucleus; gh, gathering hairs. G, cross-section of tongue without gathering hairs, × 400 times; sh, sheath; b, blood space; t, trachea: ng, gustatory nerve; cd, central duct; sd, lateral duct; pm, plaited membrane. H, same as G, but magnified two hundred times, and with pm, plaited membrane, turned outwards; h, closing hairs; lp, labial palpi; b, blood; n, nucleus; r, rod; h, closing hairs. I, small portion of the sheath; lettering as before. K, extremity of the tongue, with spoon; b, branching hairs for gathering.—After Cheshire.

82Cheshire then settles the question which has been in dispute since the time of Swammerdam, whether the bee’s tongue is solid or tubular. He agrees with Wolff that the duct is a trough and not a tube, and proves it by a satisfactory experiment. He remarks:

Fig. 87.—Longitudinal section through the head of the honey bee, ♀, just outside of right antenna: ant, antenna with three muscles attached to mes, mesocephalic pillar; cl, clypeus; lbr, labrum; 1, chyle-gland (system no. 1, of Siebold); o, opening of the same; oc, ocellus; br, brain; n, neck; th, thorax; oe, œsophagus; s.d2, s.d3, common salivary ducts of systems 2 and 3; v, salivary valve; c, cardo; ph, pharynx; mx′, labium; mx.′p, labial palpi; mt, mentum; mx, maxilla; hyp, hypopharynx; s, bouton.—After Cheshire.

“Bees have the power, by driving blood into the tongue, of forcing the rod out from the sheath, and distending the wrinkled membrane so that in section it appears as at H, Fig. 86, the membrane assuming the form of a pouch, given in full length at A. It will be seen at once that this disposition of parts abolishes the side ducts, but brings the central duct to the external surface. The object of this curious capability on the part of the bee is, in my opinion, to permit of cleaning away any pollen grains, or other impediment that may collect in the side ducts. The membrane is greasy in nature, and substances or fluids can be removed from it as easily as water from polished metal. If, now, the sides of a needle, previously dipped into clove oil in which rosanilin (magenta) has been dissolved, so as to stain it strongly red, be touched on the centre of the rod, the oil immediately enters, and passes rapidly upwards and downwards, filling the trough.”

Does the hypopharynx represent a distinct segment?—The facts which suggest that the hypopharynx may possibly represent a highly modified pair of appendages, arising from a distinct intermaxillary segment, are these: Heymons plainly shows that, in the embryo of Lepisma, the hypopharynx originates as a transverse segment-like fold in front of the 2d maxillary segment, and larger than it, and though he does not mention it in his text, it appears like the rudiment of a distinct segment; the hypopharynx of Ephemeridæ; arises and remains separate in the nymph from the labium (see Heymons’ Fig. 29, and there are two lateral projections; see also Fig. 72, and Vayssiere’s view that it may represent a pair of appendages; Kolbe also regards it as representing a third pair of maxillæ, his endolabium, p. 213). Though what is called an unpaired 83organ, it is composed of, or supported by, two bilaterally symmetrical styles, both in Myriopods (Fig. 6, labiella, stil) and in insects (Fig. 77, etc.). On the other hand, in the embryo of pterygote insects, an intermaxillary segment has not been yet detected.

LITERATURE OF THE MOUTH-PARTS OR BUCCAL APPENDAGES

a. General

Savigny, Jules-César. Mémoires sur les animaux sans vertèbres. 1re Part. Description et classification des animaux invertébrés et articulés, etc. Fasc. 1re. Mém. 1–2. Théorie des organes de la bouche des crustacés et des insectes. 12 Pl., Paris, 1816, pp. 1–117.

Gerstfeld, Georg. Ueber die Mundteile der saugenden Insekten. Dorpat, 1853.

Olfers, Ernestus V. Annotationes ad anatomiam Podurarum. Berolini, 1862, 4 Pls.

Gerstaecker, Carl Eduard Adolph. Zur Morphologie der Orthoptera amphibiotica. (Festschrift zur Feier des hundertjährigen Bestehens der Gesellschaft naturf. Freunde zu Berlin. 4º, 1873, pp. 39–59, 1 Taf.)

Muhr, Joseph. Die Mundteile der Orthoptera. Ein Beitrag zur vergleichenden Anatomie. (Jahrbuch “Lotos.” Prag, 1877, pp. 40–71, 8 Taf.)

Burgess, Edward. The anatomy of the head and the structure of the maxilla in the Psocidæ. (Proc. Boston Soc. Nat. Hist., xix, 1878, pp. 291–296, 1 Pl.)

Meinert, Fr. Sur la conformation de la tête et sur l’interpretation des organes buccaux chez les insectes, ainsi que sur la systématique de cette ordre. (Ent. Tidsskr., 1. Arg., 1880, pp. 147–150.)

—— Tungens udskydelighed hos Steninerne, en slaegt af Staphylinernes familie. (Vidensk. meddel. fra den naturh. Foren, 1884–1886, pp. 180–207, 2 Pls. Also Zool. Anzeiger, 1887, pp. 136–139.)

Müller, A. Vergleichend-anatomische Darstellung der Mundteile der Insekten. Villach, 1881, 3 Taf.

Kraepelin, Karl. Ueber die Mundwerkzeuge der saugenden Insekten. (Zool. Anzeiger, 1882, pp. 574–579.)

Dewitz, H. Ueber die führung an den Körperanhangen der Insekten. (Berlin. Zeitschr. xxvi., 1882, pp. 51–68, Figs.)

Wolter, Max. Die Mundbildung der Orthopteren mit specieller Berücksichtigung der Ephemeriden. 4 Taf. Greifswald, 1883.

Oudemans, J. T. Beiträge zur Kenntniss der Thysanura und Collembola. (Bijdragen tot de Dierkunde, pp. 149–226. Amsterdam, 1888, 3 Taf.)

Smith, John B. An essay on the development of the mouth-parts of certain insects. (Trans. Amer. Philosophical Soc., xix, pp. 175–198, 3 Pls.)

Also articles by Chatin, McLachlan, Riley, Wood-Mason.

b. Thysanoptera (Physapoda)

Jordan, Karl. Anatomie und biologie der Physapoda. (Zeitschr. f. wissens. Zool., xlvii, pp. 541–620, 3 Taf. 1888.)

Garman, H. The mouth-parts of the Thysanoptera. (Bull. Essex Inst., xxii, 4 pp., Fig. 1890.)

—— The asymmetry of the mouth-parts of Thysanoptera. (Amer. Naturalist, July, 1896, pp. 591–593, Fig.)

Bohls, J. Die Mundwerkzeuge der Physapoden. Dissertation Göttingen, 1891, pp. 1–36.

84Uzel, Heinrich. Monographie der Ordnung Thysanoptera. Königgrätz, 1895, pp. 472, 10 Taf., 9 Figs.

c. Hemiptera

Léon, N. Beiträge zur Kenntniss der Mundteile der Hemipteren. Jena, 1887, pp. 47, 1 Taf.

—— Labialtaster bei Hemipteren. (Zool. Anzeiger, pp. 145–147, 1892, 1 Fig.)

—— Beiträge zur Kenntniss des Labiums der Hydrocoren. (Zool. Anzeiger, März 29, 1897, pp. 73–77, Figs. 1–5.)

Geise, O. Mundteile der Rhynchoten. (Archiv f. Naturgesch., xlix, 1883, pp. 315–373, 1 Taf.)

Wedde, Hermann. Beiträge zur Kenntniss des Rhynchotenrüssels. (Archiv f. Naturgesch., li Jahrg., 1 Bd., 1885, pp. 113–148, 2 Taf.)

Smith, John B. The structure of the hemipterous mouth. (Science, April 1, 1892, pp. 189–190, Figs. 1–5.)

d. Coleoptera

Smith, John B. The mouth-parts of Copris carolina; with notes on the homologies of the mandibles. (Trans. Amer. Ent. Soc., xix, April, 1892, pp. 83–87, 2 Pls.)

e. Lepidoptera

Kirbach, P. Ueber die Mundwerkzeuge der Schmetterlinge. (Zool. Anzeiger, vi Jahrg., 1883, pp. 553–558, 2 Figs.)

—— Ueber die Mundwerkzeuge der Schmetterlinge. (Archiv f. Naturgeschichte, 1884, pp. 78–119, 2 Taf.)

Walter, Alfred. Palpus maxillaris Lepidopterorum. (Jenaische Zeitschr. f. Naturwiss, xviii, 1884, pp. 121–173, Taf.)

—— Beiträge zur Morphologie der Lepidoptera. I, Mundteile. (Jenaische Zeitschr. f. Naturwiss, xviii, 1885, pp. 751–807, 2 Taf.)

Breitenbach, W. Vorläufige Mitteilung über einige neue Untersuchungen an Schmetterlingsrüsseln. (Archiv f. mikroskop. Anatomie, xiv, 1877, pp. 308–317, 1 Taf.)

—— Untersuchungen an Schmetterlingsrüsseln. (Ibid., xv, 1878, pp. 8–29, 1 Taf.)

—— Ueber Schmetterlingsrüssel. (Entomolog. Nachr. 5 Jahrg., 1879, pp. 237–243, 1 Taf.)

—— Der Schmetterlingsrüssel. (Jenaische Zeitschr. f. Naturwiss, 1881.)

f. Siphonaptera

Kräpelin, K. Ueber die systematische Stellung der Puliciden. (Festschrift z. 50 jahr. Jubil. d. Realgymnas. Iohanneum, Hamburg, pp. 17, 1 Taf. 1884.)

Kellogg, V. L. The mouth-parts of the Lepidoptera. (Amer. Nat., xxix, 1895, pp. 546–556, 1 Pl. and Fig.)

g. Diptera

Menzbier, Michael Alexander. Ueber das Kopfskelett und die Mundwerkzeuge der Zweiflügler. (Bull. Soc. Imp. Natur. de Moscou, lv, 1880, pp. 8–71, 2 Taf.)

Dimmock, George. The anatomy of the mouth-parts and of the sucking apparatus of some Diptera. Boston, 1881, pp. 48, 4 Pls.

85Meinert, F. Fluernes Munddele. Trophi Dipterorum. Kjöbenhavn, 1881, 6 Pls.

—— Die Mundteile der Dipteren. (Zool. Anz. 1882, pp. 570–574, 599–603.)

Becher, E. Zur Kenntniss der Mundteile der Dipteren. (Denkschr. Akad. d. Wissensch. Wien., xlv, 1882, pp. 123–162, 4 Taf.)

Hansen, H. J. Fabrica oris dipterorum: Dipterernes mund: anatomisk og systematisk henseende. 1 Tabanidae, Bombyliidae, Asilidae, Thereva, Mydas, Apiocera. (Naturhist. Tidsskrift, 1883, xiv, pp. 1–186, Taf. 1–5.)

Kräpelin, Karl. Zur Anatomie und Physiologie des Rüssels von Musca. (Zeitschr. f. wissensch. Zool., xxxix, 1883, pp. 683–719, 2 Taf.)

McCloskie, George. Kraepelin’s Proboscis of the house-fly. (American Naturalist, xviii, 1884, pp. 1234–1244, Figs.)

Langhoffer, August. Beiträge zur Kenntniss der Mundtheile der Dipteren. Jena, 1888, pp. 1–32.

Smith, John B. A contribution toward a knowledge of the mouth-parts of the Diptera. (Trans. Amer. Ent. Soc., xvii, Nov. 1890, pp. 319–339, Figs. 1–22.)

h. Hymenoptera

Briant, Travers J. On the anatomy and functions of the tongue of the honey-bee (worker). (Journ. Linn. Soc., London, xvii, 1884, pp. 408–416, 2 Pls.)

Breithaupt, P. F. Ueber die Anatomie und die Funktionen der Bienenzunge. (Archiv f. Naturgesch., Jahrg. lii, 1886, pp. 47–112, 2 Taf.)

i. Larval stages

Brauer, F. Die Zweiflügler des kaiserlichen Museums zu Wien. III, Systematische Studien auf Grundlage der Dipterenlarven nebst einer Zusammenstellung von Beispielen aus der Litteratur uber dieselben und Beschreibung neuer Formen. (Denkschr. math.-naturwiss. Cl. k. Akad. Wiss. Wien, 1883, xlvii, pp. 100, 5 Taf.)

Dewitz, H. Ueber die Führung an den Körperanhangen der Insekten speziell betrachtet an der Legescheide der Acridier, dem Stachel der Meliponem und den Mundteilen der Larve von Myrmeleon, nebst Beschreibung dieser Organe. (Berliner ent. Zeitschr., xxvi, 1882, pp. 51–68.)

—— Die Mundteile der Larve von Myrmeleon. (Sitzungsber. d. Ges. naturforsch. Freunde zu Berlin, 1881, pp. 163–166.)

Redtenbacher, Josef. Uebersicht der Myrmeleonidenlarven. (Denkschrift, math.-naturwiss. Cl. k. Akad. Wiss. Wien, 1884, xlviii, pp. 335–368, 7 Taf.)

Schiödte, J. G. De metamorphosi Eleutheratorum. Bidrag til insekternes udviklingshistorie. (Kroyer’s Naturhist. Tidsskrift. Kjöbenhavn. 12 Teile mit 88 Taf., 1862–1883.)

j. Embryonic stages

Heymons, Richard. Grundzüge der Entwicklung und des Körpersbaues von Odonaten und Ephemeridem (Anhang zu den Abhandl. K. Akad. d. Wissens. Berlin, 1896, p. 22, 2 Taf. See Figs. 5, 29.)

—— Entwicklungsgeschichtliche Untersuchungen an Lepisma saccharina L. (Zeitschr. f. Wissens. Zoologie, lxii, 1897, p. 595, 2 Taf. See Fig. 10.)

86

THE THORAX AND ITS APPENDAGES

a. The thorax; its external anatomy

The middle region of the body is called the thorax, and in general consists of three segments, which are respectively named the prothorax, mesothorax, and metathorax (Figs. 88, 89, 98).

Fig. 88.—External anatomy of Melanoplus spretus, the head and thorax disjointed.

The thorax contains the muscles of flight and those of the legs, besides the fore intestine (œsophagus and proventriculus), as well as, in the winged insects, the salivary glands.

87In the more generalized orders, notably the Orthoptera, the three segments are distinct and readily identified.

Fig. 89.—Locust, Melanoplus, side view, with the thorax separated from the head and abdomen, and divided into its three segments.

Each segment consists of the tergum, pleurum, and sternum. In the prothorax these pieces are not subdivided, except the pleural; in such case the tergum is called the pronotum. The prothorax is very large in the Orthoptera and other generalized forms, as also in the Coleoptera, but small and reduced in the Diptera and Hymenoptera. In the winged forms the tergum of the mesothorax is differentiated into four pieces or plates (sclerites). These pieces were named by Audouin, passing from before backwards, the præscutum, scutum, scutellum, and postscutellum. In the nymph stage and in the wingless adults of insects such as the Mallophaga, the true lice, the wingless Diptera, ants, etc., these parts by disuse and loss of the wings are not differentiated. It is therefore apparent that their development depends on that of the muscles of flight, of which they 88form the base of attachment. The scutum is invariably present, as is the scutellum. The former in nearly all insects constitutes the larger part of the tergum, while the latter is, as its name implies, the small shield-shaped piece directly behind the scutum.

Fig. 90.—Thorax of Telea polyphemus, side view, pronotum not represented: em, epimerum of prothorax, the narrow piece above being the prothoracic episternum; ms, mesoscutum; scm, mesoscutellum; ms″, metascutum; scm‴, metascutellum; pt, a supplementary piece near the insertion of tegulæ; w, pieces situated at the insertion of the wings, and surrounded by membrane; epm″, episternum of the mesothorax; em″, epimerum of the same; epm‴, episternum of the metathorax; em‴ epimerum of the same, divided into two pieces; c′, c″, c‴, coxæ; te′, te″, te‴, trochantines; tr, tr, tr, trochanters. A, tergal view of the mesothorax of the same; prm, præscutum; ms, scutum; scm, scutellum; ptm, postscutellum; t, tegula.

The præscutum and postscutellum are usually minute and crowded down out of sight between the opposing segments. As seen in Fig. 90, the præscutum of most moths (Telea) is a small rounded piece, bent vertically down so as not to be seen from above. In Polystœchotes and also in Hepialus the præscutum is large, well-developed, triangular, and wedged in between the two halves of the scutum. The postscutellum is still smaller, usually forming a transverse ridge, and is rarely used in taxonomy.

Fig. 91.—Thorax of the house-fly: prn, pronotum; prsc, præscutum; sc′, mesoscutum; sct′, mesoscutellum; psct′, postscutellum; al, insertion of squama, extending to the insertion of the wings, which have been removed; msphr, mesophragma; h, balancer (halter); pt, tegula; mtn, metanotum; epis, epis′, epis″, episternum of pro-, meso-, and metathorax; epm′, epm″, meso- and meta-epimerum; st′, st″, meso- and metasternum; cx′, cx″, cx‴, coxæ; tr′, tr″, tr‴, trochanters of the three pairs of legs; sp′, sp″, sp‴, sp‴′, sp‴″, first to fifth spiracles; tg′, tg″, tergites of first and second abdominal segments; u′, u″, urites.

The metathorax is usually smaller and shorter than the mesothorax, being proportioned to the size of the wings. In certain Neuroptera and in Hepialidæ and some tineoid moths, where the hind wings are nearly as large as those of the anterior pair, the metathorax is more than half or nearly two-thirds as large as the mesothorax. In Hepialidæ the præscutum is large and distinct, while the scutum is divided into two widely separated pieces. The postscutellum is nearly or quite obsolete.

The pleurum in each of the three thoracic segments is divided into two pieces; the one in front is called the episternum, since it 89rests upon the sternum; the other is the epimerum. To these pieces, with the sternum in part, the legs are articulated (Fig. 89).

Between the episterna is situated the breastplate or sternum, which is very large in the more primitive forms, as the Orthoptera, and is small in the Diptera and Hymenoptera.

Fig. 92.—Prothorax of Geometra papilionaria: n, notum; p, pleura; st, sternum; pt, patagia; m, membrane; f, femur; h, a hook bent backwards and beneath, and connecting the pro- with the mesothorax.—After Cholodkowsky.

The episterna and epimera are in certain groups, Neuroptera, etc., further subdivided each into two pieces (Fig. 102). The smaller pieces, hinging upon each other and forming the attachments of the muscles of flight, differ much in shape and size in insects of different orders. The difference in shape and degree of differentiation of these parts of the thorax is mentioned and illustrated under each order, and reference to the figures will obviate pages of tedious description. A glance, however, at the thorax of a moth, fly, or bee, where these numerous pieces are agglutinated into a globular mass, will show that the spherical shape of the thorax in these insects is due to the enlargement of one part at the expense of another; the prothoracic and metathoracic segments being more or less atrophied, while the mesothorax is greatly enlarged to support the powerful muscles of flight, the fore wings being much larger than those appended to the metathorax. In the Diptera, whose hinder pair of wings are reduced to the condition of halteres, the reduction of the metathorax as well as prothorax is especially marked (Fig. 91).

The patagia.—On each side of the pronotum of Lepidoptera are two transversely oval, movable, concavo-convex, erectile plates, called patagia (Fig. 92). On cutting those of a dry Catocala in two, they will be seen to be hollow. Cholodkowsky[19] states that they are filled with blood and tracheal branches; and he went so far as to regard them as rudimentary prothoracic wings, in which view he was corrected by Haase,[20] who compares them with the tegulæ, regarding them also as secondary or accessory structures.

The tegulæ.—On the mesothorax are the tegulæ of Kirby (pterygodes of Latreille, paraptera of McLeay, hypoptère or squamule), which cover the base of the fore wings, and are especially developed in the Lepidoptera (Fig. 90, A, t) and in certain Hymenoptera (Fig. 95, c).

90The external opening of the spiracles just under the fore wings, is situated in a little plate called by Audouin the peritreme.

Fig. 93.—Transformation of the bumble bee, Bombus, showing the transfer of the 1st abdominal larval segment (c) to the thorax, forming the propodeum of the pupa (D) and imago; n, spiracle of the propodeum. A, larva; a, head; b, 1st thoracic; c, 1st abdominal segment. B, semipupa; g, antenna; h, maxillæ; i, 1st; j, 2d leg; k, mesoscutum; l, mesoscutellum; m, metathorax; d, urite (sternite of abdomen); e, pleurite; f, tergite; o, ovipositor; r, lingua; q, maxilla.

In the higher or aculeate Hymenoptera, besides the three segments normally composing the thorax, the basal abdominal segment is during the change from the larva to the pupa transferred to this region, making four segments. This first abdominal is called “the median segment” (Figs. 93–95). In such a case the term alitrunk has been applied to this region, i.e. the thorax, as thus constituted. Latreille wrongly stated that in the Diptera the first abdominal segment also entered into the composition of the thorax; but Brauer has fully disproved that view, as may be seen by an examination of his sketches which we have copied (Fig. 94).

91

Fig. 94.—7, 8, thorax of Tipula gigantea; 9, of Leptis; 10, thorax of Tabanus bromius after the removal of the abdomen, in order to bring into view the inner mesophragma (f), and to show the extension of the metathorax g and g′; tr, trochanter; 11, hind end of the mesothorax, the entire metathorax, and the 1st and 2d abdominal segments of Volucella zonaria, seen from the side. The internal mesophragma (f), and the position of the muscle inserted in it, are indicated by the two lines M. p, Callus postalaris; pr (pz in 8), callus præalaris Osten Sacken (= “patagium” of some authors); g, metanotum; g′, metepimerum, “segment médiaire” of Latreille (wrongly considered by him to be the 1st abdominal segment); 4, metasternum (hypopleura of Osten Sacken); 5 (? “episternum of metathorax” (Brauer) = metapleura of Osten Sacken); 6, and also H, halter; st1, mesothoracic stigma; st2, metathoracic stigma; st3, first abdominal stigma; γ, dorsopleural; δ, sternopleural; ε, mesopleural sutures; h, 1st, i, 2d, abdominal segment; al, wing; alul, alula. 12, the head and the three thoracic rings, and the 1st abdominal segment of Ephemera vulgata, the connecting membranes are in white: a, prothorax; b, præscutum; c, scutum; d, scutellum; e, postscutellum; ps, postscutellum of mesothorax.—After Brauer.

92

Fig. 95.—Alitrunk of Sphex chrysis: A, dorsal aspect; a, pronotum; b, mesonotum; c, tegula; d, base of fore,—e, of hind, wing; f, g, divisions of metanotum; h, median (true first abdominal) segment; i, its spiracle; k, second abdominal segment, usually called the petiole or first abdominal segment. B, posterior aspect of the median segment; a, upper part; b, superior,—c, inferior, abdominal foramen; d, ventral plate of median segment; e, coxa.—After Sharp.

The sternum is in rare cases subdivided into two halves, as in the meso- and metathorax of the cockroach; in Forficula the prosternum is divided into four pieces besides the sternum proper (Fig. 96); and in Embia, also, the sternites, according to Sharp, are complex.

Fig. 96.—Sternal view of pro-, meso-, and metathorax of Forficula tæniata: pst, præsternum, divided into 4 pieces; st, pro-, st′, meso-, st″, metasternum; cx, coxa; not, notum.

Fig. 97.A, under surface of prothorax, or prosternum, of Dyticus circumflexis: 2.g, prosternum; 2.f, episternum; 2.h, epimerum; 2.s, antefurca or entothorax.

93

Fig. 98.—Meso- (G2) and metathoracic ganglia (G1), with the apodemes of Gryllotalpa.—After Graber.

Fig. 99.—Parts of the mesothorax of Dyticus: A, mesosternum; 3.a, præscutum; 3.b, scutum; 3.c, scutellum; 3.d, postscutellum; 3.e, parapteron; 3.g, mesosternum; 3.f, episternum; 3.h, epimerum; 3.s, medifurca or entothorax.

Fig. 100.—Parts of the metathorax of Dyticus: A, metasternum; 4.a, præscutum; 4.b, scutum; 4.c, scutellum; 4.d, postscutellum; 4.e, parapteron; 4.f, episternum; 4.g, metasternum; 4.h, epimerum; 4.s, postfurca.—This and Figs. 97 and 99 from Audouin, after Newport.

The apodemes.—The thorax is supported within by beam-like processes, or apodemes, which pass inward and also form attachments for the muscles. Those passing up from the sternum form the entothorax of Audouin, and the process of each thoracic segment is called respectively the antefurca, medifurca, and postfurca. In the Orthoptera (Caloptenus and Anabrus), the antefurca is large, thin, flattened, directed forward, and bounds each side of the prothoracic ganglion. In the Coleoptera two plates (Fig. 97, 2.s) arise from the inside of the sternum and “form a collar or leave a circular hole between them for the passage of the nervous cord” (Newport). The medifurca is a pair of flat processes which diverge and bridge the commissure, while the postfurca is situated under the commissure. In beetles (Dyticus) Newport states that it is expanded into two broad plates, to which the muscles of the posterior legs are attached. Graber also notices in the mole cricket between the apodemes of the meso- and metathorax, a flattened spine (Fig. 98, do) with two perforations through which pass the commissures connecting the ganglia. Besides these processes there are large, thin, longitudinal partitions passing down from the tergum (or dorsum), called phragmas; they are most developed in those insects which fly best, i.e. in Coleoptera (Figs. 97–101), Lepidoptera, Diptera, and Hymenoptera, none being developed in the prothorax. (The term phragma has also been applied to a partition formed by the inflexed hinder edge of this segment, and is present only in those insects in which the prothorax is movable.—Century Dictionary.) All these ingrowths 94may be in general termed apodemes. There are similar structures in Crustacea and also in Limulus; but Sharp restricts this term to minute projections in beetles (Goliathus) situated at the sides of the thorax near the wings. (Insecta, p. 103, Fig. 57.) The internal processes arising from the sternal region have been called endosternites.

Fig. 101.—Internal skeleton of Lucanus cervus, ♂, head: A, antenna; f, mandible; d, mentum; 2, 4, tendons of mandible; f, u, t, parts of the tentorium; 3 e, labial muscles. Thorax: 2, prothorax; 3, 4, meso- and metathorax fused solidly together; 3 r, acetabulum of prothorax, into which the coxa is inserted; 2 s, sternum; 3t, acetabulum of mesothorax, 4r, of metathorax; 3 s, mesothoracic sternum fused with that of the metathorax (4g); 4 s, apodeme.—After Newport.

The acetabula.—These are the cavities in which the legs are inserted. They are situated on each side of the posterior part of the sternum, in each of the thoracic segments. They are, in general, formed by an approximation of the sternum and epimerum, and sometimes, also, of the episternum, as in Dyticus (Fig. 97, A). This consolidation of parts, says Newport, gives an amazing increase of strength to the segments, and is one of the circumstances which enables the insect to exert an astonishing degree of muscular power.

95
Tabular View of the Segments, Pieces, and Appendages of the Thorax
 
Name of Segment Pieces (Sclerites) Appendages
1. Prothorax Pronotum, sometimes differentiated into  
Scutum 1st pair of legs
Scutellum Patagia
Episternum  
Epimerum  
Sternum  
Antefurca  
     
2. Mesothorax Præscutum  
Scutum 2d pair of legs
Scutellum 1st pair of wings
Proscutellum Tegulæ
Episternum Squamæ (Alulæ)
Epimerum Peritreme
Sternum  
Mesofurca  
Mesophragma  
Apodemes  
     
3. Metathorax Præscutum  
Scutum 3d pair of legs
Scutellum 2d pair of wings
Postscutellum (Halteres of Diptera)
Episternum  
Epimerum  
Sternum  
Postfurca  
Metaphragma  
Apodemes  
96

Fig. 102.—External anatomy of the trunk of Hydröus piceus: A, sternal—B, tergal aspect; 2, pronotum; 2 a, prosternum; 2 f, episternum; 3 a, præscutum; 3 b, scutum; 3 c, scutellum; 3 d, postscutellum; 3 g, mesosternum; 3 h, episternum; 3 f, epimerum; 3 i, crest of the mesosternum; 3 a, parapteron; 3 k, coxa; 4 a, metapræscutum; 4 b, metascutum; 4 c, metascutellum; 4 d, postscutellum; 4 e, tegula; 4 f, episternum; 4 h, epimerum; 4 g, metasternum; 4 i, crest of metasternum; 4 k and l, coxa; 4 m, trochanter; n, femur; o, tibia; p, tarsus; q, unguis; 7–11, abdominal segments.—After Newport.

b. The legs: their structure and functions

The mode of insertion of the legs to the thorax is seen in Figs. 90, 97, 101, and 103. They are articulated to the episternum, epimerum, and sternum, taken together, and consist of five segments. The basal segment or joint is the coxa, situated between the episternum and trochanter. The coxa usually has a posterior subdivision or projection, the trochantine; sometimes, as in Mantispa (Fig. 103), the trochantine is obsolete. We had previously supposed that the trochantine was a separate joint, but now doubt whether it represents a distinct segment of the leg, and regard it as only a subdivision of the coxa. It is attached to the epimerum, and is best developed in Panorpidæ, Trichoptera, and Lepidoptera. In the Thysanura the trochantine is wanting, and in the cockroach it merely forms a subdivision of the coxa, its use being to support the latter. The second segment is the trochanter, a more or less short spherical joint on which the leg proper turns; in the parasitic groups (Ichneumonidæ, etc., Fig. 104) it is usually divided into two pieces, though there are some exceptions. The trochanter is succeeded by the femur, tibia, and tarsus, the latter consisting of from one to five segments, the normal number being five. Tuffen West believed that the pulvillus is the homologue of an additional tarsal joint, “a sixth tarsal joint.” The last tarsal segment ends in a pair of freely movable claws (ungues), which are modified setæ; between the claws is a cushion-like 97pad or adhesive lobe, called the empodium or pulvillus (Fig. 105, also variously called arolium, palmula, plantula, onychium, its appendage being called paronychium and also pseudonychium). It is cleft or bilobate in many flies, but in Sargus trilobate. All these parts vary greatly in shape and relative size in insects of different groups, especially Trichoptera, Lepidoptera, Diptera, and Hymenoptera. In certain flies (e.g. Leptogaster) the empodium is wanting (Kolbe). By some writers the middle lobe is called the empodium and the two others pulvilli.

Fig. 103.—Side view of meso- and metathorax of Mantispa brunnea, showing the upper and lower divisions of the epimerum (s. em′, s. em″, i. em′, i. em″); s. epis, i. epis″, the same of the episternum.

Fig. 104.—Divided (ditrochous) trochanter of an ichneumon: cx, coxa; tr, the two divisions of the trochanter; f, femur.—After Sharp.

The fore legs are usually directed forward to drag the body along, while the middle and hind legs are directed outward and backward to push the body onwards. While arachnids walk on the tip ends of their feet, myriopods, Thysanura, and all larval insects walk on the ends of the claws, but insects generally, especially the adults, are, so to speak, plantigrade, since they walk on all the tarsal joints. In the aquatic forms the middle and hind tarsi are more or less flattened, oar-like, and edged with setæ. In leaping insects, as the locusts and grasshoppers, and certain chrysomelids, the hind femora are greatly swollen owing to the development of the muscles within. The tibia, besides bearing large, lateral, external spines, occasionally bears at the end one or more spines or spurs called calcaria. The fore tibia also in ants, etc., bear tactile hairs, and chordotonal organs, as well as other isolated sense-organs (Janet), and, in grasshoppers, ears.

In the Carabidæ the legs are provided with combs for cleaning the antennæ (Fig. 107), and in the bees and ants these cleansing organs are more specialized, the pectinated spine (calcar) being opposed by a tarsal comb (Fig. 106, d; for the wax-pincers of bees, see g). In general the insects use their more or less spiny legs for cleansing the head, antennæ, palpi, wings, etc., and the 98adaptations for that end are the bristles or spinules on the legs, especially the tibiæ.

Fig. 105.—Foot of honey-bee, with the pulvillus in use: A, under view of foot; t, t, 3d–5th tarsal joints; a n, unguis; f h, tactile hairs; p v, pulvillus; cr, curved rod. B, side view of foot. C, central part of sole; pd, pad; cr, curved rod; pv, pulvillus unopened.—After Cheshire.

Fig. 106.—Modifications of the legs of different bees. A, Apis: a, wax-pincer and outer view of hind leg; b, inner aspect of wax-pincer and leg, with the nine pollen-brushes or rows of hairs; c, compound hairs holding grains of pollen; d, anterior leg, showing antenna-cleaner; e, spur on tibia of middle leg. B, Melipona: f, peculiar group of spines at apex of tibia of hind leg; g, inner aspect of wax-pincer and first tarsal joint. C, Bombus: h, wax-pincer; i, inner view of the same and first tarsal joint, all enlarged.—From Insect Life, U. S. Div. Ent.

Osten Sacken states that among Diptera the aerial forms (Bombylidæ, etc.) with their large eyes or holoptic heads, which carry with them the power of hovering or poising, have weak legs, principally fit for alighting. On the other hand, the pedestrian or walking Diptera (Asilidæ, etc.) “use the legs not for alighting only, but for running, and all kinds of other work, seizing their prey, carrying 99it, climbing, digging, etc.; their legs are provided not only with spines and bristles, but with still other appendages, which may be useful, or only ornamental, as secondary sexual characters.”

Fig. 107.—End of tibia and tarsal joints of Anophthalmus; c, comb.

Tenent hairs.—Projecting from the lower surface of the empodium are the numerous “tenent hairs,” or holding hairs, which are modified glandular setæ swollen at the end and which give out a minute quantity of a clear adhesive fluid (Figs. 108, 109, 130, 134). In larval insects, and the adults of certain beetles, Coccidæ, Aphidæ, and Collembola, which have no empodium, there are one or more of these tenent hairs present. They enable the insect to adhere to smooth surfaces.

Fig. 108.—Transverse section through a tarsal joint of Telephorus, a beetle: ch, cuticula of the upper side; m, its matrix; ch′, the sole; m′, its matrix; h, adhesive hair; h′, tactile hair, supplied with a nerve (n′), and arising from a main nerve (n); n″, ganglion of a tactile hair; t, section of main trachea, from which arises a branch (t′); dr, glands which open into the adhesive hairs, and form the sticky secretion; e, chitinous thickening; s, sinew; b, membrane dividing the hollow space of the tarsal joint into compartments. See p. 111.—After Dewitz.

Striking sexual secondary characters appear in the fore legs of the male Hydrophilus, the insect, as Tuffen West observes, walking on the end of the tibia alone and dragging the tarsus after it. The last tarsal joint is enlarged into the form of an irregular hollow shield. The most completely suctorial feet of insects are those of the anterior pair of Dyticus (Fig. 132). The under side of the three basal joints is fused together and enlarged into a single broad and nearly circular shield, which is convex above and fringed with fine branching hairs, and covered beneath with suckers, of which two are exceptionally large; by this apparatus of 100suckers the male is enabled to adhere to the back of its mate during copulation. The line branching hairs around the edge prevent the water from penetrating and thus destroying the vacuum, “while if the female struggle out of the water, by retaining the fluid for some time around the sucker, they will in like manner under these altered conditions equally tend to preserve the effectual contact.” (Tuffen West.)

Fig. 109.—Cross-section through tarsus of a locust: ch, cuticula of upper side,—ch′, ch″, ch‴, of sole; ch, tubulated layer; ch″, lamellate layer; ch‴, inner projections of ch″. Other lettering as in Fig. 101. See p. 113.—After Dewitz.

In the saw-flies (Uroceridæ and Tenthredinidæ) and other insects, there are small membranous oval cushions (arolia, Figs. 109 and 131) beneath each or nearly each tarsal joint.

The triunguline larvæ of the Meloidæ are so called from apparently having three ungues, but in reality there is only a single claw, with a claw-like bristle on each side.

Why do insects have but six legs?—Embryology shows that the ancestors of insects were polypodous, and the question arises to what cause is due the process of elimination of legs in the ancestors of existing insects, so that at present there are no functional legs on the abdomen, these being invariably restricted (except in caterpillars) to the thorax, and the number never being more than six. It is evident that the number of six legs was fixed by heredity in the Thysanura, before the appearance of winged insects. We had thought that this restriction of legs to the thorax was in part due to the fact that this is the centre of gravity, and also because abdominal legs are not necessary in locomotion, since the fore legs are used in dragging the insect forwards, while the two hinder pairs support and push the body on. Synchronously with this elimination by disuse of the abdominal legs, the body became shortened, and subdivided into three regions. On the other hand, as in caterpillars, with their long bodies, the abdominal legs of the embryo persist; or if it be granted that the prop-legs are secondary structures, then they were developed in larval life to prop up and move the abdominal region.

The constancy of the number of six legs is explained by Dahl as being in relation to their function as climbing organs. One leg, he says, will almost always be perpendicular to the plane when the animal is moving up a vertical surface; and, on the other hand, we know that three is the smallest number with which stable equilibrium is possible; an insect must therefore have twice this number, and the great numerical superiority of the class may be associated with this mechanical advantage. (This numerical superiority of insects, however, seems to us to be rather due to the acquisition of wings, as we have already stated on pages 2 and 120.)

101Loss of limbs by disuse.—Not only are one or both claws of a single pair, or those of all the feet atrophied by disuse, but this process of reduction may extend to the entire limb.

In a few insects one of the claws of each foot is atrophied, as in the feet of the Pediculidæ, of many Mallophaga, all of the Coccidæ, in Bittacus, Hybusa (Orthoptera), several beetles of the family Pselaphidæ, and a weevil (Brachybamus). Hoplia, etc., bear but a single claw on the hind feet, while the allied Gymnoloma has only a single claw on all the feet. Cybister has in general a single immovable claw on the hind feet, but Cybister scutellaris has, according to Sharp, on the same feet an outer small and movable claw. In the water bugs, Belostoma, etc., the fore feet end in a single claw, while in others (Corisa) both claws are wanting on the fore feet. Corisa also has no claws on the hind feet; Notonecta has two claws on the anterior four feet, but none on the hind pair. In Diplonychus, however, there are two small claws present. (Kolbe.)

Fig. 110.—Last tarsal joint of Melolontha vulgaris, drawn as if transparent to show the inner mechanism: un, claws; str, extensor plate; s, tendon of the flexor muscle; vb, elastic membrane between the extensor plate and the sliding surface u; krh, process of the ungual joint; emp, extensor spine, and th, its two tactile hairs.—After Ockler, from Kolbe.

Among the Scarabæidæ, the individuals of both sexes of the fossorial genus Ateuchus (A. sacer) and eight other genera, among them Deltochilum gibbosum of the United States, have no tarsi on the anterior feet in either sex. The American genera Phanæus (Fig. 111), Gromphas, and Streblopus have no tarsal joints in the male, but they are present in the female, though much reduced in size, and also wanting, Kolbe states, in many species of Phanæus. The peculiar genus Stenosternus not only lacks the anterior feet, but also those of the second and third pair of legs are each reduced to a vestige in the shape of a simple, spur-like, clawless joint. The ungual joint is wanting in the weevil Anoplus, and becomes small and not easily seen in four other genera.

Ryder states that the evidence that the absence of fore tarsi in Ateuchus is due to the inheritance of their loss by mutilation is uncertain. Dr. Horn suggests that cases like Ateuchus and Deltochilum, etc., “might be used as an evidence of the persistence of a character gradually acquired through repeated mutilation, that is, a loss of the tarsus by the digging which these insects perform.” On the other hand, the numerous species of Phanæus do quite as much digging, and the anterior tarsi of the male only are wanting. “It is true,” he adds, “that many females are seen which have lost their anterior tarsi by digging; have, in fact, worn them off; but in recently developed specimens the 102front tarsi are always absent in the males and present in the females. If repeated mutilation has resulted in the entire disappearance of the tarsi in one fossorial insect, it is reasonable to infer that the same results should follow in a related insect in both sexes, if at all, and not in the male only. It is evident that some other cause than inherited mutilation must be sought for to explain the loss of the tarsi in these insects.” (Proc. Amer. Phil. Soc., Philadelphia, 1889, pp. 529, 542.)

Fig. 111.—Fore tibia of Phanæus carnifex, ♂, showing no trace of the tarsus.

Fig. 112.—Fore leg of the mole-cricket: A, outer, B, inner, aspect; e, ear-slit.—After Sharp.

The loss of tarsi may be due to disuse rather than to the inheritance of mutilations. Judging by the enlarged fore tibiæ, which seem admirably adapted for digging, it would appear as if tarsi, even more or less reduced, would be in the way, and thus would be useless to the beetles in digging. Careful observations on the habits of these beetles might throw light on this point. It may be added that the fore tarsi in the more fossorial Carabidæ, such as Clivina and Scarites, as well as those of the larva of Cicada and those of the mole crickets (Fig. 112), are more or less reduced; there is a hypertrophy of the tibiæ and their spines. The shape of the tibia in these insects, which are flattened with several broad triangular spines, bears a strong resemblance to the nails or claws of the fossorial limbs of those mammals which dig in hard soil, such as the armadillo, manis, aardvark, and Echidna. The principle of modification by disuse is well illustrated in the following cases.

In many butterflies the fore legs are small and shortened, and of little use, and held pressed against the breast. In the Lycænidæ the fore tarsi are without claws; in Erycinidæ and Libytheidæ the fore legs of the males are shortened, but completely developed in the females, while in the Nymphalidæ the fore legs in both sexes are shortened, consisting in the males of one or two joints, the claws being absent in the females. Among moths loss of the fore tarsi is less frequent. J. B. Smith[21] notices the lack of the fore tarsi in the male of a deltoid, Litognatha nubilifasciata (Fig. 113), while the hind feet of 103Hepialus hectus are shortened. In an aphid (Mastopoda pteridis, Esl.) all the tarsi are reduced to a single vestigial joint (Fig. 114).

Fig. 113.—Leg of Litognatha: cx, coxa; f, femur; t, tibia; ep, its epiphysis, and sh, its shield-like process. The tarsus entirely wanting.—After Smith.

Entirely legless adult insects are rare, and the loss is clearly seen to be an adaptation due to disuse; such are the females of the Psychidæ, the females of several genera of Coccidæ (Mytilaspis, etc.), and the females of the Stylopidæ.

Apodous larval insects are common, and the loss of legs is plainly seen to be a secondary adaptive feature, since there are annectant forms with one or two pairs of thoracic legs. All dipterous and siphonapterous larvæ, those of all the Hymenoptera except the saw-flies, a few lepidopterous larvæ, some coleopterous, as those of the Rhyncophora, Buprestidæ, Eucnemidæ, and other families, and many Cerambycidæ are without any legs. In Eupsalis minuta, belonging to the Brenthidæ, the thoracic legs are minute.

The legs of larvæ end in a single claw, upon the tips of which the insect stands in walking.

c. Locomotion (walking, climbing, and swimming)

Mechanics of walking.—To Graber we owe the best exposition of the mechanics of walking in insects.

“The first segment of the insect leg,” he says, “upon which the weight of the body rests first of all, is the coxa. Its method of articulation is very different from that of the other joints. The enarthrosis affords the most extensive play, particularly in the Hymenoptera and Diptera.”

In the former the development of their social conditions is very closely connected with the freest possible use of the legs, which serve as hands. In the beetles, however, which are very compactly built, there exists a solid articulation whereby the entire hip rests in a tent-like excavation of the thorax, and can only be turned round a single axis, as may be seen in Fig. 115, where c represents the imaginary revolving axis and d the coxa. In the case we are supposing, therefore, only a backward and forward movement of the coxa is possible, the extent of the play of which depends on the size of the coxal pan, as well as certain groin or bar-like structures which limit further rotation. In the very dissimilar arrangement which draws in the fore, middle, and hind legs toward the body it is self-evident that their extent of action is also different. This arrangement seems to be most yielding on the fore legs, where the hips, to confine ourselves to the stag-beetles, can be turned backward and forward 60° from 104the middle or normal position, and therefore describe on the whole a curve of 120°. The angle of turning on the middle leg hardly exceeds a legitimate limit, yet a forward as well as a backward rotation takes place. The former is entirely wanting in the hind hips; they can only be moved backward.

Fig. 114.—Leg of an Aphid, with the tarsus (t) much reduced: 1, 2, 3, legs of 1st, 2d, and 3d pairs.

The number and strength of the muscles on which the rotation of the hips depends, correspond with these varying movements of the individual legs. Thus, according to Straus Durckheim, the fore coxa of many beetles possesses five separate muscles and four forward and one backward roll; the middle coxa a like number of muscles but only two forward rolls, while the hind hips succeed in accomplishing each of the motions named with a single muscle.

One can best see how these muscles undertake their work, and above all how they are situated, if he lays bare the prothorax of the stag beetle (Fig. 116). Here may be seen first the thick muscle which turns to the front the rotating axis in its cylindrical pan, and thus helps to extend the leg, while two other tendons, which take the opposite direction, are fitted for reflex movements.

Fig. 115.—Mechanics of an insect’s leg: d, coxa,—c, axis of revolution; a and b, the coxal muscles; e, trochanter muscle (elevator of the femur); f, extensor,—g, flexor, of the tibia (pn); n, tibial spine; h, flexor.—i, extensor, of the foot; k, extensor,—l, flexor, of the claw; po, place of flexure of the tibia; p1q, leg after being turned back by the coxa.—p1r, by the simultaneous flexure of the tibia. The resulting motion of the end of the tibia, through the simultaneous movement (no) and revolution (nq), indicates the curve nr.—After Graber.

In Fig. 115 the muscles mentioned above, and their modes of working, may be distinguished by the arrows a and b.

In order to simplify matters, we will imagine the second component part 105of the normal insect leg, i.e. the trochanter (Figs. 116, 117, r), as grown together with the third lever, i.e. the femur, as the movement of both parts mostly takes place uniformly.

Fig. 116.—Section of the fore leg of a stag-beetle, showing the muscles: S, extensor,—B, flexor, of the leg; s, extensor,—b, flexor, of the femur; o, femur; u, tibia; f, tarsus; k, claw; 109, s, extensor,—b, flexor, of the femoro-tibial joint, both enlarged.—After Graber.

The pulling of the small trochanter muscle works against the weight of the body when this is carried over on to the trochanter by means of the coxa, as seen at the arrow e in Fig. 115. It may be designated as the femoral lever.

The plane of direction in which the femur, as seen by the rotation just mentioned, is moved, exactly coincides in insects with that of the tibia and the foot, while all can be simultaneously raised or dropped, or, as the case may be, stretched out or retracted. Therein, therefore, lies an essential difference from the fully developed extremities of vertebrates among which, even on the lever arms which are stationary at the end, an extensive turning is possible.

The muscles which move the tibia, and indirectly the femur, also consist of an extensor muscle which is situated in the upper side of the femur (Fig. 116, s, Fig. 115, f), and of a flexor (Fig. 116, b, Fig. 115, g), which lies under the former.

The stilt-like spines on the point (Figs. 115 and 118, L3n) on which this segment is directly supported are important parts of the tibia. (Graber.)

Fig. 117.—Left fore leg of a cerambycid beetle: h, coxa; r, trochanter; o, femur; u, tibia; f, tarsus; k, claw.—After Graber.

Considering the respective positions of the individual levers of the leg and the nature of the materials of which they are made, the legs of insects may be likened, as Graber states, to elastic bows, which, when pressed down together from above, their own indwelling elasticity is able to raise again and thus keep the body upright.

This is very plainly shown in certain stilt-legged bark-beetles, in which, as in a rubber doll, as soon as the body is pressed down on the ground, the organs of motion extend again without the intervention of muscles; indeed this experiment succeeds even with dead, but not yet wholly stiff, insects.

Graber then turns to the analysis of the movements of insect legs when in motion, 106and the mode of walking of these insects in general. This subject had been but slightly investigated until Graber made a series of observations and experiments, of which we can give only the most important results.

The locomotion of insects is an extremely complicated subject.

Let us consider, Graber says, first, a running or carabid beetle, when walking merely with the fore and hind legs. The former will be bent forward and the latter backward.

“Let us begin with the left fore leg (Fig. 118, L1). Let the same be extended and fixed on the ground by means of its sharp claws and its pointed heel. Now what happens when the tibial flexors draw together? As the foot, and therefore the tibia also, have a firm position, then the contraction of the muscles named must cause the femur to approach the tibia, whereby the whole body is drawn along with it. This individual act of motion may be well studied in grasshoppers when they are climbing on a twig by stretching out their long fore leg directly forward, and then drawing up the body through the shortening of the tibial flexors until the middle leg also reaches the branch.

“But while the fore legs advance the body by drawing the free lever to the fixed leg-segment, the hind legs do this in exactly the opposite way. The hind leg, namely, seeks to stretch out the tibia, and thus to increase the angle of the knee (R3), thereby giving a push on the ground, by means of which the body is shoved forward a bit.

“Though it might be supposed that the feet would remain stationary during the extension or retraction of the limbs, this never occurs in actual walking. Not merely the upper, but also the lower, thigh is either drawn in or stretched out, as the case may be. The latter then describes a straight line with its point during this scraping or scratching motion (Fig. 115, no), which is obviously the chord to that quadrant which would be drawn by the tibia or foot in a yielding medium, as water, for instance. But even this motion results extremely rarely, and never in actual walking. If we fix our eye anew upon the fore leg at the very moment when it is again retracted, after the resultant ‘fixing,’ we shall then observe that the hip also is simultaneously turned backward in a definite angle. The tibia would describe the arc nq (Fig. 115) by means of the latter alone.

“This plane, in conjunction with the rectilinear ‘movement’ (no) obtained by the retraction of the tibia, produces a path (nr), and this is what is actually described by a painted foot upon a properly prepared surface, as a sheet of paper;[22] supposing, however, that the body in the meantime is not moved forward by other forces. In the last case, and this indeed always takes place in running, the trunk is moved a bit forward, together with the leg which is just describing its curve with a rapidity corresponding to the momentum obtained; the result of this is that the curve of the foot from its beginning (n) to its end (a) bends round close to itself, just as a man who, when on board a ship in motion, walks across it diagonally, and yet on the whole moves forward, because his line of march, uniting with that of the ship, results in a change of position in space.

107“The case is the same in the middle and hind legs, which must make a double course also, yet in such a way that the straight line is drawn, not during the retraction, but during the extension; during which, however, quite as in the fore leg, the members mentioned (R3) gradually approach the body.

“When the legs have reached the maximum of their retraction, or of their extension, as the case may be, and therefore the end of their active course for that time, then begins the opposite or backward movement; that is, the fore legs are again extended, while their levers draw the remaining legs together again.

Fig. 118.—A Carabus beetle in the act of walking or running: three legs (L1, R2, L3) are directed forward, while the others (R1, L2, R3), which are directed backward toward the tail, have ended their activity; ab, cd, and ef are curves described by the end of the tibiæ, and passing back to the end of the body; bh, di, and fg are curves described by the same legs during their passive change of position.—After Graber.

“At the same time, as we may see by the uniting leg, the limb is either a little raised, that there may be no unnecessary friction, or it remains during the passive step also, with its means of locomotion in slight contact with the ground.

“The curve of two steps, as inscribed by the end of the tibia of the left fore leg of a stag-beetle, affords an instructive summary of the conditions of which we have been speaking (Fig. 121, B). We see two curves. The thick one (ab), directed toward the axis of the body, corresponds to the effective act of a single 108walking function, which brings the body a bit forward; the thinner, on the other hand, or we might say the hair line (bc), which, however, is but rarely made quite clearly, is produced by the ineffectual backward movement, by which the insect again approaches its working posture (c). It is at first placed at some distance from the body, in order that (like c also) it may draw near to the body again; but in such a way, naturally, that it coincides with the starting-point of the following active curve (cd). It is evident that even the passive curve is not the imprint of the movement accomplished exclusively by the leg, for this latter, while struggling to reach its resting-place, is really involuntarily carried forward with the rest of the body.

“The scroll-like lines drawn by the swimming beetle (Dyticus), with the large, sharp points of its hind tibia, are also very instructive (Fig. 119, A).

Fig. 119.A, trail curves described by the tibial spines of the right and left hind limb of Dyticus. B, the same made by the right hind leg (r3) alone. Natural size.—After Graber.

Fig. 120.—The same by the two hind legs of Melolontha: a, the active and thickened section of the curve. Natural size.

Fig. 121.A, track curves of two of the tibial spines of the left, middle legs of a stag-beetle. Natural size. B, the same enlarged; fg, the longitudinal axis of the trunk; cd and ab, the active curve passing inward,—bc and de, the passive going outward. C, two curves described by the left hind legs; in this case, the curves are not inwards or backwards, but partly directly inward (b), and in part obliquely forwards (a).

“The diversions and modifications in the course of the active step, as furnished by the moving factor of the remaining legs, are already clearly illustrated by the curves shown by the joints of the hind tibia of a May-beetle (Fig. 120) and a stag-beetle (Fig. 121, c). The actual faint line in this case does not run from the front toward the back, as would correspond to the active leg-motion, but either directly inward (Fig. 121, cb), or even somewhat to the front. In the May-beetles, and even more in the running garden-beetle, the curves of the hind legs present themselves as screw-like lines (Fig. 122, l3), while the scrawling of the remaining members (l1, l2) is much simpler.

“Inasmuch as we now have a cursory knowledge of the movements made by each individual leg for itself,—movements, however, which plainly occur very differently according to the structure of these appendages,—the question now is of the combined play, the total effect of all the legs taken together, and therefore of the walk and measure of the united work of the foot.

“In opposition to the caterpillars and many other crawling animals which extend their legs in pairs and really swing them by the worm-like mode of contraction of the dermo-muscular tube, the legs of fully grown insects are moved in the contrary direction and in no sense in pairs, but alternately—or, more strictly speaking, in a diagonal direction.

109“For an examination of the gait of insects, we choose, for obvious reasons, those which have very long legs and which at the same time are slow walkers.

“Insects may be called ‘double-three-footed,’ from the manner in which they alternately place their legs. There are always three legs set in motion at the same time, or nearly so, while in the meantime the remaining legs support the body, after which they change places.

Fig. 122.—The same by the left fore (l1), middle (l2), and hind, leg (l3) of a Carabus. Natural size.

Fig. 123.-Tracks of a Blaps mortisaga marked by the differently painted tibial points: ●, tracks of fore, —○, middle, —/, hind leg. Natural size.

Fig. 124.—Tracks of Necrophorus vespilio. Natural size.

“To be more exact, it is usually thus: At first (Fig. 118) the left fore leg (L1) steps out, then follows the right middle leg (R2), and the left hind leg (L3). Then while the left fore leg begins to retract and thus make the backward movement, the right fore leg is extended, whereupon the left middle leg and the right hind leg are raised in the same order as the first three feet.”

Graber[23] painted the feet of beetles and let them run over paper, and goes on to say:

“Let us first pursue the tracks of the Blaps, for example (Fig. 123). Let the insect begin its motion. The left fore leg stands at a, the right middle leg at β, and the left hind leg at c. The corresponding number of the other set of three feet at α, b, γ. At the first step the three feet first mentioned advance to a′β′c′, the second set on the other hand to α′b′γ′. Thereby the tracks made by the successive steps fall quite, or almost quite, on each other, as appear also in the tracks of a burying beetle (Fig. 124).

“As the fore legs are directed forward and the hind legs backward, while the middle legs are placed obliquely, the reason of the more marked impressions of the latter is evident.

“The highest testimony to the precise exactitude and accuracy of the walking mechanism of insects is furnished by the fact that in most insects, and particularly in those most fleet of foot, which, whether they are running away or chasing their prey, must be able to rely entirely upon their means of locomotion;—the fact, we say, that whether they desire to move slowly or more 110quickly, the distances of the steps, measured by the length as well as by the cross-direction, hardly differ a hair’s breadth from one another, and this is also the case when the tarsi are cut off and the insects are obliged to run on the points of their heels (tibiæ).

“Thence, inasmuch as the trunk of insects is carried by two legs and by one on each side alternately, it may surely be concluded a priori that when walking it is inclined now to the right and now to the left, and that the track, too, which is left behind by a precise point of the leg, can in no wise be a straight line; and in reality this is not the case.

“A plainly marked regular curve, which approaches a sinuous line, as seen in Fig. 125, is often obtained by painting many insects, for example Trichodes, Meloë, etc., which, when running, either bring the end of their hind body near to the ground or into contact with it.

Fig. 125.—Tracks of Trichodes; the middle sinuous line is made by the tip of the abdomen. Natural size.

Fig. 126.—Tracks of another insect which, in running, can only use three legs (r1, l4, r3) which become indicated differently from normal conditions. Natural size.

Fig. 127.—The same of an insect crossing over a surface inclined 30° from the horizon, whereby the placing of the feet becomes changed. Natural size.—This and Figs. 120–126 after Graber.

“The locomotive machine of insects may be called, to a certain extent, a double set of three feet each, as most insects, and particularly those provided with a broad trunk, are able to balance themselves with one of these two sets of feet, and indeed when walking, as well as when standing still, can move about even better with one set of these feet than with four legs. In the latter case, that is, if one cuts off a pair of legs from an insect, the trunk can balance itself only with extreme difficulty, and there is therefore little prospect that insects will ever become four-footed.

“But if one compels insects to run on three legs, he will thus make the interesting discovery that to make up the deficiency they place the remaining feet and bring them to the ground somewhat differently than when the second set of feet is active. Figs. 124 and 126 may be compared for this purpose. The former shows the footprints of a burying beetle running with all six legs, the latter the track of the same insect, which, however, has at its disposal only the right fore leg, the left middle leg, and the right hind leg. One may plainly see here that the track of the hind leg on the right side (r3) approaches the track of the middle leg on the left side, and then further, that the right fore leg (r1) steps out more to the right to make up for the deficiency of the middle leg.

111“A similar adaptation of the position of the legs, which is entirely dependent on the choice of the insect, may also be observed there, if one compels insects which are not provided with corresponding adhesive lobes to run away over crooked surfaces. Fig. 123 shows the footprints of a Blaps when running upon a horizontal plane. Fig. 127, on the contrary, shows the tracks of the legs when going diagonally over a gradually inclined surface. Here, also, the insect holds on with his fore and middle legs (r1, r2) stretched upward, whereby also the impressions on both sides come to lie farther apart than in the normal mode of walking.

“It will not surprise the reader who is familiar with the gait of crabs, to hear that many insects also understand the laudable art of going backward, wherein the hind legs simply change places with the fore legs.

“The jumping motion of insects may be best studied in grasshoppers. When these insects are preparing for a jump, they stretch out the upper thigh horizontally, clap the tibiæ together, and also retract the foot-segment. After a slight pause for rest, during which they are getting ready for the jump, they then jerk the tibiæ suddenly backward and against the ground with all their strength by means of the extensor muscles.”

The correctness of Graber’s views has been confirmed by Marey by instantaneous photographs (Figs. 128, 129).

Locomotion on smooth surfaces.—How flies and other insects are able to walk up, or run with the body inverted, on hard surfaces has been lately discovered by Dewitz, Dahl, and others. All authors are agreed that this power is due to the presence of the specialized empodium of each tarsus.

Dewitz confirmed the opinion of Blackwell, that a glutinous liquid is exuded from the apices of the tenent hairs which fringe the empodium. By fastening insects feet uppermost on the under side of a covering glass which projects from a glass slide, the hairs which clothe the empodia of the foot of a fly (Musca erythrocephala) may be seen to be tipped with drops of transparent liquid. On the leg being drawn back from the glass, a transparent thread is drawn out, and drops are found to be left on the glass. In cases where these hairs are wanting, as in the Hemiptera, the adhesive fluid exudes directly from pores in the foot. In the beetles (Telephorus dispar) and other insects the tenent hairs on the foot end in sharp points, below which are placed the openings of the canals. The glands, Dewitz states, are chiefly flask-shaped and unicellular, situated in the hypodermis of the chitinous coat; each gland opening into one of the hairs (Fig. 108); they are each invested by a structureless tunica propria, and contain granular protoplasm, a nucleus placed at the inner side, and a vesicle, prolonged into a tube which, traversing the neck of the gland, is attached to the root of the hair; the vesicle receiving the secretion. Each gland is connected with a fine nerve-twig, and secretion is probably voluntary. Among the tenent 112hairs of the empodium are others which must be supplied with a nerve, forming tactile hairs, as they each proceed from a unicellular ganglion (Fig. 108, n″). The secretion is forced out of the gland by the contraction of the protoplasm, Dewitz having seen the secretion driven out from the internal vesicle into its neck.

Fig. 128.—The walk of an orthopterous insect: series to be followed from right to left.—After Marey.

Fig. 129.—Beetle walking: series to be followed from left to right.—After Marey.

113

Fig. 130.—A, end of an adhesive hair of a weevil (Eupolus): i′, canal: i‴, its external opening at the end of the hair. B, end of a similar hair of Telephorus with drops of the secretion.—After Dewitz.

In the spherical last tarsal joint of Orthoptera (Fig. 109), which is without these tenent hairs, nearly all the cells of the hypodermis are converted into unicellular glands, each of which sends out a long, fine, chitinous tubule, which is connected with its fellows by very fine hairs and is continuous with the chitinous coat of the foot and opens through it. The sole of the foot is elastic and adapts itself to minute inequalities of surfaces, while the anterior of each tarsal joint is almost entirely occupied by an enlargement of the trachea, which acts on the elastic sole like an air chamber, rendering it tense and at the same time pliant. Dewitz adds that the apparatus situated on the front legs of the male of Stenobothrus sibiricus (Fig. 131) must have the function of causing the legs to adhere closely to the female by the excretion of an adhesive material. The hairs of the anterior tarsi of male Carabi also appear to possess the power of adhesion. In the house-fly the empodia seem to be only called into action when the insect has to walk on vertical smooth surfaces, as at other times they hang loosely down.

Burmeister observed the use of a glutinous secretion for walking in dipterous larvæ, and Dewitz found that the larva of a Musca used for this purpose a liquid ejected from the mouth. The larvæ of another fly (Leucopis puncticornis) perform their loop-like walk by emitting a fluid from both mouth and anus. A Cecidomyia larva is able to leap by fixing its anterior end by means of an adhesive fluid. The larva of the leaf-beetle, Galeruca, moves by drawing up its hinder end, fixing it thus, and carrying the anterior part of the body forward with its feet until fully extended, when it breaks the glutinous adhesion. The abdominal legs of some saw-fly larvæ have the same power.

Dahl could not detect in the foot of the hornet (Vespa crabro) any space which could be considered as a vacuum.

Fig. 131.Stenobothrus sibiricus pairing: A. the ♂, fore tarsus (t) greatly enlarged; ar, arolia; p, pulvillus.—After Pagenstecher.

Simmermacher states that in most cases of climbing beetles the tubular tenent hairs pour out a secretion (Figs. 133, 134), “and it is probable that we have here to do with the phenomena not of actual attachment by, as it were, gluing, but of adhesion; the orifice of the tubes is divided obliquely, and the tubes are, at this point, extremely delicate and flexible, so as to adhere by their lower surface; 114in this adhesion they are aided by the secreted fluid.” In the case of the Diptera he does not accept the theory by which the movement of the fly along smooth surfaces is ascribed to an alternate fixation and separation, but believes in a process of adhesion, aided by a secretion, as in many Coleoptera. (In the Cerambycidæ there is no secretion, and the tubules are merely sucking organs, like those observed in the male Silphidæ.) “The attaching lobes, closely beset with chitinous hairs, are enabled, in consequence of the pressure of the foot, to completely lie along any smooth surface; this expels the air beneath the lobes, which are then acted on by the pressure of the outer air.” (Journ. Roy. Micr. Soc., 1884, p. 736.) Another writer (Rombouts) thinks this power is due to capillary adhesion.

Fig. 132.—Fore leg of ♂ Dyticus, under side, with sucker, formed of 3 enlarged tarsal joints: with a small cupule highly magnified. × 120.—After Miall.

The action of the pulvillus and claws when at rest or in use by the honey-bee is well shown by Cheshire (Fig. 135, B). In ascending a rough surface, “the points of the claws catch (as at B) and the pulvillus is saved from any contact, but if the surface be smooth, so that the claws get no grip, they slide back and are drawn beneath the foot (as at A), which change of position applies the pulvillus, so that it immediately clings. It is the character of the surface, then, and not the will of the bee, that determines whether claw or pulvillus shall be used in sustaining it. But another contrivance, equally beautiful, remains to be noticed. The pulvillus is carried folded in the middle (as at C, Fig. 105), but opens out when applied to a surface; for it has at its upper part an elastic and curved rod (cr, Figs. 105 and 135), which straightens 115as the pulvillus is pressed down; C and D, Fig. 135, making this clear. The flattened-out pulvillus thus holds strongly while pulled, by the weight of the bee, along the surface, to which it adheres, but comes up at once if lifted and rolled off from its opposite sides, just as we should pull a wet postage stamp from an envelope. The bee, then, is held securely till it attempts to lift the leg, when it is freed at once; and, by this exquisite yet simple plan, it can fix and release each foot at least twenty times per second.” (Bees and Bee-keeping, p. 127.)

Fig. 133.—Cross-section through a tarsal joint of fore leg of Dyticus, ♂, showing the stalked chitinous suckers (s), with a marginal bristle on each side: t, trachea; a, an isolated tubule or sucker of Loricera,—b, of Chlænius,—c, of Cicindela; d, two views of one of Necrophorus germanicus, ♂.

Fig. 134.—Section through the tarsus of a Staphylinid beetle; the glandular or tenent hairs arising from chitinous processes. A, section through the tarsal joint of the pine weevil, Hylobius abietis, showing the crowded, bulbous, glandular, or tenent hairs arising from unicellular glands.—This and Fig. 133 after Simmermacher.

Ockler divides the normal two-clawed foot into three subtypes: 116(1) with an unpaired median empodium; (2) with two outer lateral adhesive lobes; (3) with two adhesive lobes below the claws; the latter is the chief type and forms either a climbing or a clasping foot. The amount of movement possessed by the claws is limited, and what there is, is effected by means of an elastic membrane and the extensor plate (Fig. 110). The “extensor sole” which is always present in insects with an unpaired median fixing or adhesive organ (empodium) is to be regarded as a modification of the extensor seta. The extensor plate is peculiar to an insect’s foot. Ockler states that the so-called “pressure plate” of Dahl is only a movably articulated, skeletal, supporting plate for the median fixing lobule.

Fig. 135.—Honey-bee’s foot in the act of climbing, showing the automatic action of the pulvillus, × 30: A, position of foot in climbing on a slippery surface, or glass; pv, pulvillus; fh, tactile hairs; un, unguis; t, last tarsal joint. B, position of foot in climbing rough surface. C, section of pulvillus just touching flat surface; cr, curved rod. D, the same applied to the surface.—After Cheshire.

Climbing.—In certain respects the power of climbing supplies the want of wings, and even exists often in house-flies among which there is shown a many-sided motion that is quite unheard of in other groups of insects.

The best climbers are obviously those insects which live on trees and bushes, as, for example, longicorn beetles and grasshoppers. These may be accurately called the monkeys of the insect kind, even if their movements take place less gracefully, and indeed rather stiffly and woodenly. We already know what are the proper climbing organs; that is, the sharp easily movable claws on the foot. With the help of these claws certain insects, May-beetles for example, can hang upon one another like a chain; indeed, bees and ants in this manner bind themselves together into living garlands and bridges. There are still added to the chitinous hooks flaps and balls of a sticky nature, by help of which likewise the insects glue themselves together. To facilitate the spanning of still thicker twigs, the climbing foot of insects has a greater movability even than when it only serves as a sole. (Graber.)

The mode of swimming of insects.—To study the swimming movements of insects, let us examine a Dyticus. It will appear, as Graber states, to be wonderfully adapted to its element.

117“The body resembles a boat. There is nowhere a projecting point or a sharp corner which would offer unnecessary resistance to motion; bulging out in the middle and pointed at the end, it cuts through the resistance of the water like a wedge. The movable parts, the oars, seem to be as well fitted for their purpose as the burden to be moved by them. That the hind legs must bear the brunt of this follows from their position exactly in the middle of the body, where it is widest. In other insects also these legs are used for the same purpose as soon as the insects are put in the water. But the swimming legs of water-beetles are oars of quite peculiar construction. They are not turned about in the coxæ, as are other legs, but at the foot-joint. The coxa, namely, has grown entirely together with the thoracic partition. The muscles we have mentioned, exceeding in strength all the soft parts taken together, take hold directly of the large wing-shaped tendons of the upper thigh, and extend and retract the leg in one of the planes lying close to the abdominal partition. The foot forms the oar, however. It is very much lengthened and still more widened, and can be turned and bent in by separate muscles in such a way that in the passive movement, that is, the retraction, the narrow edge is turned to the fore, and therefore to the medium to be dislodged; however, as soon as the active push is to be performed and the leg is extended with greater force, it cuts down through the water with its whole width. These effective oar-blades are still considerably enlarged by the hairs arising on the side of the foot, which spread out at the decisive moment.

“Every one knows that the oar-blades of swimming beetles always go up and down simultaneously and in regular time. On the other hand, as soon as one puts a Dyticus on the dry land, i.e. on an unyielding medium, it uses its hind legs entirely after the manner of other land insects; that is, they are drawn in and extended again alternately, as takes place clearly enough from the footsteps in Fig. 119, A. We learn from this that water insects have not yet, from want of practice, forgotten the mode of walking of land insects.

“The forcing up of the water as a propelling power is added to the repulsion produced by the strong strokes of the oars. If the beetle stood up horizontally in the water, he would be lifted up.

“As the trunk, however, assumes an oblique position when the insect wishes to swim, one can then imagine the driving up of the water as being divided into two forces, one of which drives the body forward in a horizontal direction, while the other, that is, the vertical component, is supplied by the moving of the oars. The swimming insect is thus, as it were, a snake flying in the water.

“The long streamer-like hind legs of many water-bugs, for example Notonecta, approach more nearly our artificial oars. These legs are turned out from the bottom.

“There is no doubt but that the legs of insects, as regards the many-sidedness and exactitude of their locomotive actions, place the similar contrivances of other animals far in the shade. We shall be forced to admire these ingenious levers still more, however, when we take into consideration their energy and strength. That the force with which the locomotive muscles of insects is drawn together is enormous compared with that of vertebrates, we may learn if we try to subdue the rhythmical movements of the thorax of a large butterfly by the pressure of our finger or to open against the insect’s will the closed jumping leg of a grasshopper, or the fossorial shovel of a mole-cricket.”

118

LITERATURE ON LEGS AND FEET

MacLeay, W. S. On the structure of the tarsus in the tetramerous Coleoptera of the French entomologists. (Trans. Linn. Soc. London, xv, 1825, pp. 63–73.)

Speyer, O. Untersuchung der Beine der Schmetterlinge. (Isis, 1843, pp. 161–207, 243–264.)

Pokorsky Joravko, A. von. Quelques remarques sur le dernier article du tarse des Hyménoptères. (Bull. Soc. imp. Natur. Moscou, 1844, xvii, pp. 140–159. Ref. in Isis, 1848, v, p. 347.)

Rossmassler, E. A. Das Bein der Insekten. (Aus der Heimath, 1860, 3 kap., pp. 327–334, Fig.)

West, Tuffen. The foot of the fly; its structure and action; elucidated by comparison with the feet of other insects, etc. Part I. (Trans. Linn. Soc. London, xxiii, 1861, pp. 393–421, 1 Pl.)

Sundevall, C. On insektenas extremiteter samt deras hufoud och munddelar. (Kongl. Vetenskaps Akad. Handlingar. iii, Nr. 9, 1861.)

Lindemann, C. Notizen zur Lehre vom ausseren Skelete der Insekten (Gelenke und Muskeln der Füsse). 1 Taf. (Bull. Soc. imp. d. Natur. Moscou, xxxvii, 1864, pp. 426–432.)

Liebe, O. Die Gelenke der Insekten. Chemnitz, 1873. 4º. 1 Taf.

Canestrini, J. Ueber ein sonderbares Organ der Hymenopteren. (Zool. Anzeiger, 1880, pp. 421, 422.)

Dahl, F. Beiträge zur Kenntnis des Baues und der Funktionen der Insektenbeine. (Archiv f. Naturgesch. 1 Jahrg., 1884, pp. 146–193, 3 Taf. Sep., 48 pp. Vorlauf. Mitteil, in Zool. Anz., 1884, pp. 38–41.)

Langer, K. Ueber den Gelenkbau bei den Arthrozoen. Vierter Beitrag zur vergleichenden Anatomie und Mechanik der Gelenke. (Denkschriften der Akad. d. Wissensch. Wien, xviii, Bd. Physikal.-mathem. Classe, pp. 99–140. 3 Taf.)

Graber, Vitus. Ueber die Mechanik des Insektenkörpers. (Biolog. Centralbl., iv, 1884, pp. 560–570.)

—— Die ausseren mechanischen Werkzeuge der Tiere, ii Teil. Wirbellose Tiere, 1886, pp. 175–182, 208–210.

Dewitz, H. Ueber die Fortbewegung der Tiere an senkrechten glatten Flächen vermittelst eines Sekretes. 3 Taf. (Pflüger’s Archiv f. d. ges. Physiologie, xxxiii, 1884, pp. 440–481.)

Ockler, A. Das Krallenglied am Insektenfuss. (Archiv f. Naturgesch., 1890, pp. 221–262, 2 Taf.)

LITERATURE OF LOCOMOTION (WALKING, ETC.)

Carlet, G. Sur le mode de locomotion des chenilles. (Compt, rend. Acad. Paris, 1888, cvii, pp. 131–134. Naturwiss. Rundschau, iii Jahrg., 1888, No. 42, p. 543.)

—— De la marche d’un insecte rendu tetrapode par la suppression d’une paire de pattes. (Ibid., pp. 565, 566.)

—— Sur la locomotion des insectes et des arachnides. (Ibid., 1879, T. 89, pp. 1124, 1125.)

—— Ueber den Gang eines vierfüssig gemachten Insekts. (Naturwiss. Rundschau, viii Jahrg., 1888, pp. 666–667; Compt. rend. 1888, cvii.)

119Demoor, J. Recherches sur la marche des insectes et des arachnides. Étude experimentale d’Anatomie et de Physiologie comparées. (Archiv de Biologie, Liège, 1880, 42 pp. 3 Pls.)

—— Ueber das Gehen der Arthropoden mit Berücksichtigung der Schwankungen des Körpers. (Compt. rend. Acad. d. Sc. Paris, 1890, cxi, pp. 839–840.)

Osten-Sacken, C. R. von. Ueber das Betragen des kalifornischen flügellosen Bittacus (apterus McLachl.). (Wiener Ent. Zeit., 1882, pp. 123.)

Dixon, H. H. Preliminary note on the walking of some of the Arthropoda. (Proc. R. Dublin Soc. vii, pp. 574–578, 1892. Also Nature, 1897.)

Also the works of Graber, Marey, Cheshire, etc.

LITERATURE OF WALKING ON SMOOTH SURFACES

Blackwell, J. Remarks on the pulvilli of insects. (Trans. Linn. Soc. London, xvi, 1831, pp. 487–492, 767–770.)

Lowne, B. T. On the so-called suckers of Dytiscus and the pulvilli of insects. (Trans. Roy. Micr. Soc., pp. 267–271, 1871, 1 Pl.)

West, Tuffen. On certain appendages to the feet of insects subservient to holding or climbing. (Journ. of the Proceed. Linn. Soc. London, Zoölogy, vi, 1862, pp. 26–88.)

Dewitz, H. Ueber die Fortbewegung der Tiere an senkrechten, glatten Flächen vermittelst eines Sekrets. (Pflüger’s Archiv f. d. ges. Physiologie, xxxiii, 1884, pp. 440–481. 3 Taf. Also Zool. Anzeiger, 1884, pp. 400–405.)

—— Wie ist es den Stubenfliegen und anderen Insekten möglich, an senkrechten Glaswanden emporzulaufen. (Sitzungsb. Ges. naturf. Freunde zu Berlin, 1882, pp. 5–7.)

—— Weitere Mitteilungen über den Klettern der Insekten (Ibid., 1882, pp. 109–113).

—— Die Befestigung durch einen klebenden schleim beim springen gegen senkrechte Flächen. (Zool. Anzeiger, 1883, pp. 273, 274.)

—— Ueber die Wirkung der Haftlappchen toter Fliegen. (Ent. Nachr., x Jahrg., 1884, pp. 286, 287.)

—— Weitere Mitteilungen über das Klettern der Insekten an glatten senkrechten Flächen. (Zoolog. Anzeiger, 1885. viii Jahrg., pp. 157–159.)

—— Richtigstellung der behauptungen des Herrn F. Dahl. (Archiv f. mikroskop. Anat., 1885, xxvi, pp. 125–128.)

Rombouts, J. E. Ueber die Fortbewegung der Fliegen an glatten Flächen. (Zool. Anzeiger, 1884, pp. 619–623.)

—— De la faculté qu’out les mouches de se mouvoir sur le verre et sur les autres corps polis. (Archiv Museum Teyler (2), 4 Part, pp. 16. Fig.)

Simmermacher, G. Untersuchungen über Haftapparate an Tarsalgliedern von Insekten. (Zeitschr. f. wissensch. Zool. xl, 1884, pp. 481–556. 3 Taf., 2 Figs. Also Zoolog. Anzeiger, vii Jahrg., 1884, pp. 225–228.)

—— Antwort an Herrn Dr. H. Dewitz. (Ibid., pp. 513–517.)

Dahl, F. Die Fussdrüsen der Insekten. (Archiv f. mikroskop. Anat., 1885, xxv, pp. 236–263. 2 Taf. See also p. 118.)

Emery, C. Fortbewegung von Tieren an senkrechten und überhangenden glatten Flächen. (Biolog. Centralbl., 1884, 4 Bd., pp. 438–443.)

Léon, N. Disposition anatomique des organes de succion chez les Hydrocores et les Géocores. (Bull. Soc. des Medec. et Natur, de Jassy., 1888.)

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d. The wings and their structure

The insects differ from all other animals except birds in possessing wings, and as we at the outset have claimed, it is evidently owing to them that insects are numerically so superior to any other class of animals, since their power of flight enables them to live in the air out of reach of many of their enemies, the greatest destruction to insect life occurring in the wingless larval and pupal stages.

The presence of wings has exerted a profound influence on the shape and structure of the body, and it is apparently due to their existence that the body is so distinctly triregional, since this feature is least marked in the synapterous insects. The wings are thin, broad leaf-like folds of the integument, attached to the thorax and moved by powerful muscles which occupy the greater part of the thoracic cavity. The two pairs of wings are outgrowths of the middle and hinder part of the thorax, the anterior pair being attached to the mesothoracic and the hinder pair to the metathoracic segment. The larger pair is developed from the middle segment of the thorax. The differentiation of the tergites into scutum, scutellum, etc., is the result of the appearance of wings, because these sclerites are more or less reduced or effaced in wingless insects, such as apterous Orthoptera and moths, ants, etc.

The size of the hinder thoracic segments is closely related to that of the wings they bear. In those Orthoptera which have hind wings larger than those of the fore pair, the metathorax is larger than the mesothorax. In such Neuroptera as have the hind wings nearly or quite as large as the anterior pair, or in the Trichoptera and in the Hepialidæ, the metathorax is nearly as large as the mesothorax, while in Coleoptera the metathorax is as large and often much larger. In the Ephemeridæ, Diptera, and Hymenoptera, which have either only rudimentary (halteres) or small hind wings, the metathorax is correspondingly reduced in size.

The wings morphologically, as their development shows, are simple sac-like outgrowths of the integument, i.e. of the free hinder edge of the tergal plates, their place of origin being apparently above the upper edge of the epimera or pleural sclerites. Calvert[24] however, regards the upper lamina of the wing as tergal, and the lower, pleural.

The wings in most insects are attached to the thorax by a membrane containing several little plates of chitin called by Audouin articulatory epidemes.

121The wings, then, are simple, very thin chitinous lamellate expansions of the integument, which are supported and strengthened by an internal framework of hollow chitinous tubes.

The veins.—The so-called “veins” or “nervures,” which are situated between the upper and under layers of the wing are so disposed as to give the greatest lightness and strength to the wings. Hagen has shown that in the freshly formed wings these two layers can be separated, when it can be seen that the veins pass through each layer.

These veins are in reality quite complex, consisting of a minute central trachea enclosed within a larger tube which at the instant the insect emerges from the nymph, or pupa, as the case may be, is filled with blood (Fig. 136). Since these tubes at first contain blood, which has been observed to circulate through them, and since the heart can be most easily injected through them, they may more properly be called veins than nervures. The shape and venation of the wings afford excellent ordinal as well as family and generic characters, while they also enable the systematist to exactly locate the spots and other markings of the wings. The spaces enclosed by the veins and their cross-branches are called cells, and their shape often affords valuable generic and specific characters.

Fig. 136.—Cross-section of wing of Pronuba.—After Spuler.

Fig. 137.—Cross-section of wing of Pieris: s, insertions of scales.-After Spuler.

The structure of a complete vein is described by Spuler. In a cross-section of a noctuid moth (Triphæna pronuba, Fig. 136) the chitinous walls are seen to consist of two layers, an outer (U) and inner (c), the latter of which takes a stain and lies next to the hypodermis (hy). In the cavity of the vein is the trachea (tr), which shows more or less distinctly the so-called spiral thread; within the cavity are also Semper’s “rib” (r) and blood-corpuscles (bc), which proves that the blood circulates in the veins of the completely formed wing, though this does not apply to all Lepidoptera with hard mature wings. We have been able to observe the same structure in sections of the wing of Zygæna.

122A cross-section of a vein of Pieris brassicæ shows that the large trachea is first formed, and that it extends along the track between the protoplasmic threads connecting the two hypodermal layers.

The main tracheæ throw off on both sides a number of secondary branches showing at their end a cell with an intracellular tracheal structure; these accessory tracheæ afterwards branch out. The accessory or transverse tracheæ often disappear, though in some moths they remain permanently. Fig. 137 tr2 represents these secondary veins in the edge of the fore wing of Laverna vanella, arising from a main trachea (tr) passing through vein I (v), two of the twigs extending to the centre, showing that the latter has no homology with a true vein. Only rarely and in strongly developed thick folds are the transverse tracheæ provided with a chitinous thickening, as for example in Cossus ligniperda. Since from such accessory tracheæ the transverse veins in lepidopterous wings are developed, we can recognize in them the homologies of the net-veins in reticulated venations. There is no sharply defined difference between reticulated and non-reticulated venations; no genetic difference exists between the two kinds of venation, since there occur true Blattidæ both with and without a reticulated venation (Spuler).

In the fore wings of Odonata, Psocina, Mantispidæ, and most Hymenoptera is an usually opaque colored area between the costal edge and the median vein, called the pterostigma.

In shape the wings are either triangular or linear oval, and at the front edge the main veins are closer together than elsewhere, thus strengthening the wings and affording the greatest resistance to the air in making the downward stroke during flight. It is noticeable that when the veins are in part aborted from partial disuse of the wings, they disappear first from the hinder and middle edge, those on the costal region persisting. This is seen in the wings of Embiidæ (Oligotoma), Cynipidæ, Proctotrupidæ, Chalcids, ants, etc.

The front edge of the wing is called the costal, its termination in the outer angle of the wing is called the apex; the outer edge (termen) is situated between the apex and the inner or anal angle, between which and the base of the wing is the inner or internal edge.

While in Orthoptera, dragon-flies, Termitidæ, and Neuroptera the wings are not attached to each other, in many Lepidoptera they are loosely connected by the loop and frenulum, or in Hymenoptera by a series of strong hooks. These hooks are arranged, says Newport, “in a slightly twisted or spiral direction along the margin of the wing, so as to resemble a screw, and when the wings are expanded attach themselves to a little fold on the posterior margin 123of the anterior wing, along which they play very freely when the wings are in motion, slipping to and fro like the rings on the rod of a window curtain.”

At the base of the hind wings of Trichoptera and in the lepidopterous Micropteryx there is an angular fold (jugum) at the base of each wing (Fig. 138); that of the anterior wings is retained in Eriocephala and Hepialidæ.

Fig. 138.—Venation of fore and hind wings of Micropteryx purpurella: j, jugum, on each wing; d, discal vein; the Roman numerals indicate veins I.-VIII. and their branches.

In the wings of Orthoptera as well as other insects, the fore wings, especially, are divided into three well-marked areas, the costal, median, and internal; of these the median area is the largest, and in grasshoppers and crickets is more or less modified to form the musical apparatus, consisting of the drum-like resonant area, with the file or bow.

The squamæ.—In the calyptrate Muscidæ, a large scale-like membranous broad orbicular whitish process is situated beneath the base of the wing, above the halter; (Fig. 94, 10 sq.) it is either small or wanting in the acalyptrate muscids. Kirby and Spence state that when the insect is at rest the two divisions of this double lobe are folded over each other, but are extended during flight. Their exact use is unknown. Kolbe, following other German authors, considers the term squama as applicable to the whole structure, restricting the term alula to the other lobe-like division.

124More recently (1890 and 1897) Osten-Sacken recommends “squamæ; in the plural, as a designation for both of these organs taken together; squama, in the singular, would mean the posterior squama alone, and antisquama the anterior squama alone;” the strip of membrane running in some cases between them, or connecting the squama with the scutellum, should be called the post-alar membrane. By a mistake Loew, and others following him, used the word tegula for squama, but this term should be restricted to the sclerite of the mesothorax previously so designated (Fig. 90, A, t). The squama or its two subdivisions has also by various authors been termed alula, calypta, squamula, lobulus, axillary lobe, aileron, cuilleron, schuppen, and scale. (Berlin Ent. Zeitschrift, xli, 1896, pp. 285–288, 328, 338.)

The halteres.—In the Diptera the hind wings are modified to form the halteres or balancers, which are present in all the species, even in Nycteribia, but are absent in Braula.

Meinert finds structures in the Lepidoptera which he considers as the homologues of the halteres of Diptera. “In the Noctuidæ,” he remarks, “I find arising from the fourth thoracic segment (segment médiaire), but covered by hair, an organ like the halter of Diptera.” (Ent. Tidskrift., i, 1880, p. 168.) He gives no details.

In the Stylopidæ, on the contrary, the fore wings are reduced to little narrow pads, while the hind wings are of great size.

The thyridium is a whitish spot marking a break in the cubital vein of the fore wing of Trichoptera; these minute thyridia occur in the fore wings of the saw-flies; there is also an intercostal thyridium on the costal part of the wings of Dermaptera.

The fore wings of Orthoptera are thicker than the hinder ones, and serve to protect the hind-body when the wings are folded; they are sometimes called tegmina. It is noteworthy, that, according to Scudder, in all the paleozoic cockroaches the fore wings (tegmina) were as distinctly veined as the hinder pair, “and could not in any sense be called coriaceous.” (Pretertiary Insects of N. A., p. 39.) Scudder also observes that in the paleozoic insects as a rule the fore and hind wings were similar in shape and venation, “heterogeneity making its appearance in mesozoic times.” In the heteropterous Hemiptera, also, the basal half of the fore wings is thick and coriaceous or parchment-like, and also protects the body when they are folded; these wings are called hemelytra. In the Dermaptera the small short fore wings are thickened and elytriform.

The elytra.—This thickening of the fore wings is carried out to its fullest extent in the fore wings of beetles, where they form the sheaths, shards, or elytra, under which the hind wings are folded. The indexed costal edge is called the epipleurum, being wide in the Tenebrionidæ. During flight “the elytra are opened so as to form an 125angle with the body and admit of the free play of the wings” (Kirby and Spence). In the running beetles (Carabidæ), also in the weevils and in many Ptinidae, the hind wings are wanting, through disuse, and often the elytra are firmly united, forming a single hard shell or case. The firmness of the elytra is due both to the thickness of the chitinous deposit and to the presence of minute chitinous rods or pillars connecting the upper and lower chitinous surfaces.

Fig. 139.—Longitudinal section through the edge of the elytrum of Lina ænea: gl, glands; r, reservoir; fb, fat-body; m, matrix; u, upper,—l, lower, lamella.—After Hoffbauer.

Hoffbauer finds that in the elytra of beetles of different families the venation characteristic of the hind wings is wanting, the main tracheæ being irregular or arranged in closely parallel longitudinal lines, and nerve-fibres pass along near them, sense-organs being also present. The fat-bodies in the cavity of the elytra, which is lined with a matrix layer, besides nerves, tracheæ, and blood, contain secretory vesicles filled with uric-acid concretions such as occur in the fat-body of Lampyris. There are also a great many glands varying much in structure and position, such occurring also in the pronotum (Fig. 139).

Meinert considers the elytra of Coleoptera to be the homologues of the tegulæ of Lepidoptera and of Hymenoptera. He also calls attention to the alula observed in Dyticus, situated at the base of the elytra, but which is totally covered by the latter. The alulæ of these beetles he regards as the homologues of the anterior wings of Hymenoptera and Diptera. No details are given in support of these views. (Ent. Tidskrift, i, 1880, p. 168.)

Hoffbauer (1892) also has suggested that the elytra are not the homologues of the fore wings of other insects, but of the tegulæ.

Kolbe describes the alula of Dyticus as a delicate, membranous lobe at the base of the elytra, but not visible when they are closed: its fringed edge in Dyticus is bordered by a thickening forming a tube which contains a fluid. The alula is united with the inner basal portion and articulation of the wing-cover, forming a continuation of them. Dufour considered that the humming noise made by these beetles is produced by the alulets.

Hoffbauer finds no structural resemblances in the alulæ of Dyticus to the elytra. He does not find “the least trace of veins.” They are more like appendages of the elytra. Lacordaire considered that their function is to prevent 126the disarticulation of the elytra, but Hoffbauer thinks that they serve as contrivances to retain the air which the beetle carries down with it under the surface, since he almost always found a bubble of air concealed under it; besides, their folded and fringed edge seems especially fitted for taking in and retaining air. Hoffbauer then describes the tegulæ of the hornet and finds them to be, not as Cholodkowsky states, hard, solid, chitinous plates, but hollow. They are inserted immediately over the base or insertion of the fore wings, being articulated by a hinge-joint, the upper lamella extending into a cavity of the side of the mesothorax, and connected by a hinge-like, articulating membrane with the lower projection of the bag or cavity. The lower lamella becomes thinner towards the place of insertion, is slightly folded, and merges without any articulation into the thin, thoracic wall at a point situated over the insertion of the fore wing. The tegulæ also differ from the wings in having no muscles to move them, the actual movements being of a passive nature, and due to the upward and downward strokes of the wings.

Comstock adopts Meinert’s view that the elytra are not true fore wings, but gives no reasons. (Manual, p. 495.)

Dr. Sharp,[25] however, after examining Dyticus and Cybister, affirms that this structure is only a part of the elytron, to which it is extensively attached, and that it corresponds with the angle at the base of the wing seen in so many insects that fold their front wings against the body. He does not think that the alula affords any support to the view that the elytra of beetles correspond with the tegulæ of Hymenoptera rather than with the fore wings.

That the elytra are modified paraptera (tegulæ) is negatived by the fact that the latter have no muscles, and that the elytra contain tracheæ whose irregular arrangement may be part of the modified degenerate structure of the elytra. Kolbe finds evidences of veins. The question may also be settled by an examination of the structure of the pupal wings. A study of a series of sections of both pairs of wings of the pupa of Doryphora and of a Clytus convinces us that the elytra are the homologues of the fore wings of other insects.

e. Development and mode of origin of the wings

Embryonic development of the wings.—The wings of insects are essentially simple dorsal outgrowths of the integument, being evaginations of the hypodermis. They begin to form in the embryo before hatching, first appearing as folds, buds, or evaginations, of the hypodermis, which lie in pouches, called peripodal cavities. They are not visible externally until rather late in larval life, after the insect, such as a grasshopper, has moulted twice or more times; while in holometabolous insects they are not seen externally until the pupa state is attained.

The subject of their origin is in a less satisfactory state than desirable from the fact that at the outset the development of the wings of the most generalized insects, such as Orthoptera, Termes, etc., was not first examined, that of the most highly modified of any insects, i.e. the Muscidæ, having actually been first studied.

127In the course of his embryological studies on the Muscidæ (Musca comitoria and Sarcophaga carnaria) Weismann (1864) in examining the larvæ of these flies just before pupation, found that the wings, as well as the legs and mouth-appendages, developed from microscopic masses of indifferent cells, which he called “imaginal discs.” From the six imaginal discs or buds in the lower part of the thorax arise the legs, while from four dorsal discs, two in the meso- and two in the metathoracic segment, arise the fore and hind wings (Fig. 141.) These imaginal buds, as we prefer to call these germs, usually appear at the close of embryonic life, being found in freshly hatched larvæ.

Fig. 140.—Imaginal buds in Musca,—A, in Corethra,—B, in Melophagus,—C, in embryo of Melophagus; dorsal view of the head; b, bud; p, peripodal membrane; c, cord; hy, hypodermis; cl, cuticula; st, stomodæum; v, ventral cephalic, behind are the two dorsal cephalic buds.—After Pratt.

As first observed by Weismann, the buds are, like those of the appendages, simply attached to tracheæ and sometimes to nerves, in the former case appearing as minute folds or swellings of the peritoneal membrane of certain of the tracheæ. In Volucella the imaginal buds were, however, found by Künckel d’Herculais to be in union with the hypodermis. Dewitz detected a delicate thread-like stalk connecting the peripodal membrane with the hypodermis, and Van Rees has since proved in Musca, and Pratt in Melophagus, the connection of the imaginal buds with the hypodermis (Fig. 140). These tracheal enlargements increase in size, and become differentiated into a solid mass which corresponds to the upper part of the mesothorax, while a tongue-shaped continuation becomes the rudiment of the wing. During larval life the rudiments of the wings 128crumple, thus forming a cavity. While the larva is transforming into the pupa, the sheath or peripodal membranes of the rudimentary wings are drawn back, the blood presses in, and thus the wings are everted out of the peripodal cavities.

Due credit, however, should be given to Herold, as the pioneer in these studies, who first described in his excellent work on the development of Pieris brassicæ (1815) the wing-germs in the caterpillar after the third moult. This discovery has been overlooked by recent writers, with the exception of Gonin, whose statement of Herold’s views we have verified. Herold states that the germs of the wings appear on the inside of the second and third thoracic segments, and are recognized by their attachment to the “protoplasmic network” (schleimnetz), which we take to be the hypodermis, the net-like appearance of this structure being due to the cell-walls of the elements of the hypodermal membrane. These germs are, says Herold, also distinguished from the flakes of the fat-body by their regular symmetrical form. Fine tracheæ are attached to the wing-germs, in the same way as to the flakes of the fat-body. It thus appears that Herold in a vague way attributes the origin of these wing-germs, and also the germs of the leg, to the hypodermis, since his schleimnetz is the membrane which builds up the new skin. Herold also studied the later development of the wings, and discovered the mode of origin of the veins, and in a vague way traced the origin of the scales and hairs of the body, as well as that of the colors of the butterfly.

Herold also says that as the caterpillar grows larger, and also the wing-germs, “the larval skin in the region under which they lie hidden is spotted and swollen,” and he adds in a footnote: “This is the case with all smooth caterpillars marked with bright colors. In dark and hairy caterpillars the swelling of the skin through the growth of the underlying wing-germs is less distinct or not visible at all” (pp. 29, 30).

It should be added that Malpighi, Swammerdam, and also Réaumur had detected the rudiments of the wings in the caterpillar just before pupation under the old larval skin. Lyonet (1760) also describes and figures the four wing-germs situated in the second and third thoracic segments, but was uncertain as to their nature. Each of these masses, he says, is “situated in the fatty body without being united to it, and is attached to the skin in a deep fold which it makes there.” He could throw no certain light on their nature, but says: “their number and situation leads to the supposition that they may be the rudiments of the wings of the moth” (pp. 449, 450).

During the transformation into the pupa the imaginal buds unite and grow out or extend along their edges, while the enveloping membrane disappears. The rudimentary wings are now like little sacs, and soon show a fusion of the two wing-membranes or laminæ with the veins, while the tracheæ disappear, the places occupied by the tracheæ becoming the veins. “Very early, as soon as the scales are indicated, begin in a very peculiar way the fusion of the wing-laminæ. There occur openings in the hypodermis into which the cells extend longitudinally and then laterally give way to each other. Hence no complete opening is found, but the epithelium appears by sections through a straight line sharply bordered along the wingcavity. 129It is a continuous membrane formed of plasma which I will call the ground membrane of the epithelium. Through this ground membrane pass blood-corpuscles as well as blood-lymph.” (Schaeffer.)

Fig. 141.—Anterior part of young larva of Simulium sericea, showing the thoracic imaginal buds: p, prothoracic bud (only one not embryonic); w, w′, fore and hind wing-buds; l, l′, l″, leg-buds; n, nervous system; br, brain; e, eye; sd, salivary duct; p, prothoracic foot.—After Weismann.

Afterwards (1866) Weismann studied the development of the wings in Corethra plumicornis, which is a much more primitive and generalized form than Musca, and in which the process of development of the wings is much simpler, and, as since discovered, more as in other holometabolous insects. He also examined those of Simulium (Fig. 141).

In Corethra, after the fourth and last larval moulting, there arises at first by evagination and afterwards by invagination a cup-shaped depression on each side in the upper part of the mesothoracic segment within which the rudiment of the wings lies like a plug. The wings without other change simply increase in size until, in the transformation into the pupa by the withdrawal of the hypodermis, the wings project out and become filled with blood, the tracheæ now being wholly wanting, and other tissues being sparingly present.

Fig. 142.—Section through thorax of a Tineid larva on sycamore, passing through the 1st pair of wings (w): ht, heart; i, œsophagus; s, salivary gland: ut, urinary tube; nc, nervous cord; m, recti muscles; a part of the fat body overlies the heart. A, right wing-germ enlarged.

130These observations on two widely separate groups of Diptera were confirmed by Landois, and afterwards by Pancritius, for the Lepidoptera, by Ganin for the Hymenoptera, by Dewitz for Hymenoptera (ants) and Trichoptera; also for the Neuroptera by Pancritius. In the ant-lion (Myrmeleon formicarius) Pancritius found no rudiments of the wings in larvæ a year old, but they were detected in the second year of larval life, and do not differ much histologically or in shape from those of Lepidoptera. In the Coleoptera and Hymenoptera the imaginal buds appear rather late in larval life, yet their structure is like that of Lepidoptera. In Cimbex the rudiments of the wings are not found in the young larva, but are seen in the semipupa, which stage lasts over six weeks.

Fig. 143.—Section of the same specimen as in Fig. 142, but cut through the second pair of wings (w): i, mid-intestine; h, heart; fb, fat-body; l, leg; n, nervous cord.

The general relation of the rudiments (imaginal buds) of the wings of a tineid moth to the rest of the body near the end of larval life may be seen in Figs. 142, 143 (Tinea?), the sections not, however, showing their connection with the hypodermis, which has been torn away during the process of cutting. That the wing is but a fold of the hypodermis is well seen in Fig. 144, of Datana, which represents a much later stage of development than in Figs. 142 and 143, the larva just entering on the semipupa stage.

In caterpillars of stage I, 3 to 4 mm. in length, Gonin found the wing-germs as in Fig. 145, A being a thickening of the hypodermis, with the embryonic cells, i.e. of Verson, on the convex border. The two leaves, or sides of the wing, begin to differentiate in stage II (C, D), and in stage III the envelope is formed (E), while the tracheæ begin to proliferate, and the capillary tracheæ or tracheoles at this time arise (Fig. 145, tc). The wall of the principal trachea 131appears to be resolved into filaments, and all the secondary branches assume the appearance of bundles of twine. Landois regarded them as the product of a transformation of the nuclei, but Gonin thinks they arise from the entire cells, stating that from each cell arises a ball (peloton) of small twisted tubes.

Fig. 144.—Section through mesothoracic segment of Datana ministra, passing through the wings (w): c, cuticula; hyp, hypodermis: ap, apodeme; dm, dorsal longitudinal.—vm, ventral longitudinal. muscles; dmt, depressor muscle of tergum; t, trachea; n, nerve cords; i, intestine; u, urinary tubes; l, insertion of legs.

As the large branches penetrate into the wing, the balls (pelotons) of fine tracheal threads tend to unroll, and each of the new ramifications of the secondary tracheal system is accompanied in its course by a bundle of capillary tubes. This secondary system of wing-tracheæ, then, arises from the mother trachea at the end of the third stage, when we find already formed the chitinous tunic, which will persist through the fourth stage up to pupation. It differs from the tracheoles in not communicating with the air-passage; it possesses no spiral membrane at the origin, and takes no part in respiration.

Gonin thus sums up the nature of the two tracheal systems in the rudimentary wing, which he calls the provisional and permanent systems. “The first, appearing in the second stage of the larva, comprises all the capillary tubes, and arising from numerous branches passes off from the lateral trunk of the thorax before reaching the wing; the second is formed a little later by the direct ramification of the principal branch.

132“These two systems are absolutely independent of each other within the wing. Their existence is simultaneous but not conjoint. One is functionally active after the third moult; the other waits the final transformation before becoming active.”

Fig. 145.A, section of wing-bud of larva of Pieris brassicæ of stage I, in front of the invagination pit. B, section passing through the invagination pit. C, section of same in stage II, through the invagination pit;—D, behind it, making the bud appear independent of the thoracic wall. E, wing-bud at the beginning of the 3d larval stage, section passing almost through the pedicel or hypodermic insertion, the traces of which appear at hi; h, hypodermis; t or tr, trachea; i, opening of invagination; ec, embryonic cells; l, external layer or envelope; in, internal wall of the wing; ex, external wall; s, cell of a tactile hair; tc, capillary tubes; c, cavity of invagination.—After Gonin.

Evagination of the wing outside of the body.—We have seen that the alary germs arise as invaginations of the hypodermis; we will now, with the aid of Gonin’s account, briefly describe, so far as is known, the mode of evagination of the wings. During the fourth and last stage of the caterpillar of Pieris, the wings grow very rapidly, and undergo important changes.

Six or seven days after the last larval moult the chitinous wall is formed, the wing remaining transparent. It grows rapidly and its lower edge extends near the legs. It is now much crumpled on the edge, owing to its rapid growth within the limits of its own segment. Partly from being somewhat retracted, and partly owing to the irregularity of its surface, the wing gradually separates from its envelope, and the cavity of invagination (Fig. 145, c) becomes more like a distinct or real space. The outer opening of the alary sac enlarges quite plainly, though without reaching the level of the edge of the wing.

This condition of things does not still exactly explain how the wing passes to the outside of the body. Gonin compares these conditions to those exhibited by a series of sections of the larva, made forty-eight hours later, on a caterpillar which had just spun its girdle of silk. At this time the wings have become entirely external, but, 133says Gonin, we do not see the why or the how. The partition of the sac has disappeared, and with it the cavity and the leaf of the envelope.

It appears probable that the partition has been destroyed, because the space between the two teguments is strewn with numerous bits, many of which adhere to the chitinous integument, while others are scattered along the edges of the wings, in their folds, or between the wings and the wall of the thorax.

Another series of sections showed that the exit of the fore wings had been accomplished, while the hinder pair was undergoing the process of eversion. In this case the partition showed signs of degeneration: deformation of the nuclei, indistinct cellular limits, pigmentation, granular leucocytes, and fatty globules.

After the destruction of the partition, what remains of the layer of the envelope is destined to make a part of the thoracic wall and undergoes for this purpose a superficial desquamation. The layer of flattened cells is removed and replaced by a firmer epithelium like that covering the other regions. It is this renewed hypodermis which conceals the wing within, serves to separate it from the cavity of the body, and gives the illusion of a complete change in its situation. Other changes occur, all forming a complete regeneration, but which does not accord with the description of Van Rees for the Muscidæ. Finally, Gonin concludes that the débris scattered about the wing comes from the two layers of the partition of the sac, from the flattened hypodermis of the renewed envelope, from the chitinous cuticle of the wing, and from the inner surface of the chitinous integument.

He thinks that the metamorphosis of Pieris is intermediate between the two types of Corethra and of Musca, established by Weismann, as follows:

Corethra.—The wing is formed in a simple depression of the hypodermic wall. No destruction.

Pieris.—The rudiment is concealed in a sac attached to the hypodermis by a short pedicel. Destruction of the partition and its replacement by a part of the thoracic wall by means of the imaginal epithelium.

Musca.—The pedicel is represented by a cord of variable length, whose cavity may be obliterated (Van Rees). The imaginal hypodermis is substituted for the larval hypodermis, which has completely disappeared, either by desquamation (Viallanes), or by histolytic resorption (Van Rees).

Extension of the wing; drawing out of the tracheoles.—When it is disengaged from the cavity, the wing greatly elongates and the creases on its surface are smoothed out; the blood penetrates between the two walls, and the cellular fibres, before relaxed and sinuous, are now firmly extended.

Of the two tracheal systems, the large branches are sinuous, and they are rendered more distinct by the presence of a spiral membrane; but the two tunics are not separated as in the other tracheæ of the thorax; moreover, the mouth choked up with débris does not yet communicate with that of the principal trunk. The bundles of tracheoles on their part form straight lines, as if the folds of the organ had had no influence on them. As they have remained bound together, apart from the chitinous membrane of the tracheal trunk, 134they become drawn out with this membrane, at the time of exuviation, i.e. of pupation, and are drawn out of the neighboring spiracle.

Fig. 146.—Full-grown larva of Pieris brassicæ, opened along the dorsal line: d, digestive canal; s, silk-gland; g, brain; st I, prothoracic stigma; st IV, 1st abdominal stigma; a, a′, germs (buds) of fore and hind wings; p, bud of prothoracic segment;—those of the third pair are concealed under the silk-glands; I–III, thoracic rings.—After Gonin.

“This is a very curious phenomenon, which can be verified experimentally: if we cut off the wing, while sparing the larval integument around the thoracic spiracles, we preserve the two tracheal systems; the same operation performed after complete removal of the larval skin does not give the secondary tracheal system.” (Gonin.) Deceived by the appearance of the tracheoles while still undeveloped, Landois and Pancritius, who have not mentioned the drawing out of the capillaries of the larva, affirm that they are destroyed by resorption in the chrysalis.

135“The study of the tracheæ is closely connected with that of the veins (nervures). It is well to guard against the error of Verson, who mistakes for these last the large tracheal branches of the wing. This confusion is easily explained; it proves that Verson had, with us, recognized that the secondary system is, in the larva, exempt from all respiratory function. Landois thought that the pupal period was the time of formation of the veins. It seems to me probable that they are derived from the sheath of the peritracheal spaces.” (Gonin, pp. 30–33.)

Fig. 147.—Left anterior wing of a larva 3 days before pupation. The posterior part is rolled up: st, prothoracic stigma; tr. i., internal tracheal trunk; tr. e., tr. e.′, external tracheal trunk; p, cavity of a thoracic leg, with the imaginal bud b.—After Gonin.

The appearance of the wing-germs in the fully grown caterpillar, as revealed by simple dissection, is shown at Fig. 146; Fig. 147 136represents a wing of a larva three days before pupation, with the germ of a thoracic leg.

Fig. 148.—Graber’s diagrams for explaining the origin and primary invagination of the hypodermis to form the germs of the leg (b), and wings (f, A-C), and afterwards their evagination D, so that they lie on the outside of the body. E, stage B, showing the hypodermal cavities (f) and stalks connecting the germs with the hypodermis (z).—After Graber.

Fig. 149.—Section lengthwise through the left wing of mature larva in Pieris rapæ: t, trachea; hyp, hypodermis; c, cuticula.—After Mayer.

A. G. Mayer has examined the late development of the wings in Pieris rapæ. Fig. 149 represents a frontal section through the left wing of a mature larva and shows the rudiment of the wing, lying in its hypodermal pocket or peripodal cavity. How the trachea passes into the rudimentary wing, and eventually becomes divided into the branches, around which the main veins afterwards form, is seen in Figs. 144, 147, 159.

The histological condition of the wing at this time is represented by Fig. 151, the spindle-like hypodermal cells forming the two walls being separated by the ground-membrane of Semper.

“While in the pupa state,” says Mayer, “the wing-membrane is thrown into a very regular series of closely compressed folds, a single scale being inserted upon the crest of each fold. When the butterfly 137issues from the chrysalis, these folds in the pupal wings flatten out, and it is this flattening which causes the expansion of the wings.... It is evident that the wings after emergence undergo a great stretching and flattening. The mechanics of the operation appears to be as follows. The hæmolymph, or blood, within the wings is under considerable pressure, and this pressure would naturally tend to enlarge the freshly emerged wing into a balloon-shaped bag; but the hypodermal fibres (h) hold the upper and lower walls of the wing-membrane closely together, and so, instead of becoming a swollen bag, the wing becomes a thin flat one. And thus it is that the little thick corrugated sac-like wings of the freshly emerged insect become the large, thin, flat wings of the imago.... The area of the wing of the imago of Danais plexippus is 8.6 times that of the pupa. Now, as the wing of the young pupa has about 60 times the area of the wing in the mature larva, it is evident that in passing from the larval state to maturity the area of the wings increases more than 500 times.”

Fig. 150.—Diagrammatic reproduction of Fig. 149 showing the wing-germ in its peripodal cavity (p): h’drm, hypodermis; tr, trachea; cta, cuticula; a, anterior end.—After Mayer.

Fig. 151.—Section of the wing-germ, the upper and lower sides connected by spindle-like hypodermic cells (h), forming the rods of the adult wing; mbr, ground-membrane of Semper.—After Mayer.

f. The primitive origin of the wings

Farther observations are needed to connect the mode of formation of the wings in the holometabolous insects with the more primitive mode of origin seen in the hemimetabolous orders, but the former mode is evidently inherited from the latter. Pancritius remarks that the development of the rudiments of the wing in a hypodermal cavity is in the holometabolic insects to be regarded as a later inherited character, the external conditions causing it being unknown.

138Fritz Müller was the first to investigate the mode of development of the wings of the hemimetabolic insects, examining the young nymphs of Termites. He regards the wings as evaginations of the hypodermis, which externally appear as thoracic scale-like projections, into which enter rather late in nymphal life tracheæ which correspond to the veins which afterward arise.

Fig. 152.—Rudimentary wing of young nymph of Blatta, with the five principal veins developed.

The primitive mode of origin of the wings may, therefore, be best understood by observing the early stages of those insects, such as the Orthoptera and Hemiptera, which have an incomplete metamorphosis. If the student will examine the nymphs of any locust in their successive stages, he will see that the wings arise as simple expansions downward and backward of the lateral edges of the meso- and metanotum. In the second nymphal stage this change begins to take place, but it does not become marked until the succeeding stage, when the indications of veins begin to appear, and the lobe-like expansion of the notum is plainly enough a rudimentary wing.

Graber[26] thus describes the mode of development of the wings in the nymph of the cockroach:

“If one is looking only at the exterior of the process, he will perceive sooner or later on the sides of the meso- and metathorax pouch-like sacs, which increase in extent with the dorsal integument and at the same time are more and more separated from the body. These wing-covers either keep the same position as in the flat-bodied Blattidæ, or in insects with bodies more compressed the first rudiments hang down over the sides of the thorax. As soon as they have exceeded a certain length, these wing-covers are laid over on the back. However, if we study the process of development of the wings with a microscope, by means of sections made obliquely through the thorax, the process appears still more simple. The chief force of all evolution is and remains the power of growth in a definite direction. In regard to the skin this growth is possible in insects only in this way; namely, that the outer layer of cells is increased by the folds which are forced into the superficial chitinous skin. These folds naturally grow from one moult to another in proportion to the multiplication of the cells, and are not smoothed out until after the moulting, when the outer resistance is overcome.

139

Fig. 153.—Partial metamorphosis of Melanoplus femur-rubrum, showing the five nymph stages, and the gradual growth of the wings, which are first visible externally in 3, 3b, 3c.—Emerton del.

“As, however, the first wing-layers depend upon the wrinkling of the general integument of the body through the increase in the upper layer, the further growth of the wings depends in the later stages upon the wrinkling of the epidermis of the wing-membrane even, which fact we also observe under the microscope when the new wings drawn forth from the old covers appear at 140first to be quite creased together. These wing-like wrinkles in the skin are not empty pouches, but contain tissues and organs within, which are connected with the skin, as the fat of the body, the network of tracheæ, muscles, etc. Alongside the tracheæ, running through the former wing-pouches and accompanied by the nerves, there are canals through which the blood flows in and out.

Fig. 154.—Stages in the growth of the wings of the nymph of Termes flavipes: A, young; a, a wing enlarged. B, older nymph; b, fore wing; n, a vein. C, wings more advanced;—D, mature.

Fig. 155.—Wings of nymph of Psocus.

“After the last moult, however, when the supply of moisture is very much reduced in the wing-pouches, which are contracted at the bottom, their two layers become closely united, and afterward grow into one single, solid wing-membrane.

“These thick-walled blood-tubes arising above and beneath the upper and lower membrane of the wing are the veins of the wings; the development of 141the creased wings in the pupa of butterflies is exactly like that of cockroaches and bugs. The difference is only that the folds of integument furnishing the wings with an ample store of material for their construction reach in a relatively shorter time, that is the space of time between two moults, the same extent that they would otherwise attain only in the course of several periods of growth in the ametabolous insects.”

Fig. 156.—Nymph of Aphrophora permutata, with enlarged view of the wings and the veins: pro, pronotum; sc, mesoscutum; 1ab, 1st abdominal segment.

Ignorant of Graber’s paper, we had arrived at the same result, after an examination of the early nymph-stages of the cockroach, as well as the locusts, Termites, and various Hemiptera. In all these forms it is plainly to be seen that the wings are simply expansions, either horizontal or partly vertical (where, as in locusts, etc., the body is compressed, and the meso- and metanota are rounded downwards), of the hinder and outer edge of the meso- and metanotum. As will be seen by reference to the accompanying figures, the wings are notal (tergal) outgrowths from the dorsal arch of the two hinder segments of the thorax. At first, as seen in the young pupal cockroach (Fig. 152) and locust (Fig. 153, also Figs. 154 and 156) the rudiments of the wings are continuous with the notum. Late in nymphal life a suture and a hinge-joint appear at the base of the wing, and thus there is some movement of the wing upon the notum; finally, the tracheæ are well developed in the wings, and numerous small sclerites are differentiated at the base of the wing, to which the 142special muscles of flight are attached, and thus the wings, after the last nymphal moult, have the power of flapping, and of sustaining the insect in the air; they thus become true organs of flight.

It is to be observed, then, that the wings in all hemimetabolous insects are outgrowths from the notum, and not from the flanks or pleurum of the thorax. There is, then, no structure in any other part of the body with which they are homologous.

Fig. 157.—Development of wings of Trichoptera: A, portion of body-wall of young larva of Trichostegia; ch, cuticula, forming at r a projection into the hypodermis, m; r, and d, forming thus the first rudiment of the wing. B, the parts in a larva of nearly full size; a, c, d, b, the well-developed hypodermis of the wing-germ separated into two parts by r, the penetrating extension of the cuticula; v, mesoderm, C, wing-pad of another Phryganeid freed from its case at its change to the pupa: b, d, outer layer of the hypodermis (m) of the body-wall; v, inner layer within nuclei.—After Dewitz, from Sharp.

The same may be said of the true Neuroptera, Trichoptera (Fig. 157), the Coleoptera, and the Diptera, Lepidoptera, and Hymenoptera. As we have observed in the house fly,[27] the wings are evidently outgrowths of the meso- and metanotum; we have also observed this to be most probably the case in the Lepidoptera, from observations on a Tortrix in different stages of metamorphosis. It is also the case with the Hymenoptera, as we have observed in bees and wasps;[28] and in these forms, and probably all Hymenoptera, the wings are outgrowths of the scutal region of the notum.

With these facts before us we may speculate as to the probable origin of the wings of insects. The views held by some are those of Gegenbaur, also adopted by Lubbock, and originally by myself.[29] According to Gegenbaur:

“The wings must be regarded as homologous with the lamellar tracheal gills, for they do not only agree with them in origin, but also in their connection with the body, and in structure. In being limited to the second and third thoracic segments they point to a reduction in the number of the tracheal gills. It is quite clear that we must suppose that the wings did not arise as such, but were developed from organs which had another function, such as the tracheal gills; I mean to say that such a supposition is necessary, for we cannot imagine that the wings functioned as such in the lower stages of their development, and that they could have been developed by having such a function.”

143

Fig. 158.—Changes in external form of the young larva of Calotermes rugosus, showing, in A and B, the mode of origin of the wing-pads: A, newly hatched, with 9 antennal joints, × 8. B, older larva, with 10 joints, × 8. C, next stage, with 11 joints, × 8. D, larva, with twelve joints; the position of the parts of the alimentary canal are shown: v, crop; m, stomach; b, “paunch”; e, intestine; r, heart, × 16
3
.—After Fritz Müller, from Sharp.

If we examine the tracheal gills of the smaller dragon-fly (Agrion), or the May-flies, or Sialidæ, or Perlidæ, or Phryganeidæ, we see that they are developed in a very arbitrary way, either at the end of the abdomen, or on the sternum, or from the pleurum; moreover, in structure they invariably have but a single trachea, from which minute twigs branch out;[30] in the wings there are five or six main tracheæ, which give rise to the veins. Thus, in themselves, irrespective of their position, they are not the homologues of the gills. The latter are only developed in the aquatic representatives of the Neuroptera and Pseudoneuroptera, and are evidently adaptive, secondary, temporary organs, and are in no sense ancestral, primitive structures from which the wings were developed. There is no good reason to suppose that the aquatic Odonata or Ephemerids or Neuroptera were not descendants of terrestrial forms.

To these results we had arrived by a review of the above-mentioned facts, before meeting with Fritz Müller’s opinions, derived from a study of the development of the wings of Calotermes (Fig. 158). Müller[31] states that “(1) The wings of insects have not originated from ‘tracheal gills.’ The wing-shaped continuations of the youngest larvæ are in fact the only parts in which air tubes are completely wanting, while tracheæ are richly developed in all other parts of the body.[32] (2) The wings of insects have arisen from 144lateral continuations of the dorsal plates of the body-segments with which they are connected.”

Now, speculating on the primary origin of wings, we need not suppose that they originated in any aquatic form, but in some ancestral land insect related to existing cockroaches and Termes. We may imagine that the tergites (or notum) of the two hinder segments of the thorax grew out laterally in some leaping and running insect; that the expansion became of use in aiding to support the body in its longer leaps, somewhat as the lateral expansions of the body aid the flying squirrel or certain lizards in supporting the body during their leaps. By natural selection these structures would be transmitted in an improved condition until they became flexible, i.e. attached by a rude hinge-joint to the tergal plates of the meso- and metathorax. Then by continued use and attempts at flight they would grow larger, until they would become permanent organs, though still rudimentary, as in many existing Orthoptera, such as certain Blattariæ and Pezotettix. By this time a fold or hinge having been established, small chitinous pieces enclosed in membrane would appear, until we should have a hinge flexible enough to allow the wing to be folded on the back, and also to have a flapping motion. A stray tracheal twig would naturally press or grow into the base of the new structure. After the trachea running towards the base of the wing had begun to send off branches into the rudimentary structure, the number and direction of the future veins would become determined on simple mechanical principles. The rudimentary structures beating the air would need to be strengthened on the front or costal edge. Here, then, would be developed the larger number of main veins, two or three close together, and parallel. These would be the costal, subcostal, and median veins. They would throw out branches to strengthen the costal edge, while the branches sent out to the outer and hinder edges of the wings might be less numerous and farther apart. The net-veined wings of Orthoptera and Pseudoneuroptera, as compared with the wings of Hymenoptera, show that the wings of net-veined insects were largely used for respiration as well as for flight, while in beetles and bees the leading function is flight, that of respiration being quite subordinate. The blood would then supply the parts, and thus respiration or aëration of the blood would be demanded. As soon as such expansions would be of even slight use to the insect as breathing organs, the question as to their permanency would be settled. Organs so useful both for flight and aëration of the blood would be still further developed, until they would become permanent structures, genuine wings. They would thus be readily transmitted, and being of more use in adult life during the season of reproduction, they would be still further developed, and thus those insects which could fly the best, i.e. which had the strongest wings, would be most successful in the struggle for existence. Thus also, not being so much needed in larval life before the reproductive organs are developed, they would not be transmitted except in a very rudimentary way, as perhaps masses of internal indifferent cells (imaginal discs), to the larva, being the rather destined to develop late in larval and in pupal life. Thus the development of the wings and of the generative organs would go hand in hand, and become organs of adult life.[33]

The development and structure of the tracheæ and veins of the wing.—The so-called veins (“nervures”) originate from fine tracheal 145twigs which pass into the imaginal discs. A single longitudinal trachea grows down into the wing-germ (Fig. 147), this branch arising through simple budding of the large body-trachea passing under the rudiment of the wing.

Fig. 159.—Germ of a hind wing detached from its insertion, and examined in glycerine: i, pedicel of insertion to the hypodermis; tr, trachea; b, semicircular pad; e, enveloping membrane; c, bundle of capillary tracheoles; the large tracheæ of the wing not visible; they follow the course of the bundles of tracheoles.—After Gonin.

Gonin states that before the tracheæ reach the wing they divide into a great number of capillary tubes united into bundles and often tangled. This mass of tracheæ does not penetrate into the wing-germ by one of its free ends, but spreading over about a third of the surface of the wing, separates into a dozen bundles which spread out fan-like in the interior of the wing. (Fig. 159). These ramifications, as seen under the microscope, are very irregular; they form here and there knots and anastomoses. They end abruptly in tufts at a little distance from the edge of the wing. A raised semicircular ridge (b) surrounds the base of the wing, and within this the capillaries are formed, while on the other side they are covered by a cellular layer.

Landois, he says, noticed neither the pedicel of the insertion of the wing (i) nor the ridge (b). Herold only states that the tracheæ pass like roots into the wing. Landois believed that they formed an integral part of it. Dewitz and Pancritius used sections to determine their situation.

Fig. 160 will illustrate Landois’ views as to the origin of the tracheæ and veins. A represents the germ of a hind wing attached 146to a trachea; c the elongated cells, in which, as seen at B, c, a fine tangled tracheal thread (t) appears, seen to be magnified at C. The cell walls break down, and the threads become those which pass through the centre of the veins.

Fig. 160.—Origin of the wings and their veins.—After Landois.

Fig. 161.-Section of the “rib” of a vein: c, cord; b, twig.—After Schaeffer.

The wing-rods.—Semper discovered in transverse sections of the wings, what he called Flügelrippen; one such rib accompanying the trachea in each vein. He did not discover its origin, and his description of it is said to be somewhat erroneous. Schaeffer has recently examined the structure, remarking: “I have surely observed the connection of this cellular tube with the tracheæ. It is found in the base of the wing where the lumen of the tracheæ is much widened. I only describe the fully formed rib (rippe). In a cross-section it forms a usually cylindrical tube which is covered by a very thin chitinous intima which bears delicate twigs (Fig. 161). These twigs are analogous to the thickened ridge of the tracheal intima. I can see no connection between the branches of the different twigs. Through the ribs (rippen) extend a central cord (c) which shows in longitudinal section a clear longitudinal streaking. Semper regarded it as a nerve. But the connection of the tube with the trachea contradicts this view. I can only regard the cord as a separation-product of the cells of the walls.”

Fig. 162.—Parts of a vein of the cockroach, showing the nerve (n) by the side of the trachea (tr); c, blood-corpuscles.—After Moseley.

Other histological elements.—These are the blood-lymph, corpuscles, blood-building masses, and nerves. Schaeffer states that in the immature pupal wings we find besides the large tracheæ, which are more or less branched, 147and in the wing-veins at a later period, blood-corpuscles which are more or less gorged with nutritive material, and also the “balls of granules” of Weismann, which are perhaps the “single fat-body cells” detected by Semper. Schaeffer also states that into the hypodermal fold of the rudiments of the wings pass peculiar formations of the fat-body and tracheal system, and connected with the fat-body are masses of small cells which by Schaeffer are regarded as blood-building masses.

Fine nerves have also been detected within the veins, Moseley stating that a nerve-fibre accompanies the trachea in all the larger veins in the insects he has examined (Fig. 162), while it is present in Melolontha, where the trachea is absent.

LITERATURE ON THE WINGS

Jurine, L. Nouvelle méthode de classer les Hyménoptères et les Dipterès. Genève, 1807, 4º pp. 319, 14 Pls.

—— Observations sur les ailes des Hyménoptères. (Mém. acad. Turin, 1820, xxiv, pp. 177–214.)

Latreille, P. A. De la formation des ailes des Insectes. (Mém. sur divers sujets de l’histoire naturelle des Insectes, etc. Paris, 1819. Fasc. 8.)

—— De quelques appendices particuliers du thorax de divers Insectes. (Mém. du Mus. d’Hist. nat., 1821, vii, pp. 1–21, 354–363.)

Chabrier, J. Essai sur le vol des insectes. (Mém. du Mus. d’Hist. nat., 1820, vi, pp. 410–476; 1821, vii, pp. 297–372; 1822, viii, pp. 47–99, 349–403.) Separate, pp. 328, 13 Pls.

Burmeister, Hermann. Handbuch der entomologie, i, 1832, pp. 96–106, 263–267, 494–505.

—— Untersuchungen über die Flügeltypen der Coleopteren. (Abhandl. d. naturf. Ges. Halle, 1854, ii, pp. 125–140, 1 Taf.)

Romand, B. E. de. Tableau de l’aile supérieure des Hyménoptères, 1 Pl. Paris, 1839. (Revue Zool., ii, pp. 339; Bericht von Erichson für 1839, pp. 54–56.)

Lefebure, A. Communication verbale sur la ptérologie des Lépidoptères. (Annal. Soc. Ent. France, 1842, i, pp. 5–35, 3 Pls. Also Revue Zool. Paris, 1842, pp. 52–58, 1 Pl.)

Deschamps, B. Recherches microscopiques sur l’organisation des élytres des Coléoptères. (Ann. sc. nat., sér. 3, iii, 1845, pp. 354–363.)

Heer, Oswald. Die Insektenfauna der Tertiärgebilde von Oeningen und Radaboj., 1847, 1. Teil, pp. 75–94.

Newman, E. Memorandum on the wing-rays of insects. (Trans. Ent. Soc. London, ser. 2, iii, 1855, pp. 225–231.)

Westwood, J. O. Notes on the wing-veins of insects. (Trans. Ent. Soc. London, ser. 2, iv, 1857, pp. 60–64.)

Loew, H. Die Schwinger der Dipteren. (Berlin, Entom. Zeitschr., 1858, pp. 225–230.)

Saussure, H. de. Études sur l’aile des Orthoptères. (Ann. scienc. nat., 5 sér. x, p. 161.)

Schiner, J. R. Ueber das Flügelgeäder der Dipteren. (Verhdl. k. k. Zool.-bot., Ges. Wien, 1864, pp. 193–200, 1 Taf.)

148Hagen, H. A. Ueber rationelle Benennung des Geäders in den Flügeln der Insekten. (Stettin. Ent. Zeitung, 1870, xxxi, pp. 316–320, 1 Taf.)

—— Kurze Bemerkungen über das Flügelgeäder der Insekten. (Wiener Entom. Zeit., 1886, v, pp. 311, 312.)

Plateau, F. Qu’est-ce que l’aile d’un insecte? (Stett. Ent. Zeit. Jahrg. 32, 1871, pp. 33–42, 1 Taf. Journal d. Zool., ii, 1873, pp. 126–137.)

Moseley, H. N. On the circulation in the wing of Blatta orientalis and other insects, etc. (Quart. Journ. Micr. Sc. 1871, xi, pp. 389–395, 1 Pl.)

Roger, Otto. Das Flügelgeäder der Kafer. Erlangen, 1875, 90 p.

Rade, E. Die westfalischen Donacien und ihre nachsten Verwandten. 3 Taf. (Vierter Jahresber. d. Westfal Prov.-Vereins f. Wiss. u. Kunst, 1876, pp. 52–87; Flügel, pp. 61–68.)

Katter, F. Ueber Inseckten, speziell Schmetterlingsflügel. (Entom. Nachr., iv, 1878, pp. 279–281, 293–298, 304–309, 321–323.)

Hofmann, Georg v. Ueber die morphologische Deutung der Insektenflügel. (Jahresber. d. akad.-naturwiss. Vereins, Graz, v Jahrg., 1879, pp. 63–68.)

Kolbe, H. J. Das Flügelgeäder der Psociden und seine systematische Bedeutung. (Stettin. Entom. Zeitung, 1880, pp. 179–186, 1 Taf.)

—— Die Zwischenraume zwischen den Punktstreifen der punktiertgestreiften Flügeldecken der Coleoptera als rudimentare Rippen aufgefasst. (Jahresber. zool. Sektion d. Westfal. Prov.-Ver. f. Wiss. u. Kunst. Münster, 1886, pp. 57–59, 1 Taf.)

Lee, A. Bolles. Les balanciers des Diptères, leurs organes sensifères et leurs histologie. (Recueil Zool. Suisse, i, 1885, pp. 363–392, 1 Pl.)

Poppius, Alfred. Ueber das Flügelgeäder der finnischen Dendrometriden. 1 Taf. (Berl. Entom. Zeitschr., 1888, pp. 17–28.)

Comstock, J. H. On the homologies of the wing-veins of insects. (American Naturalist, xxi, 1887, pp. 932–934.)

Brauer, F. Ansichten über die paläozoischen Insekten und deren Deutung. (Annal. d. k. k. naturhist. Mus. Wien, Bd. i, 1886, pp. 86–126, 2 Taf.)

Brauer, F., und J. Redtenbacher. Ein Beitrag zur Entwicklung des Flügelgeäders der Insekten. (Zool. Anz. 1888, pp. 443–447.)

Redtenbacher, J. Vergleichende Studien über das Flügelgeäder der Insekten. 12 Taf. (Annalen d. k. k. naturhist. Hofmuseums zu Wien, 1886, i, pp. 153–231.)

Schoch, G. Miscellanea entomologica. I. Das Geäder des Insektenflügels; II. Prolegomena zur Fauna dipterorum Helvetiae, Wissenschaftl. (Beilage z. Programm d. Kantonsschule Zurich, 1889, 4º, 40 p.)

Bondsdorff, A. von. Ueber die Ableitung der Skulpturverhältnisse bei den Deckflügeln der Coleopteren. (Zool. Anz., 1890, xiii Jahrg., pp. 342–346.)

Spuler, Arnold. Zur Phylogenie und Ontogenie des Flügelgeäders der Schmetterlinge. (Zeitschr. wissens. Zool., liii, 597–646, 2 Taf., 1892.)

Also the writings of Adolph, Bugnion, Calvert, Comstock, Diez, Giraud, Gonin, Graber, Kellogg, Packard, Pratt, Scudder, Walsh.

g. Mechanism of flight

Marey’s views on the flight of insects.—As we owe more to Marey than to any one else for what exact knowledge we have of the theory of flight of insects, the following account is condensed from his work entitled “Movement.” The exceedingly complicated 149movements of the wings would lead us, he says, to suppose that there exists in insects a very complex set of muscles of flight, but in reality, he claims, there are only the two elevator and depressor muscles of each wing.[34] And Marey says that when we examine more closely the mechanical conditions of the flight of insects, we see that an upward and downward motion given by the muscles is sufficient to produce all these successive acts, so well coordinated with each other; the resistance of the air effecting all the other movements. He also refers to the experiments of Giraud which prove that the insect needs for flight a rigid main-rib and a flexible membrane.

Fig. 163.—The two upper lines are produced by the contacts of a drone’s wing on a smoked cylinder. In the middle are recorded the vibrations of a tuning-fork (250 vibrations per second) for comparison with the frequency of the wing movements. Below are seen the movements of the wing of a bee.—After Marey.

If we take off the wing of an insect, and holding it by the small joint which connects it with the thorax, expose it to a current of air, we see that the plane of the wing is inclined more and more as it is subjected to a more powerful impulse of the wind. The anterior nervure resists, but the membranous portion which is prolonged behind bends on account of its greater pliancy.

The wings of insects may be regarded simply as vibrating wires, and hence the frequency of their movements can be calculated by the note produced. Their movements can be recorded directly on a revolving cylinder, previously blackened with smoke, the slightest touch of the tip of the wing removing the black and exposing the white paper beneath; Fig. 163 was obtained in this way. By this method it was calculated that in the common fly the wings made 330 strokes per second, the bee 190, the Macroglossus 72, the dragon-fly 15028, and the butterfly (Pieris rapæ) 9. Thus the smaller the species, the more rapid are the movements of the wings.

Fig. 164.—Appearance of a wasp flying in the sun: the extremity of the wing is gilded.—After Marey.

The path or trajectory made by the tip of the wing is like a figure 8. Marey obtained this by fastening a spangle of gold-leaf to the extremity of a wasp’s wing. The insect was then seized with a pair of forceps and held in the sun in front of a dark background, the luminous trajectory shaping itself in the form of a lemniscate (Fig. 164).

To determine with accuracy the direction taken by the wing at different stages of the trajectory, a small piece of capillary glass tubing was blackened in the smoke of a candle, so that the slightest touch on the glass was sufficient to remove the black coating and show the direction of movement in each limb of the lemniscate. This experiment was arranged as shown in Fig. 165. Different points on the path of movement were tested by the smoked rod, and from the track along which the black had been removed the direction of movement was deduced. This direction is represented in the figure by means of arrows.

Fig. 165.—Experiment to test the direction of movement of an insect’s wing: a, a′, b, b′, different positions of the smoked rod.

Theory of insect flight.—“The theory of insect flight,” says Marey, “may be completely explained from the preceding experiments. The wing, in its to-and-fro movement, is bent in various directions by the resistance of the air. Its action is always that of an inclined plane striking against a fluid and utilizing that part of the resistance which is favorable to its onward progression.

“This mechanism is the same as that of a waterman’s scull, which as it moves backwards and forwards is obliquely inclined in opposite directions, each time communicating an impulse to the boat.”

The mechanism in the case of the insect’s wing is far simpler, 151however, than in the process of sculling, since “the flexible membrane which constitutes the anterior part of the wing presents a rigid border, which enables the wing to incline itself at the most favorable angle.”

“The muscles only maintain the to-and-fro movement, the resistance of the air does the rest, namely, effects those changes in surface obliquity which determine the formation of an 8–shaped trajectory by the extremity of the wing.”

Fig. 166.—Bee flying about in the chamber of the apparatus.—After Marey.

Lendenfeld has applied photography to determine the position of the wings of a dragon-fly, and Marey has carried chronophotography farther to indicate the normal trajectory of the wing, and to show the position in flight. Fig. 166 shows a bee in various phases of flight. “The insect sometimes assumes almost a horizontal position, in which case the lower part of its body is much nearer the object-glass than is its head, and yet both extremities are equally well defined in the photograph. The successive images are separated by an interval of 1
20
of a second (a long time when compared to the total time occupied by a complete wing movement, i.e. 1
190
of a second). And hence it is useless to attempt to gain a knowledge of the successive phases of movement by examining the successive photographs of a consecutive series representing an insect in flight. Nevertheless an examination of isolated images affords information of extreme interest with regard to the mechanism of flight.

“We have seen that owing to the resistance of the air the expanse of wing is distorted in various directions by atmospheric resistance. Now, as the oscillations during flight are executed in a horizontal plane, the obliquity of the wing-surface ought to diminish the apparent breadth of the wing. This appearance can be seen in Fig. 167. There is here a comparison between two Tipulæ: the one in the act of flight, the other perfectly motionless and resting against the glass window.

152

Fig. 167.—Illustration to show two Tipulæ, one of them remaining motionless on the glass, and the other moving its limbs in different directions, and setting its body at various inclinations: the illustration only represents a small part of a long series.—After Marey.

“The motionless insect maintains its wings in a position of vertical extension; the plane is therefore at right angles to the axis of the object-glass. The breadth of the wing can be seen in its entirety; the nervures can be counted, and the rounding off of the extremities of the wings is perfectly obvious. On the other hand, the flying insect moves its wings in a horizontal direction, and owing to the resistance of the air the expanse of the wings is obliquely disposed, and only the projection of its surface can be seen in the photograph. This is why the extremity of the wings appears as if it were pointed, while the other parts look much narrower than normal. The extent of the obliquity can be measured from the apparent alteration in width, for the projection of this plane with the vertical is the sine of the angle. From this it may be gathered that the right wing (Fig. 168, third image) was inclined at an angle of about 50° with the vertical, say 40° with the horizontal. This inclination necessarily varies at different points of the trajectory and must augment with the rapidity 153of movement; the obliquity reaching its maximum in those portions of the wings which move with the greatest velocity, namely, towards the extremities. The result is that the wing becomes twisted at certain periods of the movement.” (See the fourth image in Fig. 168.) The position of the balancers seems to vary according to that of the wings. (Marey’s Movement, pp. 253–257.)

Fig. 168.—Tipula in the act of flying, showing the various attitudes of the wings and the position of the balancers.

Graber’s views as to the mechanism of the wings, flight, etc.—Although in reality insects possess but four wings, nature, says Graber, evidently endeavors to make them dipteral. This end is attained in a twofold manner. In the butterflies, bees, and cicadas, the four wings never act independently of each other, as two individual pairs, but they are always joined to a single flying plate by means of peculiar hooks, rows of claws, grooved clamps, and similar contrivances proceeding from the modified edges of the wings; indeed, this connection is usually carried so far that the hind wings are entirely taken in tow by the front, and consequently possess a relatively weak mechanism of motion. The other mode of wing reduction consists in the fact that one pair is thrown entirely out of employment. We observe this for instance in bugs, beetles, grasshoppers, etc.

In the meantime, then, we may not trust to appearances. As their development indeed teaches us, the wings as well as the additional members must be regarded as actual evaginations of the common sockets of the body, and in order especially to refute the prevalent opinion that these wing-membranes are void of sensation, it should be remembered that Leydig has proved the existence, as well as one can be convinced by experiment, of a nerve-end apparatus in certain basal or radical veins 154of the wing-membrane, which is very extensive and complicated, and therefore indicates the performance of an important function, perhaps of a kind of balancing sense, and also that these same insect wings, with their delicate membrane, are very easily affected by different outside agents, as, for instance, warmth, currents of air, etc.

Usually in their inactive or passive state the wings are held off horizontally from the body during flight, and are laid upon the back again when the insect alights; but an exception occurs in most butterflies and Neuroptera, among which the wing-joint allows only one movement round the oblique and long axis of the wings. From this cause, too, the insects just mentioned can unfold their wings suddenly.

Fig. 169.—Anterior part of a Cicada for demonstrating the mechanism of the articulation of the fore wing: a, articular head; b, articular pan, frog, or cotyla; g, elastic band; c, d, e, system of elastic rods; r1, r2, 1st and 2d abdominal segments. HF, hind wings.—After Graber.

The transition of the wings from the active to the resting condition seems to be by way of a purely passive process, which, therefore, usually gives no trouble to the insect. The wing being extended by the tractive power of the muscles, flies back, when this ceases, to its former or resting posture by means of its natural elasticity, like a spiral spring disturbed from its balance. The structure of this spring joint is very different, however.

It usually consists (Fig. 169) of two parts. The wing can move itself up and down in a vertical plane by means of the forward joint, and at the same time can rotate somewhat round its long axis, because the chitinous part mentioned above is ground off after the fashion of a mandrel.

The hinder joint, at a greater distance from the body, virtually consists of a rounded piece (a) capitate towards the outside, and of a prettily hollowed socket (b) formed by the union of the thick ribs of the hind wings, which slides round 155the head joint when the wings snap back upon the back. The mechanism which causes this turning is, however, of a somewhat complicated nature. The most instrumental part of it is the powerful elastic band (g) which is stretched over from the hinder edge of the mesothorax (R2) towards that of the wings. This membrane is extended by the expansion of the wings, and draws them towards the body as soon as the contraction of the muscles relaxes. This closing band of the wings is assisted by a leverage system consisting of three little chitinous rods (c, d, e), which at its joining presses inwards on the body on one side, and on the hinder edge and head-joint of the wing on the other.

We must, however, lay great stress on a few more kinds of wing support.

Fig. 170.—Mesothoracic skeleton of a stag beetle: schi, scutellum, on each side of which is the articulation of the fore wing (V), consisting of two small styliform processes (v, h) of the base of the wing; za, tooth which fits into the cavity of the wing-lock (gr); l, edge of the right wing, passing into the corresponding groove (fa) of the left; Di, diaphragm for the attachment of the tergal muscle of the metasternum; Di1 (not explained by author); Ka, acetabulum of the coxa (); Se, chitinous process for the attachment of the coxal muscle; Fe, femur; Sch, tibia; B2, sternum.—After Graber.

The wing-cases of beetles at their return from flight are joined together like the shells of a mussel on the inside as well as to the wedge-shaped plate (Fig. 170, schi) between their bases. There is even a kind of clasp at hand for this purpose. The base of the wing, that is, bears a pair of tooth-like projections (za), which fit into the corresponding hollows of the little plate.

The commissure arising from the joining of the inner edges is characteristic. Usually the wings on both sides interlock by means of a groove, as in stag-beetles, but sometimes even, as in Chlamys, after the manner of two cog-wheels, so that we have here also an imitation of the two most prevalent methods which the cabinet-maker uses in joining boards together.

The act of folding the broad hind wings among beetles is not less significant than the arrangement of the fore wing. If we forcibly spread out the former in a beetle which has just been killed and then leave it to its own resources again, we observe the following result: According to its peculiar mode of joining, the costal vein on the fore edge approaches the mid or discoidal vein of the basal half as well as the distal half of the wing, whence arises a longitudinal fold which curves in underneath. Then the distal half snaps under like the blade of a pocket knife and lies on the plane of the costal edge of the wing, while it also draws after it the neighboring wing-area. The soft hinder-edge portion turns in simultaneously when this wing-area remains fixed to the body while the costal portion is moving towards the middle line of the body.

The wing-membranes of almost all insects have, moreover, the capability of folding themselves somewhat, and this power of extending or contracting the wing-membrane at will is of great importance in flight.

Yes, but how is the folded wing spread out again? The fact may be shown more simply and easily than one might suppose, and may be most plainly demonstrated even to a larger public by making an artificial wing exactly after the pattern of the natural one, in which bits of whalebone may take the place 156of veins and a piece of india rubber the membrane spread out between them. The reader will be patient while we just explain to him the act of unfolding of the membranous wing of the beetle. The actual impulse for this unfolding is due to the flexor muscles which pull on, and at the same time somewhat raise the vein on the costal edge. By this means the membranous fold lying directly behind the costal vein is first spread out. But since this fold is connected with the longitudinal fold of the distal end of the wing which closes like a blade, the wing-area last mentioned which is attached to the middle fold of the wing by the elastic spring-like diagonal vein becomes stretched out. The hinder rayed portion adjacent to the body is, on the other hand, simply drawn along when the wing stands off from the body.

In order to properly grasp the mechanism of the insect wing we must again examine its mode of articulation to the body somewhat more accurately.

Fig. 171.—Longitudinal section through a Tipula: a, mouth; an, antenna; k3, maxillary palpus; ol, labrum; oG, brain; uG, subœsophageal ganglion; BG, thoracic ganglion; schl, œsophagus; mD, digestive canal; Ov, ovary; vF, fore wing; sch, halter; lm, longitudinal—b-r, lateral muscles.—After Graber.

If we select the halteres of a garden gnat (Tipula) at the moment of extension, we shall find them to be formed almost exactly after the pattern of our oars, since the oblong oar-blade passes into a longitudinal handle. The pedicel of the balancer is formed by the thick longitudinal primary veins of the wing-membrane. This pedicel (Fig. 171) is implanted in the side of the thorax in such a manner that the wing may be compared to the top of a ninepin. One may think, and on the whole it is actually the fact, that the stiff pedicel of the wing is inserted in the thoracic wall, and that a short portion of it (Fig. 172), projects into the cavity of the thorax. It is true there is no actual hole to be found in the thoracic wall, as the intermediate 157space between the base or pedicel of the wing and the aperture in the thorax is lined with a thin yielding membrane, on which the wing is suspended as on an axle-tree. According to this, therefore, the insect wing, as well as any other appendage of arthropods, acts as a lever with two arms. The reader can then conjecture what may be the further mechanism of the wing machine. We only need now two muscles diametrically opposed to each other and seizing on the power arm of the wing, one of which pulls down the short wing arm, thereby raising the oar, while the other pulls up the power arm. And indeed the raising of the wing follows in the manner indicated, since a muscle (hi) is attached to the end of the wing-handle (a) which projects freely into the breast cavity by the contraction of which the power arm is drawn down.

Fig. 172.—Scheme of the flying apparatus of an insect: mnl, thoracic walls; ab, wings; c, pivot; d, point of insertion of the depressor muscle of the wing (kd);—a, that of the elevator of the wing (ai); rs, muscle for expanding,—ml, for contracting, the walls of the thorax.—After Graber.

Fig. 173.—Muscles of the fore wing of a dragon-fly (an, ax), exposed by removing the thoracic walls: h1, h2, elevators,—s1-s5, depressors, of the wings (s1, s2, rotators).—After Graber.

On the other hand, we have been entirely mistaken in reference to the mechanism which lowers the wings. The muscle concerned, that is kd, is not at all the antagonist of the elevator muscle of the wing, since it is placed close by this latter, but nearer to the thoracic wall. But then, how does it come to be the counterpart of its neighbor? In fact, the lever of the wing is situated in the projecting piece alone. The extensor muscle of the wing does not pull on the power arm, but on the resistant arm on the other side of the fulcrum (c). The illustration shows, however, how such a case is possible. The membrane of the joint fastening the wing-stalk to the thorax is turned up outwards below the stalk like a pouch. The tendon of the flexor of the wing passes through this pouch to its point of attachment (c) lying on the other side of the fulcrum (d). Thus it is very simply explained how two muscles which act in the same direction can nevertheless have an entirely contrary working power.

This is in a way the bare physical scheme of the flying machine by the help of which we shall more easily become acquainted with its further details.

Dragon-flies are unquestionably the most suitable objects for the study of the muscles pulling directly on the wing itself. If the lateral thoracic wall (Fig. 173) be removed or the thorax opened lengthwise there appears a whole storehouse of muscular cords which are spread out in an oblique direction between the base of the wing and the side of the thoracic plate. There is first to be ascertained, by the experiment of pulling the individual muscles in the line with a pincers, which ones serve for the lifting and which for the lowering 158of the wings. In dragon-flies the muscles are arranged in two rows and in such a way that the flexors or depressors (s, 1 bis) cling directly to the thoracic wall (compare also the muscle dk in Fig. 172 and se in Fig. 174), while the raiser or extensor (h 1, to h 2, Fig. 172, hi and Fig. 174 he) lie farther in. The form of the wing-muscles is sometimes cylindrical, sometimes like a prism, or even ribbon-like. However, the contracted bundles of fibres do not come directly upon the joint-process we have described, but pass over often indeed at a very considerable distance from them, into peculiar chitinous tendons. These have the form of a cap-like plate, often serrate on the edge, which is prolonged into a thread, which should be considered as the direct continuation of the base of the wings. The wings, therefore, sink down into the thoracic cavity as if they were a row of cords ending in handles where the strain of the muscles is applied.

Fig. 174.—Transverse section through the thorax of a locust (Stenobothrus): b1, leg; h, heart; ga, ventral cord; se, depressor,—he, elevator, of the wing (fl); b-r, lateral muscles which expand the thoracic walls;—lm, longitudinal muscles which contract them; shm, uhm, muscles to the legs; bg, apodemes.—After Graber.

Fig. 175.—Inner view of a portion of the left side of body of Libellula depressa, showing a part of the mechanism of flight, viz., some of the chitinous ridges at base of the upper wing, and some of the insertions of the tendons of muscles: A, line of section through the base of the upper wing, the wing being supposed to be directed backwards. C, upper portion of mechanism of the lower wing; b, lever extending between the pieces connected with the two wings.—After von Lendenfeld, from Sharp.

As may be seen in Fig. 173, the contractile section of several of the muscles of the wing (s5) is extraordinarily reduced, while its thread-like tendon is proportionately longer. This gradation being almost like that of the pipes of an organ in the length of the wing-muscles, as may so easily be observed in the large dragon-flies, plainly indicates that the strain of the individual muscles is quite different in strength, since, as the phenomenon of flight demands it, the different parts of the base of the wing become respectively relaxed in very dissimilar measure.

We have thus far discussed only the elevator and depressor muscles. Other groups (s1s3) are yet to be added, however, crossing under the first at acute 159angles, which when pulling the wing sidewise, bring about in union with the other muscles a screw-like turning of the wings.

While in dragon-flies all the muscles which are principally influential in moving the wing are directly attached to it, and thus evidently assert their strength most advantageously, the case is essentially different with all other insects. Here, as has already been superficially mentioned above, the entire set of muscles affecting the wing is analyzed into two parts of which the smaller only is usually directly joined to the wings, while the movement is indirectly influenced by the remainder (Graber).

In the dragon-fly the two wings are “brought into correlative action by means of a lever of unusual length existing amongst the chitinous pieces in the body wall at the base of the wings (Fig. 175, b). The wing-muscles are large; according to von Lendenfeld there are three elevator, five depressor, and one abductor muscles to each wing. He describes the wing-movements as the results of the correlative action of numerous muscles and ligaments, and of a great number of chitinous pieces connected in a jointed manner” (Sharp).

If again we take the longitudinal section of the thoracic cavity of gnats in Fig. 171, we shall perceive a compactly closed system of muscular bars intersecting each other almost at right angles and interlaced with a tangled mass of tracheæ, some of which muscles extend (lm) longitudinally, that is from the front to the back, while others (b-r) stretch out in a vertical direction, that is between the plates of the abdomen and back.

In order that we may more easily comprehend this important muscular apparatus we will illustrate the thoracic cavity of insects by an elastic steel ring (Fig. 172), to which we may affix artificial wings. If this ring be pressed together from above downward, along the line rs, thus imitating the pulling of the vertical or lateral thoracic muscles, then the wings on both sides spring up. This is to be explained by the fact that through this manipulation a pressure is exerted on the lifting power arm of the wings. If, on the other hand, the ring be compressed on the sides (ml), which is the same thing as if the longitudinal muscles contracted the thorax from before backward, and thus arched it more, then the wings are lowered.

Agrioninæ, according to Kolbe, can fly with the fore pair of wings or with the hind pair almost as well as with both pairs together. Also the wings of these insects can be cut off before the middle of their length without injuring their power of flight. Butterflies, Catocalæ, and Bombycidæ fly after the removal of the hind wings. Also the balancers of the Diptera must be useful in flying, since their removal lessens the power of flight.

Chabrier regarded the under sides of the shell-like extended wing-covers of the beetles as wind-catchers, which, seized by wind currents, carry the insect through the air. We may also consider the wing-covers as regulators of the centre of gravity of flight.

The observations of insects made by Poujade (Ann. Soc. Ent., France, 1887, p. 197) during flight teaches us, says Kolbe, that in respect to the movement during flight of both pairs of wings, they may be divided into two categories:—

1. Into those where both pairs of wings (together), either united, and also when separated from each other, perform flight. Such are the Libellulidæ, Perlidæ, Sialidæ, Hemerobiidæ, Mymeleonidæ, Acridiidæ, Locustidæ, Blattidæ, Termitidæ, etc.

2. Into those whose fore and hind wings act together like one wing, since they are connected by hooks (hamuli), as in certain Hymenoptera, or are attached in other ways. Here belong Hymenoptera, Lepidoptera, Trichoptera, Cicadidæ, Psocidæ, etc.

160The musculature of the mesothorax and metathorax is similar in those insects both of whose pairs of wings are like each other, and act independently during flight, viz. in the Libellulidæ. On the other hand, in the second category, where the fore and hind wings act as a single pair and the fore wings are mostly larger than the hinder (except in most of the Trichoptera), the musculature of the mesothorax is more developed than that of the metathorax.

To neither category belong the beetles, whose wing-covers are peculiar organs of flight, and not for direct use, and the Diptera, which possess but a single pair of wings. In the beetles the hind wings, in the Diptera the fore wings, serve especially as organs of flight. It may be observed that the Diptera are the best fliers, and that those insects which use both pairs of wings as a single pair fly better than those insects whose two pairs of wings work independently of each other. An exception are the swift-flying Libellulidæ, whose specially formed muscles of flight explain their unusual capabilities for flying (Kolbe).

LITERATURE ON FLIGHT

Marey, E. J. La machine animale. Locomotion terrestre et aërienne. Paris, 1874.

—— Mémoire sur le vol des insectes et des oiseaux. (Annal. Scienc. natur., 5 sér., Zool. xii, 1869, pp. 49–150; 5 sér., Zool. xv, 1872, 42 Figs.)

—— Note sur le vol des insectes. (Compt. rend. et Mém. Soc. d. Biol. Paris, 4 sér., v, 1869, C. R. pp. 136–139.)

—— Recherches sur le mécanisme du vol des insectes. (Journal de l’Anatomie et de la Physiologie, 6 Année, 1869, pp. 19–36, 337–348.)

—— Animal mechanism. New York, 1879, pp. 180–209.

—— Movement. New York, 1895, pp. 239–274.

Hartings. Ueber den Flug. (Niederland. Archiv f. Zoologie, iv, Leiden, 1877–78.)

Lucy. Le vol des oiseaux, chauvesouris et insectes. Paris.

Tatin, V. Expériences physiologiques et synthétiques sur le mécanisme du vol. (Ecole prat. d. haut. étud. Physiol. expérim. Trav. du laborat. de Marey, 1877, pp. 293–302.)

—— Expériences sur le vol mécanique. (Ibid., 1876, pp. 87–108.)

Bellesme, Jousset de. Recherches expérimentales sur les fonctions du balancier chez les insectes Diptères, Paris, 1878, 96 pp., Figs.

—— Sur une fonction de direction dans le vol des insectes. (Compt. rend., lxxxix, 1879, pp. 980–983.)

Pettigrew, J. Bell. On the mechanical appliances by which flight is attained in the animal kingdom. (Trans. Linn. Soc., 1868, xxvi, Pt. I, pp. 197–277, 4 Pl.)

—— On the physiology of wings. (Trans. Roy. Soc. Edinburgh, 1871, xxvi, pp. 321–446.)

Krarup-hansen, C. J. L. Beitrag zu einer Theorie des Fluges der Vogel, Insekten und Fledermause. (Copenhagen u. Leipzig, Fritsch, 1869, 48 pp.)

Lendenfeld, R. V. Der Flug der Libellen. (Sitzungsber. d. kais. Akad. d. Wiss. Wien, lxxxiii, 1881, pp. 289–376, 7 Taf.; Zool. Anz., 1880, p. 82.)

Girard, M. Note sur diverses expériences relatives à la fonction des ailes chez les insectes. (Ann. Soc. Ent. France, 4 sér., ii, 1862, pp. 154–162.)

Mühlhäuser, F. A. Ueber das Fliegen der Insekten. (22. bis 24., Jahresb. d. Pollichia, Dürkheim, 1866, pp. 37–42.)

Plateau, Félix. Recherches expérimentales sur la position du centre de gravité chez les insectes. (Archiv d. Scienc. phys. et natur. d. Genève, Nouv. période, xliii, 1872, pp. 5–37.)

161Plateau, Félix. Ueber die Lage des Schwerpunktes bei den Insekten. Auszug. (Naturforscher v. Sklarek, v. Jahrg., 1872, pp. 112–113.)

—— Recherches physico-chimiques sur les articulés aquatiques. (Bull. d. l’Acad. Roy. Belg., xxxiv, 1872, pp. 1–50, Fig.)

—— Qu’est-ce que l’aile d’un insecte? (Stett. Ent. Zeit., 1871, pp. 33–42, Pl.)

—— L’aile des insectes. (Journ. d. Zool., ii, 1873, pp. 126–137.)

Perez, J. Sur les causes de bourdonnement chez les insectes. (Comptes rend., lxxxvii, p. 535, Paris, 1878.)

Strasser, Hans. Mechanik des Fluges. (Archiv f. Anat. u. Phys., 1878, p. 310–350, 1 Taf.)

—— Ueber die Grundbedingungen der aktiven Locomotion. (Abhandl. d. naturf. Gesellsch., Halle, 1880, xv, pp. 121–196, Figs.)

Moleyre, L. Recherches sur les organes du vol chez les insectes de l’ordre des Hemiptères. (Compt. rend. de l’Acad. d. Scienc. de Paris, 1882, xcv, pp. 349–352.)

Amans, P. Essai sur le vol des insectes. (Revue d. Sc. Nat. Montpellier, 3 sér., ii, 1883, pp. 469–490, 2 Pl.; iii, 1884, pp. 121–139, 3 Pl.)

—— Étude de l’organe du vol chez les Hyménoptères. (Ibid., iii, pp. 485–522, 2 Pl.)

—— Comparaisons des organes du vol dans la série animale. Des organes du vol chez les insectes. (Annal. d. Scienc. nat. Zool., 6 sér., xix, pp. 1–222, 8 Pl.)

Mullenhoff, K. Die Grosse der Flugflächen. (Pflüger’s Archiv f. d. ges. Physiologie, 1884, xxxv, pp. 407–453.)

—— Die Ortsbewegungen der Tiere. (Wissensch. Beil. z. Programm d. Andreas-Realgymnas. Berlin, 1885, 19 pp.)

Poujade, G. A. Note sur les attitudes des insectes pendant le vol. (Ann. Soc. Ent. France, 1884, 6 sér., iv, pp. 197–200, 1 Pl.)

Krancher, O. Die Töne der Flügelschwingungen unserer Honigbiene. (Deutscher Bienenfreund, 1882, 18. Jahrg., pp. 197–204.)

Landois, H. Ueber das Flugvermögen der Insekten. (Natur. und Offenbarung, vi, 1860, pp. 529–540.)

Ungern-Sternberg, von. Betrachtungen über die Gesetze des Fluges. (Zeitschr. d. Deutschen Vereins z. Förderung d. Luftschiffahrt-Naturwissensch. Wochenschrift v. Potonie, iv, 1889, p. 158.)

Baudelot, E. Du mecanisme suivant lequel s’effectue chez les Coléoptères le retract des ailes inférieures sous les élytres au moment du passage a l’état de repos. (Bull. Soc. d. Scienc. nat., Strasbourg, 1 Année, 1868, pp. 137–138.)

Ris, Fr. Die schweizerischen Libellen. Schaffhausen, 1885. (Beiheft der Mitteil. d. Schweiz. Ent. Ges., vii, pp. 35–84.)

162

THE ABDOMEN AND ITS APPENDAGES

Fig. 176.—Abdomen of Termes flavipes: 1–10, the ten tergites; 1–9, the nine urites; c, cercopod.

Fig. 177.—End of abdomen of Panorpa debilis drawn out, the chitinous pieces shaded: L, lateral, D, dorsal view; c, jointed cercopoda.—Gissler del.

In the abdomen the segments are more equally developed than elsewhere, retaining the simple annular shape of embryonic life, and from their generalized nature their number can be readily distinguished (Fig. 176). The tergal and sternal pieces of each segment are of nearly the same size, the tergal often overlapping the sternal (though in the Coleoptera the sternites are larger than the tergites), while there are no pleural pieces, the lateral region being membranous when visible and bearing the stigmata (Fig. 177, L). In the terminal segments beyond the genital outlet, however, there is a reduction in and loss of segments, especially in the adults of the metabolous orders, notably the Panorpidæ (Fig. 177), Diptera, and aculeate Hymenoptera; in the Chrysididæ only three or four being usually visible, the distal segments being reduced and telescoped inward.

The typical number of abdominal segments (uromeres), i.e. that occurring in each order of insects, is ten; and in certain families of Orthoptera, eleven. In the embryos, however, of the most generalized winged orders, Orthoptera (Fig. 199), Dermaptera, and Odonata, eleven can be seen, while Heymons has recently detected twelve in blattid and Forficula embryos, and he claims that in the nymphs of certain Odonata there are twelve segments, the twelfth being 163represented by the anal or lateral plates. It thus appears that even in the embryo condition of the more generalized winged insects, the number of uromeres is slightly variable.

We have designated the abdomen as the urosome; the abdominal segments of insects and other Arthropods as uromeres, and the sternal sclerites as urosternites, farther condensed into urites. (See Third Report U. S. Entomological Commission, 1883, pp. 307, 324, 435, etc.)

Fig. 178.—Nymph of the pear tree Psylla, with its glandular hairs.—After Slingerland. Bull. Div. Ent. U. S. Dep. Agr.

The reduction takes place at the end of the abdomen, and is usually correlated with the presence or absence of the ovipositor. In the more generalized insects, as the cockroaches, the tenth segment is, in the female, completely aborted, the ventral plate being atrophied, while the dorsal plate is fused during embryonic life, as Cholodkowsky has shown, with the ninth tergite, thus forming the suranal plate.

In the advanced nymph of Psylla the hinder segments of the abdomen appear to be fused together, the traces of segmentation being obliterated, though the segments are free in the first stage and in the imago (Fig. 178). It thus recalls the abdomen of spiders, of Limulus, and the pygidium of trilobites.

The median segment.—There has been in the past much discussion as to the nature of the first abdominal segment, which, in those Hymenoptera exclusive of the phytophagous families, forms a part of the thorax, so that the latter in reality consists of four segments, what appearing to be the first abdominal segment being in reality the second.

Latreille and also Audouin considered it as the basal segment of the abdomen, the former calling it the “segment médiaire,” while Newman termed it the “propodeum.” This view was afterward held by Newport, Schiödte, Reinhard, and by the writer, as well as Osten-Sacken, Brauer, and others. The first author to attempt to prove this by a study of the transformations was Newport in 1839 (article “Insecta”). He states that while the body of the larva is in general composed of thirteen distinct segments, counting the head as the first, “the second, third, fourth, and, as we shall hereafter see, in part also the fifth, together form the thorax of the future imago” (p. 870). Although at first inclined to Audouin’s opinion, he does not appear to fully accept it, yet farther 164on (p. 921) he concludes that in the Hymenoptera the “fifth” segment (first abdominal) is not in reality a part of the true thorax, “but is sometimes connected more or less with that region, or with the abdomen, being intermediate between the two. Hence we have ventured to designate it the thoracico-abdominal segment.” Had he considered the higher Hymenoptera alone, he would undoubtedly have adopted Latreille’s view, but he saw that in the saw-flies and Lepidoptera the first abdominal segment is not entirely united with the thorax, being still connected with the abdomen as well as the thorax. Reinhard in 1865 reaffirmed Latreille’s view. In 1866 we stated from observations on the larvæ made three years earlier, that during the semipupa stage of Bombus the entire first abdominal segment is “transferred from the abdomen to the thorax with which it is intimately united in the Hymenoptera,” and we added that we deemed this to be “the most essential zoölogical character separating the Hymenoptera from all other insects.” (See Fig. 93, showing the gradual transfer and fusion of this segment with the thorax.) In the saw-flies the fusion is incomplete, as also in the Lepidoptera, while in the Diptera and all other orders the thorax consists of but three segments. (See also pp. 90–92.)

Fig. 179.—Abdomen of Machilis maritima, ♀, seen from beneath: the left half of the 8th ventral plate removed; I-IX, abdominal segments; c, cercopoda; cb, coxal glands; hs, coxal stylets; lr, ovipositor.—After Oudemans, from Lang.

The cercopoda.—We have applied this name to the pair of anal cerci appended to the tenth abdominal segment, and which are generally regarded as true abdominal legs. As is now well known, the embryos of insects of different orders have numerous temporary pairs of abdominal appendages which arise in the same manner, have the same embryonic structure, and are placed in a position homologous with those of the thorax. In the embryo of Œcanthus rudimentary legs appear, as shown by Ayers, on the first to tenth abdominal segment, the last or tenth pair becoming the cercopoda; and similar rudimentary appendages have been detected in the embryos of Coleoptera, Lepidoptera, and Hymenoptera (Apidæ). Cholodkowsky has observed eleven pairs of abdominal appendages in Phyllodromia.

They are very long and multiarticulate in the Thysanura (Fig. 179). In the Dermaptera they are not jointed and are forcep-like. It should also be observed that in the larva or Sisyra (Fig. 181) there are seven pairs of 5–jointed abdominal appendages, though these may be secondary structures or tracheal gills. In the Perlidæ 165and the Plectoptera (Ephemeridæ), they are very long, sometimes over twice as long as the body, and composed of upward of 55 joints; they also occur in the Panorpidæ (Fig. 177). In the dragon-flies the cerci are large, but not articulated, and serve as claspers or are leaf-like[35] (Fig. 180). In a few Coleoptera, as the palm-weevil (Rhynchophorus phœnicis), Cerambyx, Drilus, etc., the so-called ovipositor ends in a hairy, 1–jointed, palpiform cercus. Short 25–jointed cercopoda are present in Termitidæ, and 2–jointed ones in Embiidæ.

Fig. 180.—End of abdomen of Æschna heros, ♀: ur, urosternite; or, outer, ir, inner styles of the ovipositor; 11, 11th abdominal segment; c, cercopod.

Fig. 181.—Larva of Sisyra, from beneath. B, an abdominal appendage.—After Westwood, from Sharp.

Fig. 182.—Cercopoda (P) of Mantis.—After Lacaze-Duthiers.

The anal cerci are present in the Orthoptera and, when multiarticulate, function as abdominal antennæ. They are longest in the Mantidæ (Fig. 182); they also occur in the larva of the saw-fly, Lyda (Fig. 183). Dr. A. Dohrn has stated that the cerci of Gryllotalpa are true sensory organs, and we have called those of the cockroach abdominal antennæ, having detected about ninety sacs on the upper side of each joint of the stylets, which are supposed to be olfactory in nature, and which are larger and more numerous than similar sacs or pits in the antennæ 166of the same insect.[36] From his experiments upon decapitated cockroaches, Graber concluded that these cerci were organs of smell.

Fig. 183.—Lyda larva: a, head; b, end of body seen from above; c, from side, with cercopod.

Haase regarded these appendages, from their late development and frequent reduction, as old inherited appendages which are approaching atrophy through disuse.

Cholodkowsky states that Tridactylus, a form allied to Gryllotalpa, bears on the tenth abdominal segment two pairs of cerci (ventral and dorsal), and that the ventral pair may correspond to the atrophied appendages of the tenth embryonic segment of Phyllodromia, with which afterward the eleventh segment becomes fused.

The cercopods are not necessarily confined to the eleventh or to the tenth segment, for when there are only nine segments, with the vestige of a tenth, as in Xiphidium, they arise from the ninth uromere, and in the more modern cockroaches, as Panesthia, in which there are but seven entire segments, they are appended to the last or eighth uromere.

Fig. 184.—Anabrus, ♀, side-view, dissected; showing the relative size of the ovipositor: c, the minute cercopod.—Kingsley del.

167As to the homology and continuity of these cercopods with the ventral outgrowths of the embryo, several embryologists, notably Wheeler, are emphatic in regarding them as such. It thus appears that either the embryonic appendages of the seventh or eighth, ninth or tenth uromere may persist, and form the cercopoda of the adult.

The ovipositor.—The end of the oviduct is guarded by three pairs of chitinous, unjointed styles closely fitted together, forming a strong, powerful apparatus for boring into the ground or into leaves, stems of plants, the bodies of insects, or even into solid wood, so that the eggs may be deposited in a place of safety. In the ants, wasps, and bees the ovipositor also functions as a sting, which is further provided with a poison-sac.

Morphologically, the ovipositor is composed of three pairs of unjointed styles (rhabdites of Lacaze-Duthiers, gonapophyses of Huxley), which are closely appressed to or sheathed within each other, the eggs passing out from the end of the oviduct, which lies, as Dewitz states, between the two styles of the lowest or innermost pair, and under the cross-bars or at the base of the stylets mentioned; the styles or blades spreading apart to allow of the passage of the egg.

Fig. 185.—Saw of Hylotoma: a, lateral scale; i, saw; f, gorget; 7t, 7th tergite; 6s, 6th sternite; ov, oviduct; in, intestine.—After Lacaze-Duthiers.

The ovipositor is best developed in the Thysanura (Fig. 179, Campodea excepted), in Orthoptera (Fig. 184), in the Odonata, Hemiptera, certain Physapoda, Rhaphiidæ, and in the phytophagous Hymenoptera, where it is curiously modified to form a rather complicated saw for cutting slits in wood or leaves (Fig. 185). It is wanting or quite imperfect in Coleoptera, Diptera, and Lepidoptera.

Morphologically, the ovipositor appears to be formed out of the abdominal appendages of the seventh, eighth, and ninth segments of the female, which, instead of disappearing in the orders first mentioned, persist as permanent styles.

Wheeler asserts from his study of the embryonic development of Xiphidium “there can be no doubt concerning the direct continuity of the embryonic appendages with the gonapophyses.” He goes on to say:—

168“One embryo, which had just completed katatrepis, still showed traces of all the abdominal appendages. The pairs on the eighth, ninth, and tenth segments were somewhat enlarged. In immediately succeeding stages the appendages of the second to sixth segments disappear; the pair on the seventh disappear somewhat later. Up to the time of hatching the gonapophyses could be continuously traced, since in Xiphidium there is no flexure of the abdomen, as in other forms, to obscure the ventral view of the terminal segments. From the time of hatching Dewitz has traced the development of the ovipositor in another locustid (Locusta viridissima), so that now we have the complete history of the organ.”

Heymons, however, is inclined to believe that they are simply hypodermal outgrowths.

Fig. 186.—Ideal plan of the structure of the ovipositor to illustrate Lacaze-Duthiers’ view: b, 8th tergite; c, epimerum; a′, a, two pieces forming the outer pair of rhabdites; i, the 2d pair, or stylets; and f, the inner pair, or sting; d, support of sting; e, piece supporting the stylet; R, anus; o, outlet of oviduct. The 7th, 8th, and 9th sternites are aborted.—After Lacaze-Duthiers.

The first to study the morphology of the ovipositor was Lacaze-Duthiers, who referred their origin to the partially atrophied dorsal or ventral sclerites of one of the last abdominal segments; a view accepted by Gerstaecker[37] (Figs. 186, 187). The present writer (1866), however, showed that the sting of Bombus was not formed of the reduced pieces of the segments themselves, but arose from special outgrowths on the ventral side of the eighth and ninth abdominal segments. These appendages he did not at first regard as the homologues of the limbs, until in 1871, after studying the origin of the spring of the Podurans (Isotoma), he found that it was a true jointed appendage and therefore a homologue of a pair of the styles forming the ovipositor of the winged insects, and that the three pairs of styles of the latter were homologues of the thoracic legs and cephalic appendages. The view was stated in the Guide to the Study of Insects. (See also Amer. Nat., March, 1871, p. 6.) Kraepelin also affirms that the styles of the ovipositor are segmental appendages and homologues of the antennæ, wings (sic), and legs.

169

Fig. 187.—1, abdomen of Cynips, showing the great dorsal segment, the peduncle, and the position of the ovipositor within; 2, the entire ovipositor; a, lateral scale; a′, its valve; b, anal scale; b′, stylet; c, support of the stylet; e, base or support of sting (fi); 3, profile showing the relation of the genital armature to the rest of the abdomen, the 6th sternite having been drawn to show its full size; 4, anal scale (b) and stylet; e, i, supports and body of the stylet; c, piece uniting the two scales; 5, lateral scale (a), and a′ sheath; d, support of the sting (f); 6, transverse section of the body through the sting (diagrammatic); R, internal armature; o, oviduct; a, lateral scale; a′ its valve; e, support of the stylets (i); b, anal scale; c, piece uniting two scales; f, sting; d, its support; 7, a second section simpler and more theoretical than the first; 8, diagrammatic, all the elements of the sting have been reduced to pieces of the same form.—After Lacaze-Duthiers.

170An objection to this view is the fact that the posterior pairs of styles appear to arise both from one and the same segment,—the ninth. Dewitz questions whether the four appendages of the ninth segment represent two pairs of limbs, or one pair split into two branches, and prefers the latter view, but leaves it as a point to be settled by future investigations. As will be seen below, both Kraepelin and Bugnion observed a pair of rudiments to each of the three penultimate segments, those of the middle pair splitting in two. Wheeler maintains, erroneously we think, that the inner of the two pairs on the ninth segment represents the tenth pair of abdominal appendages; but in reality this latter pair become the cercopods. That there are probably originally in insects of all the orders provided with an ovipositor three distinct pairs of appendages, one to each segment, is proved, or at least strongly suggested, by Ganin’s researches on the three pairs of abdominal imaginal discs of the third larva of Platygaster and Polynema (Fig. 188), which are transformed into the ovipositor. He remarks that these imaginal discs have the same origin and pass through the same changes as those in front, i.e. those destined to form the thoracic legs. Dewitz has shown that the germs of the ovipositor of the honey-bee arise as buds on the two segments before the last (Fig. 189).

Fig. 188.—Third larva of Polynema: at, antenna; fl, imaginal buds of the wing; l, of the legs; tg, buds of the middle pair of stylets of the ovipositor; fk, fat-body; eg, ear-like process.—After Ganin.

Fig. 189.—Imaginal buds and papillæ of the ovipositor of the honey-bee attached to tracheæ; at different stages: b′, 1st; b″, 2d or middle; and c′, 3d pair of papillæ.—After Dewitz.

Kraepelin also detected in the larva of the honey-bee a pair of what he regarded as genuine imaginal buds on abdominal segments eight, nine, and ten; the buds on the tenth segment are divided each in two; of these four appendages the two median ones form the barbed sting (gorgeret or stachelrinne), and the two lateral stylets, the valves (stachelscheiden). The two buds of the ninth segment give rise to the vagina and to the oviducts, and these unite secondarily with the posterior end of the ovaries. The genital appendages 171of the male correspond to those of the female, and arise from four imaginal buds situated on the under side of the tenth abdominal segment.

Fig. 190.—1, sting and poison sac of the honey-bee: GD, poison gland; Gb, poison reservoir; D, accessory gland; sh, sheathing style or sting-“feeler”; Str sting; Ba, sheath; Q, quadrate plate; O, oblong piece; W, angular piece; B, base of the sting and stylets; Stb′, Stb″, the two barbed stylets or darts. 2, sting seen from the ventral face: lettering as in the other figure.—After Kraepelin, from Perrier.

In the ants, according to Dewitz, the genital armature is derived from imaginal buds situated on the under side of the seventh, eighth, and ninth abdominal segments. Bugnion has observed the formation of six imaginal buds of the genital armature in the larva of a chalcid (Encyrtus, Figs. 41, 42, 191, q1, q2, q3), the transformation of the central part of these structures into small digitiform pads, then the division of the two intermediate buds into four (?) (Fig. 191, B, q2), but was unable to trace their farther development.

The subject still needs farther investigation, since certain observers, as Haase, and, more recently, Heymons, do not believe that they are homologues of the legs, but integumental structures, though of somewhat higher value than the style of the base of the legs of Scolopendrella and Thysanura; but it is to be observed that as yet we know but little of the embryological history of these styles.

Those authors who have examined the elements of the ovipositor, and regard them as homologues (homodynamous) of the limbs, are Weismann (1866), Ganin (1869), Packard (1871), Ouljanin (1872), Kraepelin, Kowalevsky (1873), Dewitz (1875), Huxley (1877), Cholodkowsky, Bugnion (1891), and Wheeler (1892).

As shown, then, by our observations and those of Dewitz (Figs. 189 and 192), the rudiments of the ovipositor consist of three pairs of tubercles, arising, as Kraepelin and also Bugnion (Fig. 191) have shown, from three pairs of imaginal discs, situated respectively on the seventh, eighth, and ninth uromeres, or at least on the three penultimate segments of the abdomen. With the growth of the 172semipupa, the end of the abdomen decreases in size, and is gradually incurved toward the base (Fig. 193), and the three pairs of appendages approach each other so closely that the two outer ones completely ensheath the inner pair, until a complete extensible tube is formed, which, by the changes in form of the muscles within, is gradually withdrawn entirely within the body.

Fig. 191.A, end of larva of Encyrtus of 2d stage, showing the three pairs of imaginal buds of the ovipositor q1, q2, q3. B, the same in an older larva ready to transform; i, intestine; x, genital gland; a, anus.—After Bugnion.

174An excellent account of the honey-bee’s sting is given by Cheshire (Figs. 194, 195). The outermost of the three pairs of stylets forming the apparatus is the two thick, hairy “palpi” or feelers (P), these being freer from the sting proper than in the ovipositor of Orthoptera. The sting itself is composed of the two inner pairs of stylets; one of these pairs is united to form the sheath (sh), while the other pair form the two barbed darts. The sheath has three uses: first, to open the wound; second, to act as an intermediate conduit for the poison; and third, to hold in accurate position the long barbed darts. The sheath does not enclose the darts as a scabbard, but is cleft down the side presented in Fig. 194, which is below when the sting points backward. But, says Cheshire, as the darts move up and down, they would immediately slip from their position, unless prevented by a mechanical device, exhibited by B and C, giving in cross-section sheath and darts near the end, and at the middle of the former. “The darts (d) are each grooved through their entire length, while upon the sheath (sh) are fixed two guide rails, each like a prolonged dovetail, which, fitted into the groove, permits of no other movement than that directly up and down.” The darts are terminated by ten barbs of ugly form (D, Fig. 194), and much larger than those of the sheath, and as soon as the latter has established a hold, first one dart and then the other is driven forward by successive blows. These in turn are followed by the sheath, when the darts again more deeply plunge, until the murderous little tool is buried to the hilt. But these movements are the result of a muscular apparatus yet to be examined, and which has been dissected away to bring the rigid pieces into view. The dovetail guides of the sheath are continued far above its bulbous portion, as we see by E, Fig. 195; and along with these the darts are also prolonged upward, still held to the guides by the grooved arrangement before explained; but both guides and darts, in the upper part of their length, curve from each other somewhat like the arms of a Y, to the points c, c′ (A, Fig. 194), where the darts 175make attachment to two levers (i, i′). The levers (k, l and k′, l′) are provided with broad muscles, which terminate by attachment to the lower segments of the abdomen. These, by contraction, revolve the levers aforesaid round the points f, f′, so that, without relative movement of rod and groove, the points 176c, c′ approach each other. The arms of the Y straighten and shorten, so that the sheath and darts are driven from their hiding-place together and the thrust is made by which the sheath produces its incision and fixture. The sides being symmetrical, we may, for simplicity’s sake, concentrate our attention on one, say the left in the figure. A muscular contraction of a broad strap joining k and d (the dart protractor) now revolves k on l, so that a is raised, by which clearly c is made to approach d; i.e. the dart is sent forward, so that the barbs extend beyond the sheath and deepen the puncture. The other dart, and then the sheath, follow, in a sequence already explained, and which G, Fig. 195, is intended to make intelligible, a representing the entrance of the sheath, b the advance of the barbs, and c the sheath in its second position. The barb retractor muscle is attached to the outer side of i, and by it a is depressed and the barbs lifted. These movements, following one another with remarkable rapidity, are entirely reflex, and may be continued long after the sting has been torn, as is usual, from the insect. By taking a piece of wash-leather, placing it over the end of the finger, and applying it to a bee held by the wings, we may get the fullest opportunity of observing the sting movements, which the microscope will show to be kept up by continued impulses from the fifth abdominal ganglion and its multitudinous nerves (n, Fig. 194, A), which penetrate every part of the sting mechanism, and may be traced even into the darts. These facts, together with the explanation at page 49, will show why an abdomen separated many hours may be able to sting severely, as I have more than once experienced.

Fig. 192.—Base of the ovipositor of Locusta viridissima seen from beneath: c′, sheath, or outer and lower pair of stylets turned to one side to show the others; b′, upper and inner pair; b″, third or innermost, smallest pair of stylets. A, the same on one side, in section. The shaded parts show the muscular attachments. The muscles which extend the apparatus and are attached to ν, δ, and η, as also the membranes which unite the pieces from η; to γ with each other and the body, are removed, so that only the chitinous parts remain.—After Dewitz.

Fig. 193.—Development of the sting in Bombus: A, a, 1st pair on 8th sternite; b, 2d inner pair forming the darts; c, outer pair. B-E, more advanced stages. F, x, y, z, three pairs of tubercles, the germs of the male organs.

Fig. 194.—Sting of bee × 30 times: A, sting separated from its muscles; ps, poison sac; pg, poison gland; 5th g, 5th abdominal ganglion; n, n, nerves; e, external thin membrane joining sting to last abdominal segment; i, k, l, and i′, k′, l′, levers to move the darts; sh, sheath; v, vulva; p, sting-palpus or feeler, with tactile hairs and nerves. B and C, sections through the darts and sheath, × 300 times: sh, sheath; d, darts; b, barbs; p, poison-channel. D, end of a dart, × 200: o, o, openings for poison to escape into the wound.—After Cheshire.

Fig. 195.—Details of sting of bee: E, darts, sheath, and valves; pb, poison-bag duct; fo, fork; s, slide piece; va, valve; b, barbs. F, terminal abdominal segments; w, worker’s sting; q, queen’s sting; r, r′, anal plate; G, sting entering skin; sh, sheath; a, b, c, positions in first, second, and third thrusts with the sting. H, portion of poison gland, × 300; cn, cell nucleus; n, nerve; g, ganglionic cell. I, portion of the poison gland, cells removed; cd, central duct; d, individual small ducts; pr, tunica propria. K, gland of Formica rufa; cd, central duct; d, small ducts; sc, secreting cells. L, valve and support; t, trachea; va, valve; tr, truss or valve-prop.—After Cheshire.

Fig. 196.A, rudimentary ovipositor of nymph of Æschna. B, the corresponding ♂ structures; a, enlarged. C, ovipositor of nymph of Agrion; d, gill.

The male genital armature in the bees is originally composed of three pairs of tubercles, homologous with those of the female, all originally arising from three abdominal segments, two afterward being anterior, and the third pair nearer the base of the abdomen.

The ovipositor of the dragon-flies (Odonata) is essentially like that of the Orthoptera and Hymenoptera. Thus in Æschna (Fig. 196), Agrion (Fig. 196, C), and also in Cicada it consists of a pair of closely appressed ensiform processes which grow out from under the posterior edge of the eighth uromere and are embraced between two pairs of thin lamelliform pieces of similar form and structure.

The styles and genital claspers (Rhabdopoda).—Other appendages of the end of the abdomen of pterygote insects, and generally, if not always, arising from the ninth segment, are the clasping organs, or rhabdopoda as we may call them, of Ephemeridæ (Fig. 197), Neuroptera (Corydalus [Fig. 198], Myrmeleon, Rhaphidia), Trichoptera, 177Lepidoptera, Diptera, and certain phytophagous Hymenoptera. They do not appear to occur in insects which are provided with an ovipositor. In Thysanura the styles are present on segments 1–9 (Fig. 179). Those of the male Ephemeridæ, of which there are two pairs arising from the ninth segment, are remarkable, since they are jointed, and they serve to represent or may be the homologues of two of the pairs of stylets composing the ovipositor of insects of other orders. The lower pair (Fig. 197, rh) are either 2–, 3–, or 4–jointed (in Oniscigaster 5–jointed), while those of the upper pair are 2–jointed (rh′). These rhabdopods in the ephemerids are evidently very primitive structures, since they approach nearest in shape and in being jointed to the abdominal legs of Scolopendrella and the Myriopoda. The styles of the Orthoptera are survivals of the embryonic appendages of the ninth segment (Wheeler, etc.). In Mantis they are seen to have the same relations as the cerci, as shown by Heymons (Fig. 200).

Fig. 197.—Abdomen of Ephemera (Leptophlebia) cupida, ♂: c, base of cercopoda; rh, outer 3–jointed claspers or rhabdopods; rh′, inner pair. A, side view.

Fig. 198.—End of abdomen of Corydalus cornutus, ♂: vh, rhabdopod; c, cercopod.

In the Phasmidæ, in Anabrus, and in the Odonata the cercopods, which are not jointed, are converted into claspers, and in the Odonata the claspers are spiny within, so as to give a firmer hold. The suranal plate is apparently so modified as to aid in grasping the female. In nearly all the Trichoptera there are, besides the suranal plate, which is sometimes forked (Nosopus), a pair of superior and of inferior claspers, and in certain genera 178(Ascalaphomerus, Macronema, Rhyacophila, Hydropsyche, Amphipsyche, Smicridea, and Ganonema) the lower pair are 2–jointed like those of Ephemeridæ. The number of abdominal segments in the adult Trichoptera is nine, and McLachlan states that the genital armature consists of three pairs of appendages, i.e. a superior, inferior, and intermediate pair, besides the suranal plate (vestige of a tenth segment) and the penis. Judging by his figures, these three pairs of appendages arise from the last or ninth uromere, and the upper pair seem to be the homologue of the cercopoda of ephemerids. It needs still to be ascertained whether the intermediate pair is a separate set, or merely subdivisions of the upper or lower, and whether one of the latter may not arise from the penultimate segment, because we should not expect that the last segment should bear more than one pair of appendages, as we find to be the case in arthropods in general, and in the Neuroptera, from which the Trichoptera may have originated.

Fig. 199.—End of abdomen of embryo of Mantis: r, rhabdopod; c, cercopod; sp, suranal plate; st, stigma on 8th segment.—After Heymons.

Fig. 200.—End of abdomen of Periplaneta americana, ♂, side view: c, cercopod; st, stilus; p, penis; t, titillator; d, “bird’s head” (clasper?); i, “oblong plate”; IX-XI, terminal segments; X, suranal plate; XI′, 11th sternite.—After Peytoureau.

In most larvæ of the Trichoptera, especially the Rhyacophilidæ and Hydropsychidæ, the last abdominal segment bears a pair of 2–jointed legs (cercopoda), ending in either one or two claws, which under various forms, sometimes forming long processes, persist in the pupa; and there appears to be a suranal plate, the vestige of the tenth uromere. In the pupa, judging by Klapálek’s figure of Leptocerus (248,9, 25), a pair of lateral spines arise which may in the imago form one of the pairs of appendages or styles. In the pupa of Œcetis furva his figure 289 shows two pairs of 1–jointed appendages arising from the last segment; whether the long dorsal or upper styles arise from the vestige of a more distal 179segment is not distinctly shown in Klapálek’s sketch. The origin of these elements of the genital armature evidently needs further study.

Whether the abdominal legs or so-called false or prop-legs of lepidopterous larvæ are genuine legs, homologous with those of the thorax and with the cephalic appendages, or whether they are secondary adaptive structures, is a matter still under discussion. That, however, they are true legs is shown by the embryology of the Lepidoptera, where there is a pair to each abdominal segment. It may also be asked whether the anal legs of lepidopterous larvæ are not the homologues of the 2–jointed anal appendages of caddis-worms.

Fig. 201.Eriocephala calthella, ♂, side view: t, palpiform suranal plate; cl, claspers; s, inferior claspers; mxp, maxillary palpi; cx. coxa; tr, trochanter; sc, scutum; sc′, scutellum.

In Lepidoptera, notably the male of the very generalized Eriocephala calthella (Fig. 201), besides the broad unjointed claspers, which are curved upward and provided with a brush of stiff hooked setæ (this upper pair being perhaps modified cercopods), there is an accessory lower slenderer pair, while the suranal plate (t) is palpiform or clavate and also adapted to aid in the action of the claspers. The examination of the cercopods and rhabdopods in the Trichoptera and in a generalized lepidopterous form like this enables one to understand the morphology of the genital armature, since it consists, besides the suranal plate, which is often deeply forked (in Sphingidæ, Smith), of a pair of modified hook-like cercopoda, and in some cases (Eriocephala) of an additional pair of claspers which 180may be the homologues of the ephemerid rhabdopods. A pair of hooks, often strong and claw-like (harpes), are situated, one near the base on the inside of each clasper; they are especially developed in the Noctuidæ (Smith), and appear to be present in certain Trichoptera, but this remains to be proved. This complicated apparatus of claspers and hooks is utilized by those insects which pair while on the wing, and is wanting in such forms as Coleoptera and Hemiptera. Besides the forceps of Panorpa, there are two pairs of slender filiform appendages which need farther examination. In the Diptera, especially Tipulidæ, there is a pair of 2–jointed appendages or forceps, as in Limnophila (Osten Sacken). The male genital armature of Diptera appears to be on the same general plan as in Lepidoptera, but more complicated.

Fig. 202.—Male organs of generation of Athalia.—After Newport.

Notice should also be taken of the paired uncinate hooks which are modifications of the penis-sheath of the male of cockroaches (Phyllodromia), which Haase states appear to originate on the tenth ventral plate, and which probably “serve to open and dilate the vagina of the female, especially as a perforated penis, which is highly developed in Machilis, seems to be wanting in the Blattidæ.” (Haase.)

The penis.—This is a single or double median style-like structure either hollow and perforated, or solid, very variable in shape, receiving the end of the ejaculatory duct. It is usually enclosed between two lateral plates, the homologues perhaps of the inner pair of sheaths of the ovipositor. In the Coleoptera, as in Carabidæ and Melolonthidæ, the penis is a long chitinous tube, “retractile within the abdomen on the under surface as far as the anterior segments.” (Newport.) In the Hymenoptera, of which that of the saw-flies is a type, Newport states that it “consists of a short valvular projectile organ, covered externally by two pointed horny plates (i) clothed with soft hairs.” Above these are two other irregular double-jointed plates (Fig. 202, l) surrounded at their base by a chitinous ring (k); they are edged with prehensile hooked spines (i). Between these in the middle line are two elongated muscular parts (m) which enclose the penis (h), and which, like those in beetles, perhaps aid in dilating the vulva of the female.

An examination of Figs. 203–207 will aid in understanding the various modifications in beetles, etc., of this organ.

181A general study of the anatomy and homologies of the male genital armature, from a developmental point of view, together with a comparison of them with the corresponding female organs, is still needed.

Fig. 203.—End of abdomen situated under the anal lobes of Hydrophilus piceus, drawn out, seen from the ventral side: 6, sternal region of 6th segment; 7, 8, 9, segments telescoped, when retracted, in 6th segment; zw, membrane connecting 6th and 7th segments; G, intromittent apparatus; vl, external lobes; vlu, inner lobes; pn, penis.

Fig. 204.—The same as in Fig. 203, seen from the side: 6, the free 6th segment; 7–10, the four last, when at rest, retracted and telescoped within the 6th segment, with the copulatory apparatus (g); vl, outer, vlu, inner lobe; 10s, tergite of 10th segment; 10i, sternite of the same; an, anal opening.

Fig. 205.—Terminal parts of the male copulatory apparatus of Hydrophilus piceus, torn apart: vlu, the two inner lodes; pn, penis; x, membrane torn from under side of penis; ej, ejaculatory duct; os, its opening on the under side of the penis, directly under its tip. The muscles, tracheæ, and nerves are not drawn.

Velum penis.—In the locusts (Acrydiidæ) the penis is concealed by a convex plate, flap, or hood, free anteriorly and attached posteriorly and on the sides to the ridge forming the upper edge of the tenth sternite. When about to unite sexually, the tip of the abdomen is depressed, the hood is drawn backward, uncovering the chitinous penis.

The suranal plate.—This is a triangular, often thick, solid plate or area, the remnant of the tergum of the last, usually tenth, segment of the abdomen, the supra-anal or suranal plate, or anal operculum (lamina supra-analis) of Haase. In most lepidopterous larvæ this plate is well marked; in those of the Platypteridæ it is remarkably elongated, forming an approach to a flagellum-like terrifying 182appendage, and in that of Aglia tau it forms a long, prominent, sharp spine. In the cockroach, both Cholodkowsky and Haase maintain that the tenth abdominal segment is suppressed in the female, the tergal portion being fused with the suranal plate (the latter in this case, as we understand it, being the remnant of the eleventh segment of the embryo). As to the nature of the middle jointed caudal appendage in Thysanura and May-flies Heymons has satisfactorily shown that it is a hypertrophied portion of the suranal plate, being in Lepisma but a filamental elongation of the small eleventh abdominal tergite.

Fig. 206.—Copulatory organ of a weevil, Rhychophorus phœnicis, seen from above. A, vl, the lobes united into a capsule; pp, torn membrane which connects the capsule with the 9th abdominal segment; ej, ejaculatory duct. B, the same seen from the side; mu, end of the muscle of the penis. C, the same as B, without the capsule; os, opening of the ejaculatory duct (ej). Other letters as in A.

Fig. 207.A, penis (pn) of Carabus hortensis: bl, wrinkled membranous vesicle; vlu, the valves; g, part of 9th segment. B, end of penis of the same, enlarged; os, cleft-like opening; also a wrinkled vesicle, as at bl.—This and Figs. 203–205 after Kolbe.

At the base of the suranal plate of locusts (Acrydiidæ) is the suranal fork or suranal furcula (furcula supra-analis, as we have called it) (Fig. 88, 89, f).

The podical plates or paranal lobes.—In the cockroach and other insects, also in the nymphs of Odonata, the anus is bounded on each side by a more or less triangular plate, the two valves being 183noticeable in lepidopterous larvæ. They are the valvulæ of Burmeister, and podical plates of Huxley, who also regarded them as the tergites of an eleventh abdominal segment;[38] and the subanal laminæ of Heymons. They are wanting in Ephemeridæ.

The infra-anal lobe.—Our attention was first called to this lobe or flap, while examining some geometrid larvæ. It is a thick, conical, fleshy lobe, often ending in a hard, chitinous point, and situated directly beneath the vent. Its use is evidently to aid in tossing the pellets of excrement away so as to prevent their contact with the body. The end may be sharp and hard or bear a bristle. Whether this lobe is the modified ventral plate of the ninth urite, we will not undertake at present to say.

The egg-guide.—In the Acrydiidæ the external opening of the oviduct is bounded on the ventral side by a movable, triangular, acute flap, the egg-guide (Fig. 88, B, eg). Whether this occurs in other orders needs to be ascertained.

LITERATURE ON THE ABDOMEN AND ITS APPENDAGES

a. General (including the cerci, stili, etc.)

Cornelius, C. Beiträge zur naheren Kenntnis von Palingenia longicauda. (Programm d. Real- u. Gewerbeschule zu Elberfeld, 1848, pp. 1–38, 4 Taf.)

Schiödte, J. G. Bemerkungen über Myrmecophilen. Ueber den Bau des hinterleibes bei einigen Käfergattungen. (Germar’s Zeits. f. Ent., 1844.)

—— De metamorphosi Eleutheratorum observationes. (Naturhist. Tidsskr. i-xiii, 1861–1883.)

Meinert, F. Anatomia Forficularum. Anatomisk undersogelse af de Danske Orentviste, i. (Naturhist. Tidsskr. 3 raekke, ii, 1863, pp. 427–482, 1 Pl.)

—— Om dobbelte Saedgange hos insecter. (Naturhist. Tidsskrift, 3 raekke, v, 1868.)

Tullberg, Tycho. Sveriges Podurider. (Svenska Vetensk.-Akad. Handl., 1872, x, Nr. 10, 12 Pls.)

Davis, H. Notes on the pygidia and cerci of insects. (Journ. R. Micr. Soc., 1879, ii.)

Westhoff, F. Ueber den Bau des Hypopygiums der Gattung Tipula Meig. (Münster, 1882, pp. 1–62, 6 Taf.)

Saunders, Edward. Further notes on the terminal segments of aculeate Hymenoptera. (Trans. Entom. Soc. London, 1884, pp. 251–267, 1 Pl.)

Palmén, J. A. Ueber paarige Ausführungsgange der Geschlechtsorgane bei Insekten. (Helsingfors, 1884, 5 Taf.)

Haase, E. Die Abdominalanhänge der Insekten mit Berücksichtigung der Myriopoden. (Morpholog. Jahrbuch, 1889, xv, pp. 331–435, 2 Taf.)

—— Abdominalanhänge bei Hexapoden. (Sitzungsber. d. Gesellsch. naturforsch. Freunde, 1889, pp. 19–29.)

184Wheeler, William M. On the appendages of the first abdominal segment of embryo insects. (Trans. Wis. Acad. Sc., viii, 1890, pp. 87–140, 3 Pls.)

Janet, Charles. Études sur les fourmis, 5e note, sur la morphologie du squelette des segments post-thoraciques chez les Myrmicides. (Mém. Soc. Acad, de l’Oise, xv, pp. 591–611, Figs. 1–5, 1894.)

Heymons, Richard. Die Segmentirung des Insektenkörpers. (Anh. Abh. Akad. Berlin, Phys. Abh., pp. 39, Taf., 1895. See also Die Embryonalent, von Dermapteren und Orthopteren, under Embryology.)

—— Grundzüge der Entwicklung und des Körperbaues von Odonaten und Ephemeriden. (Abhandl. k. Preuss. Akad. d. Wissens. Berlin, 1896, 2 Taf., pp. 1–66.)

—— Zur Morphologie der Abdominalanhänge bei den Insecten. (Morphol. Jahrb., iv, 1896, pp. 178–203, 2 Taf.)

Peytoureau, S. A. Contribution à l’étude de la morphologie de l’armure génital des insectes. (Bordeaux, 1895, pp. 248, 22 Pls., 43 Figs. in text.)

Verhoeff, C. Cerci und styli der Tracheaten. (Ent. Nachr., xxi, pp. 166–168, 1895.)

Also the writings of Oudemans, Packard, etc.

b. The ovipositor

Lacaze-Duthiers, Henri. Recherches sur l’armure génitale femelle des insectes. (Ann. d. sc. natur., 1849, xii, pp. 353–374, 1 Pl.; 1850, xiv, pp. 17–52, 1 Pl. (Hyménoptères); 1852, xvii, pp. 206–251, 1 Pl. (Orthoptères); 1853, xix, pp. 25–88, 4 Pl. (Neuroptères, Thysanures, Coléoptères, Diptères); pp. 203–237 (Lépidoptères, Aphaniptères en general. Also separate.))

Sollmann, A. Der Bienenstachel. (Zeitschr. f. Wissens. Zool., xiii, 1863, pp. 528–540, 1 Taf.)

Fengger, H. Anatomie und Physiologie des Giftapparates bei den Hymenopteren. (Archiv f. Naturgesch., 1863, pp. 139–178, 1 Taf.)

Eaton, A. E. Remarks upon the homologies of the ovipositor. (Trans. Ent. Soc. London, 1868, pp. 141–144.)

Packard, A. S. On the structure of the ovipositor and homologous parts in the male insects. (Proc. Boston Soc. Nat. Hist., xi, 1868, pp. 393–399, Figs. 1–11.)

Lambrecht, A. Samtliche Teile des Stechapparates in Bienenkörper und ihre Verwendung zu technischen und vitalen Zwecken. (Bienemwirtsch. Centralbl., 7 Jahrg., 1871, pp. 5–11.)

Kraepelin, K. Untersuchungen über den Bau, Mechanismus und die Entwicklung des Stachels der bienenartigen Tiere. (Zeitschr. f. wiss. Zoologie, xxiii, 1873.)

Dewitz, H. Vergleichende Untersuchungen über Bau und Entwicklung des Stachels der Honigbiene und der Legescheide der grünen Heuschrecke. (Königsberg, 1874.)

—— Ueber Bau und Entwicklung des Stachels und der Legescheide einiger Hymenopteren und der grünen Heuschrecke. (Zeitschr. f. wissen. Zoologie, xxv, 1874, pp. 174–200, 2 Taf.)

—— Ueber Bau und Entwicklung des Stachels der Ameisen. (Zeitschr. f. wiss. Zoologie, 1877, xxviii, pp. 527–556, 1 Taf.)

Adler, H. Legeapparat und Eierlegen der Gallwespen. (Deutsche Entom. Zeitschr., 1877, xxi Jahrg., pp. 305–332, 1 Taf.)

Cholodkowsky, N. Ueber den Hummelstachel und seine Bedeutung für die Systematik. (Zool. Anzeiger, vii, 1884, pp. 312–316.)

185Ihering, H. von. Der Stachel der Meliponen. (Ent. Nachr., 1886, xii, Jahrg., pp. 177–188, 1 Taf.)

Meinert, F. Bidrag til de danske Myrers Naturhistorie. (Kjöbenhavn, 1890, 68 s.u. Danske Vidensk. Selsk. Skrifter, 5 Raek, v, 3 Pls.)

Beyer, O. W. Der Giftapparat von Formica rufa ein reduziertes Organ. (Jena. Zeitschr. Naturw., 1890, xxv, pp. 26–112, 2 Taf.)

Carlet, G. Mémoire sur le venin et l’aiguillon de l’abeille. (Ann. d. sc. nat. Zool., 7 sér., ix, 1890, pp. 1–17, 1 Pl.)

Künckel d’Herculais, J. Méchanisme physiologique de la ponte chez les insectes orthoptères de la famille des Acridides.—Rôle de l’air comme agent mécanique et fonctions multiples de l’armure génitale. (Compt. Rend., cxix, pp. 244–247, 1894.)

Also the writings of Verhoeff, Heymons.

c. The external genital armature

Klug, Johann C. F. Versuch einer Berichtigung der Fabriciusschen Gattungen Scolia u. Tiphia. (Ueber u. Mohr Beiträge zur Naturkunde, i, pp. 8–40, 1805, 1 Taf.)

Audouin, J. V., and Lachat. Observations sur les organes copulateurs males des Bourdons. (Annal. général, d. sc. phys., 1821, viii, pp. 285–289.)

Audouin, J. V. Lettre sur la génération des insectes. (Ann. des sc. nat., sér. 1, ii, 1824.)

Suckow, F. W. L. Geschlechtsorgane der Insekten. (Heusinger, Zeitschr. organ. Physik., 1828, ii, pp. 231–264, 1 Taf.)

Rathke, H. De Libellularum partibus genitalibus. Regiomonti, 1832, pp. 6 + 38, 3 Pls.

Siebold, C. Th. E. von. Ueber die Fortpflanzungsweise der Libellulinen. (Germar’s Zeitschr. f. Ent., 1840, ii, pp. 421–438.)

Selys-Longchamps, E. de. Monographie des Libellulidees d’Europe. 4 Pls., 1840.

Bassi, C. A. Studi sulle funzioni degli organi genitali degl’ insetti da lui osservati piu specialmente nella Bombyx mori. (Atti della 5 Riun. d. Scienz. Ital. Lucca, 1844, pp. 39–94.)

Stein, F. Vergleichende Anatomie und Physiologie der Insekten, i. Die weiblich. Geschlechtsorgane der Käfer. Berlin, 1847, 9 Taf.

Ormancey, P. Recherches sur l’étui penial considéré comme limite de l’espèce dans les coléoptères. (Ann. sc. nat., 1849, 3 sér., Zool., xii, pp. 227–242.)

Roussel, C. Recherches sur les organes génitaux des insectes coléoptères de la famille des Scarabéides. (Compt. rend. Acad. d. sc. Paris, 1860, l, pp. 158–161.)

MacLachlan, R. A monographic revision and synopsis of the Trichoptera of the European fauna. London, 1874–80, 59 Pls.

—— On the sexual apparatus of the male Acentropus. (Trans. Ent. Soc. London, 1872, pp. 157–162.)

Thomson, C. G. Nagra anmarkningar ofver arterna af slagtet Carabus. (Thomson’s Opuscula Entomologica, vii, 1857, pp. 615–729, 1 Pl.)

Dufour, L. Sur l’appareil génital male du Coræbus bifasciatus. (Thomson, Archiv ent., 1857, i, pp. 378–381.)

White, F. Buchanan. On the male genital armature in the Rhopalocera. (Trans. Linn. Soc., ser. 1, Zool., i, pp. 357–369, 1876, 3 Pls.)

Graber, V. Die Aehnlichkeit im Baue der ausseren weiblichen Geschlechtsorgane bei den Lokustiden und Akridiern auf Grund ihrer Entwicklungsgeschichte. (Sitzber. k. Akad. d. Wissensch. Wien., 1870, lxi, pp. 1–20, 1 Taf.)

186Scudder, Samuel H., and Edward Burgess. On asymmetry in the appendages of hexapod insects, especially as illustrated in the lepidopterous genus Nisoniades. (Proc. Boston Soc. Nat. Hist., 1871, xiii, pp. 282–306.)

Chadima, J. Ueber die Homologie zwischen den männlichen und weiblichen ausseren Sexualorganen der Orthoptera Saltatoria Latr. (Mitteil. d. naturwiss. Vereins f. Steiermark, 1872, pp. 25–33, 1 Taf.)

Hagens, von. Ueber die Genitalien der männlichen Bienen, besonders der Gattung Sphecodes. (Berlin Ent. Zeitschr., 1874, pp. 25–43.)

—— Ueber die männlichen Genitalien der Bienengattung Sphecodes. (Deutschen Entom. Zeitschr., 1882, pp. 209–228, 2 Taf.)

Lindenman, C. Vergleichend-anatomische Untersuchung ueber das männliche Begattungsglied der Borkenkäfer. (Bull. Soc. Imp. d. Natural. Moscou, 1875–77.)

Forel, A. Der Giftapparat und die Analdrüsen der Ameisen. (Zeitschr. f. wiss. Zoologie, xxx, suppl., 1878.)

Kraatz, G. Ueber die Wichtigkeit der Untersuchung des männlichen Begattungsgliedes der Käfer für die Systematik und Artunterscheidung. (Deutschen Entom. Zeitschr., 1881, xxv, pp. 113–126.)

—— Ueber das männliche Begattungsglied der europaischen Cetoniiden und seine Verwendbarkeit für deren scharfe spezifische Unterscheidung. (Ibid., pp. 129–149.)

Gosse, Ph. H. On the clasping-organs ancillary to generation in certain groups of the Lepidoptera. (Trans. Linn. Soc., 1882, Ser. 2, Zool., ii, pp. 265–345, 8 Pls.)

—— The prehensors of male butterflies of the genera Ornithoptera and Papilio. (Proc. Roy. Soc. London, 1881, xxxiii, pp. 23–27.)

Radoszkowski, O. Revision des armures copulatrices des males du genre Bombus. (Bull. Soc. Natur. Moscou, 1884, xlix, pp. 51–92, 4 Pls.)

—— Revision des armures copulatrices des males de la tribu Philérémides. (Ibid., 1885, lxi, pp. 359–370, 2 Pls.)

—— Revision des armures copulatrices des males de la famille des Mutillidæ. (Horæ Soc. Ent. Ross., 1885, xix, pp. 3–49, 9 Pls.)

—— Revision des armures copulatrices des males de la tribu des Chrysides. (Horæ Soc. Ent. Ross, xxiii, 1890, pp. 3–40, 6 Pls.)

Hofmann, O. Beiträge zur Kenntnis der Butaliden. (Stett. Ent. Zeit., 1888, pp. 335–347, 1 Taf.)

Driedzichi, H. Revue des espèces européennes du genre Phronia Winn. (Horæ Soc. Ent. Ross., 1889, xxiii, pp. 404–532, 10 Pls.)

Sharp, David. On the structure of the terminal segment in some male Hemiptera. (Trans. Ent. Soc. London, 1890, pp. 399–427, 3 Pls.)

Escherich, K. Die biologische bedeutung der Genitalanhänge der Insekten. (Verhandl. d. zool. bot. Ges. Wien., 1892.)

—— Anatomische Studien über das männliche Genitalsystem der Coleopteren. (Zeits. f. wissens. Zool., lvii, pp. 620–641, 1 Taf., 3 figs.)

Verhoeff, C. Zur vergleichenden Morphologie der “Abdominalanhänge” der Coleopteren. (Ent. Nachr., xx, pp. 93–96, 1894. Compare also O. Schwarz and J. Weise’s criticisms in D. Ent. Zeit., pp. 153–157; also pp. 101–109, 155–157. Zool. Anzeiger, pp. 100–106, 1894.)

—— Vergleichende—morphologische Untersuchungen ueber das Abdomen der Endomychiden, etc., und über die Musculature des Copulationsapparates von Triplax. (Archiv f. Naturg., lxi, pp. 213–287, 2 Taf., 1895.)

—— Vergleichende Untersuchungen über die abdominal Segmente der weiblichen Hemiptera-Heteroptera und Homoptera. (Verh. Nat. Ver. Bonn, l, pp. 307–374, 1894.)

187Verhoeff, C. Beiträge zur vergleichenden Morphologie des Abdomens der Coccinelliden, etc. (Archiv f. Naturg., lxi, pp. 1–80, 6 figs., 1895.)

Boas, J. E. V. Organe copulateur et accouplement der hanneton. (Oversigt over det K. Danske Vidensk. Selskab Forhand, 1892, Copenhagen, 1893, 1 Pl., pp. 239–261.)

Pérez, J. De l’organe copulateur mâle des Hyménoptères et de sa valeur taxonomique. (Ann. Soc. Ent. France, lxiii, pp. 74–81, Figs., 1894.)

Goddard, Martha Freeman. On the second abdominal segment in a few Libellulidæ. (Proc. Amer. Philos. Soc., xxxv, pp. 205–212, January 11, 1897, 2 Pls.)

Also the writings of Eaton, Emery, Fischer, Forel, Géhin, Godart, Hagen, Joly, Koletani, Loew, Meinert, Mik, Nicolet, Osten Sacken, Pictet, Roussel, Schaeffer (1754), Schaum, Schenk, J. B. Smith, Thompson, Buchanan-White, Brunner von Wattle-Wyll, Weise, Wyenbergh.

The subject of copulation has been treated by Hoffer, Hartig, Schiedeknecht, Verhoeff, etc.

THE ARMATURE OF INSECTS: SETÆ, HAIRS, SCALES, TUBERCLES, ETC.

Fig. 208.—Larva of Dryocampa rubicunda, stage II.—Bridgham del.

The cuticula.—The integument is externally either smooth and shining or variously punctured, granulated, tuberculated, striated, or hairy. In certain orders the skin is clothed with flattened setæ or scales, while many forms, as some caterpillars (Figs. 208, 209), beetles (Fig. 210), etc., are protected by spines, horns, etc., these in adult insects often forming secondary sexual characters, usually being more developed in the males than in the females.

Fig. 209.—Larva of Hyperchiria io, on hatching.

The cuticula is not always smooth, but is often finely granulated or even minutely spinulated. On the abdominal segments of Anabrus, as observed by Minot, the cuticula is armed with microscopic conical nodules scattered irregularly over it. They do not correspond, he says, in any way to hairs; for they do not rest over pores, nor did he see any specially modified cells underlying them. “As far as I have observed, they are mere local irregularities, each nodule being apparently supported by some four or six unmodified epidermal cells.” Minot adds that the whole of the cuticula, except the cones just described and the hairs, is divided into numerous minute fields, each of which corresponds 188to a single cell of the underlying hypodermis. Each field is bounded by a distinct polygonal outline, and its surface is either covered by a large number of extremely minute projecting points, as on the dorsal arch of the segment, or is smooth, as upon the articular membrane and ventral arch. Upon the sides of the dorsal arch and upon the spiracular membrane each field has a projecting spine or sometimes two or even three. (See also pp. 28, 30.)

Fig. 210.Phanæus pegasus, ♂, from Mexico.—After Graber.

Fig. 211.—Section of integument of Datana ministra: c, cuticula; hyp, hypodermis; p, outer pigmented nodulated layer.

The cuticle of lepidopterous larvæ has also been described and figured by Minot. In the caterpillars of different groups investigated by him, the cuticle was found to be rough with microscopic teeth or spinules, erect or flattened and scale-like, and either densely crowded or scattered, and affording excellent generic and specific characters. In the slug-worms (Limacodids) we have observed that the cuticula is unusually rough, especially on the spiniferous tubercle of Empretia, Parasa, etc. (Fig. 213, c). The skin of the body between the tubercles is seen to be finely shagreened, due to the presence of fine teeth, which are more or less curved and bent, these teeth arising from a very finely granulated surface (d). The cuticle of neuropterous, trichopterous, and tenthredinid larvæ will probably afford similar cases. The integument of the larva of Datana is, on the black bands, rough and nodulated, the irregular nodules being filled with a black pigment, and forming a layer (p) external to the true cuticula (Fig. 211).

Fig. 212.—Hairs of Datana: f, formative hair-cell; c, cuticula; p, pigmented layer; hy, hypodermis.

The integument of many insects contains fine canals passing through the chitinous layers and opening externally in minute pores. Certain of the pore-canals communicate with hollow setæ which sit directly over the pores; other pores form the external openings of dermal glands, but in many cases they are empty or only filled with air, and do not have any hairs connected with them. Each of these pores communicates with a hair-forming hypodermal cell, called by Graber a trichogen.

Setæ (“hairs” and bristles).—The setæ of insects are, as in worms, processes of the cuticle originating from certain of the hypodermal cells. They arise either from a ring-like pit, or from a minute tubercle, and are usually situated at the outlet of a pore-canal, which connects with an underlying cell of the hypodermis (Fig. 212). They are, then, bristle or hair-like processes arising from the hypodermis. Where the hairs or setæ are rubbed off, their site is indicated by a minute ring like a follicle in the 189chitinous integument. The cuticular hair, says Leydig, is in its first condition the secretion of the cellular element of the skin, and a thread-like continuation of the cell-body may rise up through the pore-canal into the centre of the hair, remaining there permanently.

While the setæ are usually simple, they are often branched, plumose, or spinulose, as in larval Hemerobiidæ, Anthrenus, and Dermestes, the larvæ of certain coccinellid beetles, notably Epilachna, and of Cassida, the larvæ of arctians, etc., and in bees (Anthophila, Megachile, Osmia, Colletes, Apis, etc.).

The use of these spinulose, plumose, and twisted hairs in the bees is clearly shown by J. B. Smith, who states that as these insects walk over flowers, the pollen grains adhere to the vestiture, “and this also accounts for the fact, probably noticed by every observant fruit-grower, that bees frequently bury themselves completely in the blossoms, or roll over every part of them. Such insects are after pollen, not honey, and by so rolling about, the pollen grains are brought into contact with and adhere to the surface of the insect.” The syrphid flies also pollenize flowers, the pollenizing of chrysanthemums being effected, as Smith states, by Eristalis tenax, and he adds that the body vestiture of the syrphids “is often composed of spurred and branched hairs.” (For reference to gathering hairs, see p. 45.)

Fig. 213.—Cuticular spinules of larva of Adoneta: a, b, c, d, different forms; e, e′, caltrops.

Certain remarkable spines occur in limacodid larvæ, notably Empretia and Adoneta. These we have called caltrops spines, from their resemblance to the caltrops formerly used in repelling the attacks of cavalry. They are largely concerned in producing the poisonous and irritating effects resulting from contact with the caterpillars of these moths, and are situated in scattered groups near the end of the tubercles. A group of three is represented at Fig. 213, e. They are not firmly embedded in the cuticle, but on the contrary 190appear to become very easily loosened and detached, and they probably, when brought into contact with the skin of any aggressor, burrow underneath, and are probably in part the cause of the continual itching and annoyance occasioned by these creatures. It will be seen by reference to Fig. 213, e′, that the body of the spine is spherical, with one large, elongated, conical spine arising from it, the spherical base being beset with a number of minute, somewhat obtuse spinules.

Fig. 214.—Glandular hairs of caterpillars. A, Dasylophia anguina: a, of body; b, of head; c, of prothoracic shield. B, Ceratosia tricolor: a, on body; b, on abdominal legs. C, Schizura ipomeæ: a, from third thoracic segment; b, from larva stage II; c, simple setæ from minute warts.

Glandular hairs and spines.—In some insects occur fine, minute, hollow setæ from which exude, perhaps through pore-canals of extreme fineness, droplets of a clear watery or plasma-like sticky fluid. The club-shaped tenent hairs of the feet of Collembola, and the hairs fringing the feet of Diptera, are modified glandular hairs. Here they serve to give out a sticky fluid enabling the insect to walk on smooth surfaces; they end in a vesicle-like bulbous expansion, which may contain numerous pore-canals. Those of caterpillars were first noticed by Zeller, and Dimmock has particularly described those of 191the larvæ of Pterophoridæ. They are either club-shaped, or variously forked at the end (Fig. 214, B, a). They are usually replaced after the first larval moult by ordinary, simple, solid, pointed setæ, and their use in caterpillars is as yet unknown. Whether these hairs, as seems most probable, arise from a specialized glandular hypodermal cell, or not, has not yet been discovered.

Fig. 215.A, group of setæ arising from a subdorsal tubercle: cut, the cuticle; hy, the hypodermis; sc, the enlarged and specialized cells of the hypodermis which secrete the spines themselves; pglc, the nuclei which secrete the venomous fluid which fills the cavity of the seta (s), seen at p in a broken spine. B, a short entire, and a long broken seta (s-p); pgle, four poison cells; p, the poison in the hollow of the spine.

These temporary fine glandular hairs are probably the homologues of the larger true glandular bristles and spines of the later stages of certain lepidopterous larvæ, which are brightly colored and lead an exposed life, living through a large part of the summer. In these structures the bristles or spines are hollow, filled with a poisonous secretion formed in a single large, or several smaller specialized hypodermal cells situated under the base of the spine. In the venomous spines of Lagoa crispata the poisonous fluid in the larger spines (Figs. 215, C, 216, b) is secreted in several large cells situated at the base of the spine, and this is the usual form. In the finer spines of a large tubercle (Figs. 215, A, 216) there appears to be a differentiation of the hypodermal cells into two kinds, the large, basal deep-seated, setigenous cells (216, sc) and the poison-secreting nuclei (216, pglc) situated nearer the base of the setæ. The spines being filled with poison and breaking into bits in the skin of the hands or neck, cause great irritation and smarting. These nettling or poisonous hairs or spines are especially venomous in the larva of 192Orgyia, Empretia stimulea, Hyperchiria io, the larvæ of the saturnians (Fig. 217) and lasiocampids, etc. They rarely occur in insects of other orders, though the skin of Telephorus is said by Leydig to bear glandular hairs.

Fig. 216.—Section of a subdorsal tubercle from a larva in stage 1: sc, the setigenous cells, one for each seta; pglc, nuclei by which the poison is secreted; s, seta; p, poison in middle of a broken spine; cut, cuticle; sd, tub, spinulated surface of the subdorsal tubercle.

Leydig states that in the stout bristles of Saturnia there is, as in the integument of the body, a homogeneous cuticula, under which is the cellular matrix (hypodermis), and the clear contents (hyaloplasma) are secreted from the blood. The cell-structure of the hairs consist, as in the cells of the body, of spongioplasma and hyaloplasma. Leydig has observed the droplets of the secretion of the caterpillar of Saturnia carpini oozing through distinctly observable pores, and states that there are similar openings in the hairs and scales. Dewitz found easily observable openings at the end of the hair of a large exotic weevil (Fig. 130).

The advanced nymph of Psylla is also armed with clavate glandular hairs (Fig. 178).

Fig. 217.—Armature of last four segments of Callosamia promethea: a, a dorsal seta; b, one showing the poison (p) within.

The tubercles are outgrowths of the body-walls; they are either smooth, warty, or spiny, as in many caterpillars. While the armature of insects is of little morphological 193importance, it is evidently of great biological importance, the welfare or even the life of the insect depending upon it; and it varies in each species of insect, especially in Diptera, where the position of even a single seta characterizes the species.

Fig. 218.—Section through an antennal pectination of Saturnia carpini: a, hypodermis, formative cells of the hairs (c); d, cuticula; e, trachea.—After Semper.

Fig. 219.—Flattened hairs from the lateral tufts of larva of Gastropacha americana: A, three from the lateral tuft of Heteropacha rileyana.

The mode of development of the hairs was first described by Semper. In the pectination of the antenna of Saturnia carpini he observed that the hairs arise, like the scales of the wings, from large round formative-cells lying in the cavity, which send out through the hypodermis and cuticle a long slender process which finally becomes the hair (Fig. 218).

Tactile hairs are those setæ arising over nerve cells or nerve terminations and will be discussed under the organs of sense.

Fig. 220.—The same in G. quercifolia: a, a small hair ending in two minute processes.

Scales.—In very rare cases the hairs of caterpillars (Fig. 219) are flattened and scale-like, and this passage in the same insect of cylindrical hairs into flattened scale-like ones, shows that the scales are only modified hairs. Also, as we shall see farther on, Semper has proved that their mode of origin is identical. While true scales are characteristic of Synaptera (Thysanura and Colembola), as well as Lepidoptera and Trichoptera, they also occur in the Psocidæ (Amphientomum), in 194many Coleoptera (Curculionidæ, Cleridæ, Ptinidæ, Dermestidæ, Byrrhidæ, Scarabæidæ, Elateridæ, and Cerambycidæ), and in the Culicidæ, and a few other Diptera, though they are especially characteristic of the Lepidoptera, not a species of this great order being known to be entirely destitute of them.

Fig. 221.—Flattened and spinulated hairs of tufts of larva of Acronycta hastulifera.

Fig. 222.—Scales from dorsal tuft, on second thoracic segment of larva of Gastropacha quercifolia.

The scales vary much in shape, but are more or less tile-like, attached to the surface of the body or wing by a short slender pedicel, and are more loosely connected with the integument than the hairs, which are thicker at the base or insertion than beyond.

The markings of the scales, both of Synaptera and Lepidoptera, are very elaborate, consisting of raised lines, ridges, or striæ with transverse ridges between. “The striæ of the transparent scales of Micropteryx are from about 500 to 300 to the millimetre, varying in different species. The 195opaque scales of Morpho, which show metallic reflections, have about 1400 striæ to the millimetre.” (Kellogg.)

The primary use of scales, as observed by Kellogg, is to protect the body, as seen in Synaptera and Lepidoptera. A nearly as important use is the production of colors and patterns of colors and markings, while in certain butterflies certain scales function as the external openings of dermal scent-glands, and they afford in some cases (as first claimed by Kettelhoit in 1860) generic and specific characters. Spuler has shown that the scales are strengthened by internal chitinous pillars. Burgess has observed in the scales of Danais plexippus that the under surface of the scales is usually smooth, or provided with few and poorly developed ridges, and this has been confirmed by Spuler and by Mayer (Fig. 226).

In the irised and metallic scales the ridges, says Spuler, are not divided into teeth, and they converge at the base to the pedicel and also toward the end of the scale (Micropteryx), or end in a single process beyond the middle (the brass-colored scales of Plusia chrysitis).

The arrangement of the scales on the wings is, in the generalized moths, irregular; in the more specialized forms they are arranged in bands forming groups, and in the most specialized Lepidoptera they are more thickly crowded, overlapping each other and inserted in regular rows crossing the wings, these rows either uniting with each other or running parallel. (Spuler.) The scattered irregular arrangement seen in Micropteryx is also characteristic of the Trichoptera and of Amphientomum.

Fig. 223.—Portion of a longitudinal section through one of the young pupal wings of a summer pupa of Vanessa antiopa: s, young scale; leu. cy., leucocyte; mbr. pr., ground membrane; prc, hypodermis-cells.

Fig. 224.—Portion of a longitudinal section through one wall only of the pupal wing of a specimen slightly older than that of Fig. 223; s, older scale.

Development of the scales.—The mode of origin of the scales was first worked out by Semper in 1886, who stated that in the wing of the pupal Sphinx and Saturnia they are seen, in sections, to arise from large roundish cells just under the hypodermis and which have a projection which passes out between the hypodermis (his “epidermis”) cells, expanding into a more or less spherical vesicle, the latter being the first indication of the future scale. He 196observed that the scales are not all formed at once, but arise one after another, so that on one and the same wing the scales are in different stages of development.

Fig. 225.—Portion of a longitudinal section through a pupal wing about eight days before emergence: s, formative scale-cell; upper s, a scale.

More recently Schaeffer has stated that the scales and also the hairs are evaginations of greatly enlarged hypodermis cells, and still more complete evidence has been afforded by A. G. Mayer (1896). In the wings of Lepidoptera, about three weeks before the imago emerges, certain of the hypodermis cells, which occur at regular intervals, begin to increase in size and to project slightly above the level of the hypodermis; these are Semper’s “formative cells,” and are destined to secrete the scales. They increase in length, and appear as in Fig. 223. In the next stage observed, the projections are much longer (Fig. 224). The hypodermis is now thrown up into a regular series of ridges, which run across the wing. Each ridge, says Mayer, corresponds in position with a row of formative cells, and each furrow with the interval between two adjacent rows. The scales always project from the tops of these ridges. The ground or basal membrane has not participated in this folding, and the deep processes of the hypodermis (prc) that once extended to this membrane have largely disappeared. Figure 225 represents a more advanced stage almost eight days before the emergence of the imago.

The scales are originally filled with protoplasm, which gradually withdraws, leaving behind it little chitinous bars or pillars which serve to bind together the upper and lower surfaces of the scales, and finally the scales become “merely little flattened hollow sacs containing only air.” As Mayer shows (Figs. 226, 227), from the study of scales examined four days before emergence of the butterfly (Danais), “the striations upon the upper surface of the scale are due to a series of parallel longitudinal ridges,” while the under side is usually smooth.

The mode of insertion is seen in Fig. 227. The narrow cylindrical pedicel of the scale is merely, according to Semper, inserted into a minute close-fitting socket, which perforates the wing-membrane, 197and not into a tube, as Landois supposed. Spuler describes a sort of double sac structure or follicle (Schuppenbalg) which receives the hollow pedicel of the scale. This was originally (1860) observed by F. J. Carl Mayer, but more fully examined by Spuler (Fig. 228) though not detected by A. G. Mayer.

Fig. 226.—Portion of a cross-section through the pupal wing of Danais plexippus, about six days before emergence: sg, scale; cta.al, wing-membrane; cl.frm, formative cell of the scale; mbr.pr, ground-membrane; fbr.h′drm, hypodermal fibres of pupal wings. A, portion of a longitudinal section through the pupal wing, eight or nine days before emergence; prc, processes of young hypodermis scales.—This and Figs. 223–225 after Mayer.

Spinules, hair-scales, hair-fields, and androconia.—Besides the scales, fine spinules occur on the thickened veins of the wings of the Blattidæ, where they resemble fir-cones; also in the Perlidæ, in the Trichoptera, and in the more generalized Lepidoptera (Micropterygidæ and Hepialidæ), occur, as indicated by Spuler, delicate chitinous hollow spinules scarcely one-tenth as long as, and more numerous than, the scales, which sometimes form what he calls “Haftfelds,” or holding areas. These spinules have also been noticed by Kellogg, and by myself in Micropteryx; Kellogg, and also Spuler, have observed them in certain Trichoptera (Hydropsyche). 198These also occur on the veins, and detached ones near large one-jointed hairs, or hair-scales, said by Kellogg to be striated. Kellogg has detected these scale-hairs, as he calls them, in Panorpa.

Fig. 227.—View looking down upon the upper (i.e. exposed) surface of one of the large scales situated on the veins of Danais plexippus, about four days before emergence: clm, chitinous pillars found in scales. A, a smaller scale, a, a′, sections of the scales. B, leucocyte found in the larger scale.—After Mayer.

Fig. 228.—Scale-follicles: A, of a scale of Galleria mellonella: r, neck-ring. B, the same of Polyommatus phlæas. C, the same of a hair on inner edge of hind wing of Lycæna alexis ♀.—After Spuler.

Fig. 229.A, portion of wing of a caddis-fly (Mystacides). B, enlarged, showing the androconia and hair-scales. C, a separate androconium.—After Kellogg.

The “hair-scales” of the phylogenetically older Trichoptera correspond to certain scales of Lepidoptera, especially the Psychidæ (Spuler), variously called “plumules” (Deschamps), “battledore scales,” also certain minute cylindrical hairs. To these scent-scales 199is applied the term androconia. They are found, almost without exception, on the upper side of the fore wings, occurring in limited areas, such as the discal spots, or on folds of the wings. Fritz Müller has shown that they function as scent-scales, and are confined to the males. Kellogg has detected androconia-like scales on the wings of a caddis-fly, Mystacides punctata (Fig. 229).

Fig. 280.—Cross-section of androconia surface on wing of Thecla calanus; a, androconia; gl, gland of base; s, ordinary scales; w, wing in section.—After Thomas.

Thomas has proved by sections of the wing of Danais, etc., that the androconia arise from glands situated in a fold of the wing (Fig. 230), and he states that the material elaborated by the local glands, and distributed upon the surface of the wing by the androconia, is that which gives to many of the Lepidoptera their characteristic odor. On comparing these “glands,” it is evident that they are groups of specialized formative cells of Semper (trichogens), which secrete an odorous fluid, issuing perhaps from extremely fine pore-canals at the ends of the androconia. They thus correspond to the glandular hairs, poison-hairs, and spines of caterpillars, the formative cells of which contain either a clear lymph or poison.

LITERATURE

a. Hairs, bristles, cleaning spines, calcaria, combs, etc.

Leydig, Franz. Zum feineren Bau der Arthropoden. (Müller’s Archiv f. Anat. und Phys., 1855, pp. 376–480.)

Fobel, Auguste. Les fourmis de la Suisse. Bâle, 1874.

Saunders, Edward. Remarks on the hairs of some of our British Hymenoptera. (Trans. Ent. Soc. London, 1878, pp. 169–171.)

Perez, J. Notes d’apiculture. (Bull. Soc. d’Apic. de la Gironde, Bordeaux, 1882.)

Osten Sacken, C. R. von. An essay on comparative chætotaxy, or the arrangement of characteristic bristles of Diptera. (Trans. Ent. Soc. London, 1884, pp. 497–517.)

—— Preliminary notice of a subdivision of the suborder Orthorrhapha Brachycera (Diptera) on chætotactic principles. (Berlin Ent. Zeitschr., 1896, pp. 365–373.)

200Janet, Charles. Études sur les fourmis. 8e Note. Sur l’organe de nettoyage tibio-tarsien de Myrmica rubra. (Ann. Soc. Ent. France, 1895, pp. 691–704, 6 Figs.)

See also J. B. Smith’s Economic Entomology, 1896, hairs of bees. Also the writings of De Geer, Huber, Fenger, Mayr, Forel, Canestrini, and Berlese (1880); Dahl, Cheshire, etc.

b. Glandular and poisonous setæ and spines

Ratzeburg, J. Th. Ch. Ueber entomologische Krankheiten. (Stettin Ent. Zeit., 1846, vii, pp. 35–41.)

Zeller, P. C. Revision der Pterophoriden. (Linnæa Ent., vi, pp. 319–416, 1852, at p. 356 speaks of “Drüsenhärchen.”)

Dimmock, George. On some glands which open externally on insects. (Psyche, iii, pp. 387–401, 1882.)

Goossens, Th. Des chenilles urticants. (Ann. Soc. Ent. France, 1881, pp. 231–236.) Des chenilles vésicants. (Ibid., 1886, pp. 461–464.)

Packard, A. S. Notes on some points in the external structure and phylogeny of insects. (Proc. Boston Soc. Nat. Hist., xxv, 1890, pp. 83–114, 2 Pls.)

—— A study of the transformations and anatomy of Lagoa crispata, a bombycine moth. (Proc. Amer. Phil. Soc., xxxii, pp. 275–292, 7 Pls., 1894.)

Holmgren, Emil. Studier öfner hudens och de körtelartade hudorganens morfologi hos Skandinaviska macrolepidopterlarver. (K. Svenska Vetenskaps-Akad. Handl., xxvii, pp. 1–83, Stockholm, 1895, 9 Pls.)

Also the writings of Leydig, Keller, Bach, Karsten, Scribner, Riley, etc.

(See also Literature of repugnatorial glands.)

c. Androconia

Deschamps, Bernard. Récherches microscopiques sur l’organisation des ailes der Lépidoptères. (Ann. des Sc. nat. [?], iii, pp. 111–157, 1835.)

Waufor, T. W. On certain butterfly scales characteristic of sex. (London, 1867–68.)

McIntire, S. J. Notes on the minute structure of the scales of certain insects. (London, 1871.)

Anthony, J. The markings on the battledore scales of some of the Lepidoptera. (London, 1872.)

Scudder, S. H. Antigeny or sexual dimorphism in butterflies. (Proc. Amer. Acad. Arts and Sc., xii, 1877, pp. 150–158.) Also Butterflies, etc. (New York, 1881, pp. 192–206, figs.)

Müller, Fritz. A prega costal das Hesperideas. (Archivas do Museo nac. do Rio de Janeiro, iii, pp. 41–50, 2 Pls., 1878.)

Thomas, M. B. The androconia of Lepidoptera. (Amer. Nat., xxvii, pp. 1018–1021, 2 Pls., 1893.)

d. Scales

Leydig, Franz. Zum feineren Bau der Arthropoden. (Archiv f. Anat. und Phys., 1855, pp. 376–480, 1 Taf.)

Semper, Carl. Beobachtungen über die Bildung der Flügel, Schuppen, und Haare bei den Lepidopteren. (Zeitschrift f. wissensch. Zoologie, 1857, pp. 326–339, 1 Taf.)

Mayer, F. T. Karl. Ueber den Staub der Schmetterlingsflügel. (Allgem. mediz. Centralzeitung, 1860, pp. 772–774.)

201Landois, H. Beiträge zur Entwicklungsgeschichte der Schmetterlingsflügel in der Raupe und Puppe. (Zeitschr. f. wissensch. Zoologie, xxi, 1871, pp. 305–316, 1 Taf.)

Weismann, August. Ueber Duftschuppen. (Zool. Anzeiger, i, 1878, pp. 98–99.)

Dimmock, George. Scales of Coleoptera. (Psyche, iv, pp. 1–11, 23–27, 43–47, 63–71, 1883.)

Schaeffer, Cäsar. Beiträge zur Histologie der Insekten. (Zool. Jahrbücher, Abth. f. Anat. u. Ontog., iii, pp. 611–652, 2 Pls., 1889.)

Kellogg, Vernon L. The taxonomic value of the scales of the Lepidoptera. (Kansas Univ. Quart., iii, pp. 45–89, figs. 1–17, 9 Taf., 1894.)

Mayer, Alfred G. The development of the wing-scales and their pigment in butterflies and moths. (Bull. Mus. Comp. Zool., xxix., 1896, pp. 209–236, 7 Pls.)

Spuler, Arnold. Beiträge zur Kenntniss des feineren Baues und der Phylogenie der Flügeltedeckung der Schmetterlings. (Zool. Jahrb. Abth. f. Anat. u. Ontog., viii, pp. 520–543, 1 Taf., 1895.)

—— Ueber das Vorhandensein von Schuppenbalg bei den Schmetterlingen. (Biol. Centralblatt, xvi, Sept. 15, 1896, pp. 677–679, 3 figs.)

THE COLORS OF INSECTS

The colors and bright markings of insects, especially those of butterflies, render them the most brilliant and beautiful creatures in existence, rivalling and even excelling the gay hues of our most splendidly colored birds. The subject has been but recently taken up and is in a somewhat crude condition, but the leading features have been roughly sketched out by the work of a few observers from a physical, chemical, and biological point of view.

The colors of insects, as of all other animals, are primarily due to the action of light and air; other factors are, as Hagen observes, heat and cold, moisture and dryness, as recently shown by the experiments on butterflies by Dorfmeister, Weismann, W. H. Edwards, and later observers. They have their seat in the integument. Hagen divides colors into optical and natural.

Optical colors.—“These,” says Hagen, “are produced by the interference of light, and are by no means rare among insects, but they are solely optical phenomena. Colors by the interference of light are produced in two different ways: either by thin superposed lamellæ, or by many very fine lines or small impressions in very close juxtaposition.

“1. There must be present at least two superposed lamellæ to produce colors by interference. The naked wings of Diptera, of dragon-flies, and of certain Neuroptera often show beautiful interference colors. The wings of Chrysopa and Agrion show interference colors only for a certain time, viz., as long as the membranes of the wings are soft and not firmly glued together. Afterwards such wings become simply hyaline.

202“The scales of Entimus and other Curculionidæ are well known for their brilliancy, and it is interesting to remark that when dry scales are examined with the microscope, many are found partly injured, which give in different places different colors, according to the number of layers which remain. The elytra of some Chrysomelina and other beetles with iridescent colors probably belong to the same category.

“2. When there are scales with many fine lines or small impressions close to each other, we have the second mode of producing colors.

“The fine longitudinal and transversal lines of lepidopterous scales seem to serve admirably well to produce the brilliant effect of color-changing butterflies. But there must be something more present, as most of the scales of Lepidoptera are provided with similarly fine lines, and only comparatively few species change colors. I remark purposely that the lines in the color-changing scales are not in nearer juxtaposition.” (Hagen.)

“The colors of butterflies change mostly from purple to blue, sometimes to yellow. The splendid violet color at the end of the wings of Callosune ione is brought out by a combination of the natural with interference colors. Originally the scales are colored lake-red; but a blue interference color is mixed with it; hence the violet hue results. The blue tones, i.e. the splendid varying blue of the Morpho butterflies, Schatz claims, owe their hue less to the interference of light than to a clouded layer of scales situated over the dark ground, through which the light becomes reflected on the same. The scales of the Morphids are in reality brown, as we see by transmitted light; moreover, only the upper side of the scales sends off blue reflections—the under side is simply brown. But the blue scales of Urvilliana are also shining blue beneath; by transmitted light they appear as if clear yellow. The smaragd-green scales of Priamus show by transmitted light a bright red-orange, and the orange-yellow of Crœsus a deep grass-green.” (Schatz in Kolbe.)

“Krukenberg presumes the golden-green color of Carabus auratus to be an interference color. It is not changed by the interference of light, nor was he able to extract from the elytra any green pigment with ether, benzol, carbon of sulphur, chloroform, or alcohol, even after having previously submitted the elytra to the influence of muriatic acid or ammonia. Chlorophyll is not present, whether free or combined with an acid.” (Hagen.)

Leydig has shown that the interference colors of the hairs of certain worms (Aphrodite and Eunice) may be produced by very small impressions in juxtaposition, which bring about the same effect as striæ. Such an arrangement occurs on the feathers of birds, i.e. on the necks of pigeons and elsewhere, and Hagen suggests that this kind of interference colors occurs more frequently among insects than is commonly known. At least the limbs of certain forms appear yellow, but when held in a certain position change to brown or blackish. “I know of no other explanation of this not uncommon fact on the legs of Diptera, of Hymenoptera, and of Phryganidæ.” Interference colors, he adds, may occur in the same place together with natural colors. “The mirror spots of Saturnia pernyi show besides the interference colors a white substance in the cells of the matrix, which Leydig believes to be guanin. But this fact is denied by Krukenberg for the same species and also for Attacus mylitta and Plusia chrysitis.”

203Natural colors.—These are divided by Hagen into dermal (cuticular) and hypodermal. The dermal colors are due to pigment deposited in the form of very small nuclei in the cuticula. Hagen considers them as “produced mostly by oxidation or carbonization, in consequence of a chemical process originating and accompanying the development and the transformations of insects.”

“To a certain extent the dermal colors may have been derived from hypodermal colors, as the cuticula is secreted by the hypodermis, and the colors may have been changed by oxidation and air-tight seclusion. The cuticula is in certain cases entirely colorless,—so in the green caterpillar of Sphinx ocellata; but the intensely red and black spots of the caterpillar of Papilio machaon belong to the cuticula, and only the main yellow color of the body to the hypodermis.” (Leydig, Histiol., p. 114.)

“The dermal colors are red, brown, black, and all intermediate shades, and all metallic colors, blue, green, bronze, copper, silver, and gold. The dermal colors are easily to be recognized as such, because they are persistent, never becoming obliterated or changed after death.” (Hagen.)

Minot and Burgess refer to the cuticular colors of the cotton-worm (Aletia), the dark brown color belonging to the cuticula or crust. “Upon the outside of the crust is a very thin but distinct layer, which in certain parts rises up into a great number of minute, pointed spines that look like so many dots in a surface view. Each spine is pigmented diffusely, and together they produce the brown markings. The spines are clustered in little groups, one group over each underlying hypodermal cell.” (U. S. Ent. Comm., 4th Report, p. 46.) Minot also shows that in caterpillars generally a part of the coloration is caused by pigmentation of the cuticula.

In a dull-colored insect, such as the Mormon cricket (Anabrus), the coloration, as Minot states, depends principally upon the pigment of the hypodermis shining through the cuticula. “Most of the cells contain dull, reddish-brown granules, but scattered in among them are patches of cells bright green in color. I have observed no cells intermediate in color; on the contrary, the passage is abrupt, a brown or red cell lying next a green one. Indeed, I have never seen any microscopic object more bizarre than a piece of the epidermis of Anabrus spread out and viewed from the surface.” (2d Report U. S. Ent. Comm., p. 189.)

The pigment may extend through the entire cuticula, but it is usually confined to the outermost layers, and occurs there in union with a peculiar modelling of the upper surface into microscopic figures which are of interest not only from their delicacy, but because they vary with each species. (See p. 184.)

The hypodermal colors, situated in the hypodermis, are, according to Hagen, the result of a chemical process, generating color out of substances contained in the body. They are easily recognized, since 204they fade, change, and disappear after death. But where these colors are preserved after death and enclosed in air-tight sacs, as in the elytra and scales and hairs of the body, they persist, though, as we well know, they may fade after exposure to light.

The hypodermal colors are mostly brighter and lighter than the dermal ones, being light blue or green in different shades, yellow to orange, and the numerous shades of these colors combined with white; exceptionally they are metallic, as in Cassida, and are then obliterated after death.

“The fact that such metallic colors can be retained in dead specimens by putting a drop of glycerine under the elytra, leads us to conclude that those colors are based upon fat substances. The hypodermal colors are never glossy, as far as I know; the dermal colors frequently.

“As the wings, elytra, and hairs all possess a cuticula, dermal colors are frequently to be found, together with hypodermal ones, chiefly in metallic colors. In the same place both colors may be present, or one of them alone. So we find hypodermal colors in the elytra of Lampyridæ. In the elytra of the Cicindelidæ the main metallic color is dermal, the white lines or spots are hypodermal, by which arrangement the variability in size and shape of those spots is explained.

“There occur in a number of insects external colors, that is, colors upon the cuticula, which I consider to be in fact displaced hypodermal colors: the mealy pale blue or white upon the abdomen of some Odonata, the white on many Hemiptera, the pale gray on the elytra and on the thorax of the Goliath beetle, and the yellowish powder on Lixus. Some of these colors dissolve easily by ether or melt in heat, and some of them are a kind of wax. I believe that those colors are produced in the hypodermis, and are exuded through the pore-canals.” (Hagen.)

The white colors are simply for the most part due to the inclusion of air in scales. The white mother-of-pearl spots of Argynnis are produced by a system of fine transverse pore-canals filled with air; in Hydrometra the white ventral marks have the same origin. (Leydig.)

The further statements and criticisms of Hagen regarding the relation of color to mimicry, sexual selection, and the origin of patterns are of much weight and will be referred to under those heads. Indeed, these subjects cannot well be discussed without reference to the fundamental facts stated in the masterly papers of Leydig and of Hagen, and much of the theorizing of these latter days is ill-founded, because the colors of insects and animals are attributed to natural selection, when they seem really the result of the action of the primary factors of organic evolution, such as changes of light, heat, cold, and chemical processes dependent on the former.

As to the chemical nature of color, Hagen, after quoting the results of Krukenberg and others, thinks that the colors of insects are chemically produced by a combination of fats or fat-acids with other 205acids or alkalis under the influence of air, light, and heat. He concludes:—

1. That some colors of insects can be changed or obliterated by acids.

2. That two natural colors, madder-lake and indigo, can be produced artificially by the influence of acid on fat-bodies.

3. As protein bodies in insects are changed into fat-bodies, and may be changed by acids contained in insects into fat-acids, the formation of colors in the same manner seems probable.

4. That colors can be changed by different temperatures.

5. That the pattern is originated probably by a combination of oxygen with the integument.

6. That mimicry of the hypodermal colors may be effected by a kind of photographic process.

7. Finally, color and pattern are produced by physiological processes in the interior of the bodies of insects.

Krukenberg concludes that change of color (in perfectly developed insects) is a consequence of the change of food, and can be explained by the alteration of the pigment through heat and light. His experiments were made in order to ascertain the cause of the turning of green grasshoppers in autumn into yellow and pink. He tried to answer two questions: First, does the pigment of grasshoppers originate directly out of the food, and does it consist of pure chlorophyll or of a substance containing chlorophyll, or is it to be accepted as a peculiar product of the organism? Second, is the color the consequence of only one pigment, or of several? Special analysis proves that the green color has no connection with chlorophyll. He concludes: “It is evident that the green color of the grasshopper is the consequence of several different pigments which can be separated by a chemical process.” Krukenberg believes that light has a marked influence on the color of insects and that light turns to red or pink the insects which were green during the summer. It would seem, however, more probable that cold was the agent, the change being due to the colder autumn weather.

Here we might refer to the results of the studies of Buckton and Sorby, on the changes in color of Aphides:—

“1. The purple coloring matter appears to be a quasi-living principle, and not a product of a subsequent chemical oxidizing process. Mounted in balsam or other preserving fluids, the darker species stain the fluid a fine violet.

“2. As autumn approaches and cold weather reduces the activity of the Aphides, the lively greens and yellows commonly become converted into ferruginous red, and even dark brown, which last hue in reality partakes more or less of intense violet or purple. These changes have some analogy with the brilliant hues assumed by maple and other leaves during the process of slow decay.

“3. Aqueous solutions of crushed dark brown and yellow-green varieties of Aphides originate different colors with acids and alkalies.

“4. In the generality of cases coloring-matters, such as indigo, Indian yellow, madder-lake, and the like, do not separately exist in the substance of vegetables, but the pigments are disengaged through fermentation or oxygenation. Again, alizarin itself is reddish yellow, but alkaline solutions strike it a rich 206violet just as we find them to act towards the substance which Mr. Sorby calls aphidilutein.

“5. Mr. Sorby’s four stages of the changes effected by the oxidation of aphideine produce four different substances.”

Chemical and physical nature of the pigment.—Researches in this difficult field of inquiry have been made by Landois (1864), Sorby (1871), Meldola (1871), by Krukenberg (1884), and more recently by Coste, Urech, Hopkins, and Mayer, and the subject is of fundamental importance in dealing with mimicry and protective coloration, the primary causes of which appear to be due to the action of physical and chemical agents.

Over twenty years ago Meldola observed that the yellow pigment of the sulphur-yellow butterfly (Gonopteryx rhamni) was soluble in water, and showed that its aqueous solution had an acid reaction.

Besides the yellow uranidin found by Krukenberg in different beetles and lepidopterous pupæ, still other coloring-matters, which are very constant in different species are readily recognized by the spectroscope. “Thus there appear in the brownish yellow lymph of Attacus pernyi, Callosamia promethea and Telea polyphemus, after saponification of the precipitated soap readily effected by ether, or incompletely or not removed by benzine, a chlorophane-like lipochrome; and in the yellowish green lymph of Saturnia pyri and of Platysamia cecropia besides this pigment still another whose spectrum shows a broad band on D, but which disappears with the addition of acetic acid or ammonia, as also after a long heating of the lymph up to 66° C.”

Coste, and more especially Urech, have shown that many of the pigments may be dissolved out of the scales by means of chemical reagents, giving colored solutions, and leaving the scales white or colorless. They have also shown that some of these pigments may be changed in color by the action of reagents, and then restored to their original color by other reagents. They have proved that reds, yellows, browns, and blacks are always due to pigments, and in a few cases greens, blues, violets, purples, and whites, and not, as is usually the case, to structural conditions, such as striæ on the scales (Mayer). They confined themselves solely to the chemical side of the problem, not considering the structure of the scales themselves.

Urech has also discovered a beautiful smaragd-green coloring-matter in the wings (not in the scales) of the pupa of Pieris brassicæ. It is not chlorophyll, and Urech suggests that it may be either the germinal substance of the pigments of the scales or its bearer. It is not the pigment of the blood.

Urech has also demonstrated that in many Lepidoptera the color of the urine which is voided upon emergence from the chrysalis is similar to the principal color of the scales.

207Hopkins has worked on the pigments within the scales of butterflies. The yellow pigment in Gonopteryx rhamni is a derivation of uric acid, and he calls it lepidotic acid. Its aqueous solution is strongly acid to litmus, and must be bad-tasting to birds.

Hopkins has dissolved the red pigment from the border of the hind wing of Delias eucharis, an Indian butterfly, in pure water, finding as the result a yellow solution; but if the solution be evaporated to dryness, the solid residue of pigment is red once more. He has obtained from this pigment of eucharis a silver compound which contains a percentage of metals exactly equal to that from the pigment of G. rhamni. (Nature, April 2, 1892.)

“The scales of the wings of the white butterflies (Pieridæ) are also shown by Hopkins to contain uric acid, this substance practically acting as a white pigment in these insects. A yellow pigment, widely distributed in the same family, is shown to be a derivative of uric acid, and its artificial production as a by-product of the hydrolysis of uric acid is demonstrated. That this yellow pigment is an ordinary excretory product of the butterfly is indicated by the fact that an identical substance is voided from the rectum on emergence from the pupa. These excretory pigments, which have well-marked reactions, are apparently confined to the Pieridæ, and are not found in other Rhopalocera. This fact shows that when a Pierid mimics an insect belonging to another group, the pigments of the mimicked and mimicking insects, respectively, are chemically quite distinct. Other pigments existing, not in the scales, but between the wing-membranes, are shown to be of use for ornament.” (Proc. Royal Soc., London, 1894.)

Griffiths (1892) claims that the green pigment found in several species of Papilio, Hesperia, and Limenitis, also in Noctuidæ, Geometridæ, and Sphingidæ likewise consists of a derivative of uric acid, which he calls lepidopteric acid. By prolonged boiling in HCl it is converted into uric acid.

Spuler, however, finds that green does not depend on pigmentation, but is an optical color. As remarked by Spuler, either the chitin of the scales itself is colored reddish (yellow grayish), or the pigment is secreted in the nuclei.

A. G. Mayer believes that the pigments of the scales are derived from the hæmolymph or blood of the pupa, for the following reasons: (1) He is unable to find anything but blood within the scales during the time when the pigment is formed. (2) In Lepidoptera generally the first color to appear upon the pupal wings is a dull ochre-yellow, or drab, and this is also the color assumed by the blood when it is removed from the pupa and exposed to the air. (3) He has succeeded by artificial means in manufacturing several pigments from the blood which are similar in color to various markings upon the wing of the imago; chemical reagents have the same effect upon these manufactured pigments that they do upon the similarly colored pigments of the wings. “It should be here noted,” he says, “that in 1866 Landois pointed out the fact that the color of the dried blood of many caterpillars is similar to the ground color of the wings of the mature insect.”

Ontogenetic and phylogenetic development of colors.—The colors of the wings of Lepidoptera, as is well known, are acquired at the end 208of the pupal state. The order of development of the colors in the pupal wings has been observed by Schaeffer, Van Bemmelen, Urech, Haase, Dixey, Spuler, and A. G. Mayer. The immature wings are at first transparent and full of protoplasm. The transparent condition of the wings corresponds to the period before the scales are formed, and when they are full of protoplasm; they then become whitish as the scales develop; the latter are at first filled with protoplasm, and afterwards turn whitish, being little hollow sacks filled with air. After the protoplasm has completely withdrawn from the scales, the blood of the pupa enters them, and then the coloring-matter forms. (Mayer.) He adds that “about twenty-four hours after the appearance of the dull yellow suffusion the mature colors begin to show themselves. They arise, faint at first, in places near the centre of the wings, and are distinguished by the fact that they first appear upon areas between the nervures, never upon the nervures themselves. Indeed, the last place to acquire the mature coloration are the outer and costal edges of the wings, and the nervures.”

The faint color of the scales gradually increases in intensity. “For example, if a scale be destined to become black, it first becomes pale grayish brown, and this color gradually deepens into black.”

Urech states that in Vanessa io first a white, and in V. urticæ a pale reddish hue, are spread over the entire wings, and then successively arise other colors in the following order: yellow, yellow to brown, red, brown and black.

Spuler, however, claims that the differentiation of colors and markings do not follow one another, but arise simultaneously, and that his view is confirmed by Fischer. This may be the case with the highly specialized and diversely marked butterflies, but certainly taking the Lepidoptera as a whole the yellows and drabs must have been the primitive hues, the other colors being gradually added in the later more specialized forms.

It is noticeable that the most generalized moths, such as the species of Micropteryx, Tinea, Psychidæ, Hepialidæ (in general), etc., are dull brown or yellow-drab without bars, stripes, or spots of bright hues. These shades prevail in others of the more primitive Lepidoptera, such as many bombycine moths, and they even appear to a slight extent in certain caddis-flies. The authors mentioned, especially Mayer, whom we quote, claim that “dull ochre-yellows and drabs are, phylogenetically speaking, the oldest pigmental colors in the Lepidoptera; for these are the colors that are assumed by the hæmolymph upon mere exposure to the air. The more brilliant pigmental colors, 209such as bright yellow, reds, greens, etc., are derived by more complex chemical processes. We find that dull ochre-yellow and drabs are at the present day the prevalent colors among the less differentiated nocturnal moths. The diurnal forms of Lepidoptera have almost a monopoly of the brilliant colorations, but even in these diurnal forms one finds that dull yellow or drab colors are still quite common upon those parts of their wings that are hidden from view.”

The more primitive moths being more or less uniformly yellowish or drab, the next step was the formation of bars, stripes, finally spots, and eyed spots, these markings in the later forms appearing simultaneously in one and the same species of certain highly specialized moths and butterflies. All that has been said will prepare the reader for the consideration of the subject of insect coloration. The origin of such markings has been discussed by Weismann, Eimer, Haase, Dixey, Fischer, and others.

LITERATURE

Heer, O. Einfluss des Alpenklimas auf die Farbe der Insecten. (Froebel u. Heer, Mitth. aus dem Gebiete der theoret. Erdkunde, 1836, i, pp. 161–170.)

Goureau. Mémoire sur l’irisation des ailes des insectes. (Ann. Soc. Ent. France, 2 sér., i, 1848, pp. 201–215.)

Laboulbène, A., et M. Follin. Note sur la matière pulvérulente qui recouvre la surface du corps des Lixus et de quelques autres insectes. (Ann. Soc. Ent. de France, 1848, vi, pp. 301–305, Fig.)

Coquerel, Ch. Note sur la prétendue poussière cryptogamique qui recouvre le corps de certains insectes. (Ann. Soc. Ent. France, 1850, viii, pp. 13–15.)

Brauer, F. Beobachtungen in Bezug auf den Farbenwechsel bei Chrysopa vulgaris. (Verhandl. k. k. zool.-botan. Gesellsch. Wien., 1852, pp. 12–14.)

Prittwitz, O. F. W. v. Bemerkungen über die geographische Farbenverteilung unter den Lepidopteren. (Stett. Ent. Zeit., 1855, xvi, pp. 175–185.)

Latham, A. G. The causes of the metallic lustre of the scales on the wings of certain moths. (Proc. Lit. and Phil. Soc. Manchester, iii, 1864, pp. 198–199. Quart. Journ. Micr. Sc., new ser., iv, 1864, pp. 48–49.)

Sorby, H. C. On the coloring matter of some Aphides. (Quart. Journ. Micr. Sc., new ser., xi, 1871, pp. 352–361.)

Leydig, Franz. Bemerkungen über Farben der Hautdecke und Nerven der Drüsen bei Insekten. (Archiv f. mikr. Anatomie, xii, 1876, pp. 536–550, 1 Taf.)

Weismann, A. Studien zur Descendenz-Theorie, ii, 1876.

Hemmerling, Hermann. Ueber die Hautfarbe der Insecten. (Bonn, 1878, p. 27.)

Buckton, C. B. Monograph of the British Aphides. (London, 1879, ii, p. 167.)

Cameron, P. Notes on the coloration and development of insects. (Trans. Ent. Soc. London, 1880, pp. 69–79.)

Hagen, Hermann A. On the color and pattern of insects. (Proc. Amer. Acad. Arts and Sc., 1882, pp. 234–267.)

210Poulton, Edward Bagnall. The essential nature of the colouring of phytophagous larvæ (and their pupæ), etc. (Proc. Roy. Soc. London, xxxviii, pp. 269–315, 1884–1885.)

—— An inquiry into the cause and extent of a special colour-relation between certain exposed lepidopterous pupæ and the surfaces which immediately surround them. (Phil. Trans. Roy. Soc. London, clxxviii, pp. 311–441, 1 Pl., 1887.)

Krukenberg, C. Fr. W. Grundzüge einer vergleichenden Physiologie der Farbstoffe und der Farben. (Heidelberg, 1884, pp. 102.)

McMunn, C. A. Krukenberg’s chromatological speculation. (Nature, xxxi, p. 217, 1885.)

Müller, Fritz, and Dr. H. A. Hagen. The color and pattern of insects. (Kosmos, xiii, 1886, pp. 466–469.)

Slater, J. W. On the presence of tannin in insects and its influence on their colors. (Trans. Ent. Soc. London, 1887, iii, Proceed., pp. 32–34.)

Bemmelen, J. F. van. Ueber die Entwicklung der Farben und Adern auf den Schmetterlingsflügeln. (Tijdschrift der nederland. Dierkundige Vereeniging, ser. 2, pp. 235–247, 1889.)

Hopkins, F. G. Uric acid derivatives functioning as pigments in butterflies. (Proc. Chem. Soc. London, 1889, p. 117; also Nature, xl, p. 335.)

—— Pigment in yellow butterflies. (Nature, xlv, p. 197, 1891.)

—— The pigments of the Pieridæ. (Proc. Roy. Soc. London, lvii, No. 340, pp. 5, 6, 1894. Phil. Trans. Roy. Soc. London, clxxxvi, pp. 661–682, 1896.)

Coste, F. H. P. Contributions to the chemistry of insect colors. (The Entomologist, xxiii, 1890; xxiv, 1891, pp. 9–15, etc. Nature, xlv., pp. 513–517, 541–542, 605.)

Urech, F. Beobachtungen über die verschiedenen Schuppenfarben und die zeitliche Succession ihres Auftretens. (Zool. Anzeiger, xiv, pp. 466–473, 1891; Ibid., August 1, 1892.)

—— Beiträge zur Kenntniss der Farbe von Insektenschuppen. (Zeits. f. Wissens. Zool., lvii, pp. 306–384, 1893.)

Griffiths, A. B. Recherches sur les couleurs de quelques insectes. (C. R. Acad. Sc. Paris, cxv, pp. 958, 959.)

Mayer, Alfred Goldsborough. On the color and color-patterns of moths and butterflies. (Proc. Bost. Soc. Nat. Hist., xxvii., March, 1897, pp. 243–330, 10 Pls. See also p. 201 under Mayer.)

Also the writings of Bates, Beddard, Belt, Butler, Darwin, Dimmock, Dixey, Eimer, Haase, Higgins, Müller, Poulton, Seitz, Wallace, Weismann.

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2. INTERNAL ANATOMY

THE MUSCULAR SYSTEM

In its general arrangement the muscular system of insects corresponds to the segmented structure of the body. Of the muscles belonging to a single segment, some extend from the front edge of one segment to that of the next behind it, and others to the hinder edge; there are also sets of dorsal and ventral muscles passing in an oblique or vertical course (Figs. 16–18). As Lang observes, “the greater part of the muscles of the body can be traced back to a paired system of dorsal and ventral intersegmental longitudinal muscles.” The muscular system is simplest in larval insects, such as caterpillars, where the musculature is serially repeated in each segment.

In the larva of Cossus Lyonet found on one side of the body 217 dorsal, 154 lateral, 369 ventral, and in the thoracic legs 63, or 803 muscles in all. “Adding to this number the 12 small muscles of the second segment, and 8 others of the third, which he did not describe, there would be for all the muscles on one side of the caterpillar 823. This would make for the entire body 1646, without counting a small single muscle which occurs in the subdivision of the last segment,” and also those of the internal organs as well as those of the head, so that the total number probably amounts to about 2000, not 3000, as usually stated in the books. Lubbock admits that Lyonet was right in his mode of estimating the number. In the larva of Pygærci bucephala he found that “the large muscles scarcely vary at all,” though certain smaller ones are very variable. Lubbock observed that certain of the longitudinal muscles in the caterpillar of Diloba split up into numerous, not less than ten, separate fascicles. “This separation of the fibres composing a muscle into separate fascicles is carried on to a much greater extent in the larvæ of Coleoptera. Of course in the imago the number of thoracic muscles is greatly increased, or at least in Dyticus and the wood-feeding Lamellicorns, which alone I have examined. In these two groups each of the larger muscles is represented by at least twenty separate fascicles, which makes it far more difficult to distinguish the arrangement of the muscles.”

The muscles are whitish or colorless and transparent, those in the thorax being yellowish or pale brown; and of a soft, almost gelatinous consistence. In form they are simply flat and thin, straight, band-like, or in rare cases pyramidal, barrel or feather shaped. They act variously as rotators, elevators, depressors, retractors, protractors, flexors, and extensors.

212

Fig. 231.—Diagram of the muscles and nerves of the ventral surface of the segments in the larva of Sphinx ligustri: A, A, recti muscles; 1, 2, ventral recti muscles (1, recti majores; 2, recti minores); 3, ridge giving origin to recti muscles of one segment, and insertion to the same of the adjoining segment; 4, ridge for attachment of muscle; 5, retractor ventriculi, connecting the mid-intestine with the outer integument of the body. B, 6, first oblique,—7, second oblique,—9, 10, third oblique, muscles; 11, fourth oblique,—12. third rectus,—13, fifth oblique,—14, triangularis, muscle; 15, transversus medius; 16, transverse ridge; 17, transversi abdominales; 18, abdominales anteriores; 19, 20, abdominales laterales, some (20) longer than others; 21, obliquus posterior; 22, postero-laterales obliqui; 23, transversus lateralis; 24, second transversus lateralis; 25, retractor spiraculi, or constrictor of the spiracles, attached by a long tendon (26); 27, retractor valvulæ.

Nerves: a, ganglion,—c, transverse nerves, of which p is the first, q the second, r the third,
and s the fourth branch; t, the main trunk, which crosses the great longitudinal trachea, receives a
filament from the transverse nerve (n), and divides into two branches (t);—some of these branches
form a small plexus (u); the nerve t divides in two divisions (p and v). The second division ends
in w and x; the branch q divides into y and z. For other explanations, see Newport, art. Insecta.—After
Newport.

213

Fig. 232.—Musculature of the European cockchafer, Melolontha vulgaris: a, a, levatores capitis; b, depressores capitis; c, rotatores capitis; d, depressors externi; e, retractor or flexor of the jugular plate; f, oblique extensor of the jugular plate; g, the other retractor of the jugular plate; h, retractor prothoracis superior; i, inferior retractor, the proper depressor of the prothorax; k, elevator prothoracis; l, one of the rotatores prothoracis; m, n, o, flexors of the coxa; x, great depressor muscle of the wing; y, y, elevators and protractors attached to the metaphragma and base of the postfurca; z, second flexor of hind leg; a, a, extensors of hind leg; c, c, dorsal recti of abdomen. Q, ejaculatory duct; R, penis; S, its prepuce. M, rectum.—After Straus-Durckheim, from Newport.

Our knowledge of the muscular system of insects is still very imperfect. To work it out thoroughly one should begin first with that of Scolopendrella, then some generalized synapterous form, as Japyx or Lepisma, then passing to that of a caterpillar, and ending with some of the more highly specialized forms, such as a beetle, etc. Thus far our knowledge is confined to that of the caterpillars (Lyonet, Newport, and Lubbock) and the beetle (Straus-Durckheim) and ants (Forel, Lubbock, and Janet).

Musculature of a caterpillar.—Newport’s account of that of the larva of Sphinx ligustri is the most useful (Fig. 231). The muscles here present, he says, great uniformity of size and distribution in every segment, the motions of each of these divisions of the body being almost precisely similar, especially in the 4th to 9th trunk segments. In these segments the first layer seen on removing the fat and viscera are the flat straight recti muscles. They are the most powerful of all the trunk muscles, and are those which are most concerned in shortening the body, in effecting the duplicature of the external teguments during the changes of the insect, and which during the larval state mainly assist in locomotion. There are four sets, two dorsal and two ventral (Fig. 231, A, A). Without entering into farther details, the reader is referred to the works of Newport and to Fig. 231.

Musculature of a beetle.—The best general account of the musculature of a perfect insect is that of Straus-Durckheim in his famous work on the Melolontha. 214We will copy the summary of Newport, who adopted the nomenclature applied to these parts by Burmeister:—

“The muscles that connect the head with the thorax are contained within the prothorax (Fig. 232, 2), and are of three kinds, extensors, flexors, and retractors. The extensors, levatores capitis (a, a), consist of two pairs, one of which arises from the middle line of the pronotum, and diverging laterally from its fellow of the opposite side, passes directly forwards, and is inserted by a narrow tendon into the anterior superior margin of the occipital foramen. The other arises further back from the prophragma. It is a long, narrow muscle that passes directly forwards through the prothorax, and is inserted by a tendon near the superior median line of the foramen; so that, while this muscle and its fellow of the opposite side elevate the head almost in a straight line, the one first described, when acting alone or singly, draws the head a little on one side; but when the whole of these muscles act in unison, they simply elevate the head upon the prothorax. The depressors or flexors, depressores capitis (b), are exceedingly short muscles, which arise from the jugular plate, or, when that part does not exist, from the border of the prosternum, and are attached to the inferior margin of the occipital foramen. They simply flex the head on the prothorax. The lateral flexors, depressores externi (d), are two little muscles that arise from the same point as the preceding, and are attached to the lateral inferior margin of the occipital foramen. The rotatory muscles, rotatores capitis (c), are two flat muscles like the elevators, which arise, one at the side of the antefurca and the other from the posterior jugular plate, and passing upwards and outwards are attached to the lateral margin of the occipital foramen. The retractor or flexor of the jugular plate is a small muscle (e) that arises from the margin of the antefurca, and passing directly forwards is inserted by a small tendon into the middle of the jugular piece. The oblique extensor of the jugular plate is a long, slender muscle (f) that arises from the external margin of the pronotum, and passing obliquely downwards and forwards traverses the prothorax and is inserted by a narrow tendon to the jugular plate immediately before the retractor. The other retractor (g) arises from the anterior superior boundary of the pronotum, and passing downwards is inserted into the jugular plate between the larger levator and flexor capitis.

“The muscles proper to the prothorax consist of four pairs, by which it is united to the succeeding segments. The first of these, the superior retractor, retractor prothoracis superior (h), arises by a broad, fleshy head from the anterior external margin of the pronotum, and passing directly backwards is inserted by a tendon into the prophragma, a little on one side of the median line. The next muscle of importance, the inferior retractor (i), arises from the anterior border of the medifurca, and is united to the posterior of the antefurca, thus forming with that muscle part of the great recti of the larva. This muscle must be considered as the proper depressor of the prothorax. The elevator prothoracis (k) is narrow, pyramidal, and arises fleshy from the lateral surface of the prophragma. It passes downwards and is attached by a narrow tendon to the superior portion of the antefurca. The rotatores prothoracis are the largest of all the muscles of this segment. They arise, one on each side (l), by a narrow head from the posterior part of the pronotum, and passing beneath the prophragma are considerably enlarged and attached to the tegument between the two segments, and also to the anterior portion of the mesothorax. The remaining muscle proper to the prothorax is the closer of the spiracle, an exceedingly small muscle not shown in the drawing.

“The other muscles of this segment are those of the legs, which are of considerable size. There are three distinct flexors of the coxa (m, n, o). The first of 215these arises from the superior lateral border of the pronotum, the second from the superior posterior border, the third from the sides of the prothorax, and the fourth a little nearer posteriorly, and the whole of them are attached by narrow tendons to the sides of the coxa. But there is only one extensor muscle to this part. In like manner, the extensor of the trochanter is formed of three portions (Fig. 233, a, b, c); but there is only one flexor (d), and one abductor (e). In the femur, there is one extensor (f),—a long penniform muscle that occupies the superior part of the thigh, and is attached by a tendon to the anterior-posterior margin of the joint formed by the end of the tibia. There is also but one flexor (g) in the femur, which, like the preceding muscle, is penniform, and occupies the inferior portion of the femur, and its tendon is attached to the inferior border of the tibia. In the tibia itself there is also one flexor and one extensor. The flexor (i) occupies the superior portion of the limb, and ends in a long tendon (l) that passes directly through the joints of the tarsus, on their inferior surface, and is attached to the inferior margin of the claw (g). The extensor (h) occupies the inferior portion of the tibia, and is shorter than the preceding muscle, like which it ends in a long tendon that is attached to the upper margin of the claw. Besides these muscles, which are common to the joints of the tarsus, there are two others belonging to the claw, situated in the last joint. The first of these, the extensor (m), is short, and occupies the superior portion of the last phalanx of the tarsus, and the other, the flexor (n), is a much longer penniform muscle, which occupies nearly the whole of the upper and under surface of the posterior part of the phalanx, and is attached, like the long flexor of the tarsus, to the inferior part of the claw.”

Fig. 233.—Muscles of the fore leg of Melolontha vulgaris: a, b, c, three divisions of the extensor of the trochanter; d, flexor,—e, abductor, of the trochanter; f, extensor of the femur; g, flexor of the femur; h, extensor of the tibia; i, flexor of the tibia; l, tendon attached to the lower edge of the claw (g); m, extensor,—n, flexor, of the claw.—After Straus-Durckheim, from Newport.

These are the muscles of the prothorax, and its organs of locomotion. The reader is referred for a further account of the muscles of the hinder thoracic and of the abdominal segments to Straus-Durckheim’s original work.

Minute structure of the muscles.—The muscular fibres of insects are striated (Figs. 235–238), even those of the alimentary canal; the only notable exception being the alary muscles of the pericardial septum, while Lowne states that certain of the thoracic muscles of the blow-fly are not striated (Miall and Denny).

216

Fig. 234.-Section through the prothorax of Diapheromera femoratum: prov, proventriculus; tr, trachea; n. c, nervous cord; s. gl, salivary gland; hyp, hypodermis; ur. t, urinary tube; ht, heart; m, m″, m‴, muscles for lowering and raising the tergum; m′, another muscle, its use unknown.

Fig. 235.—Striated muscular fibre of Hydrophilus: A and B, two fibrillæ in a state of extension; a, thick disk; b, thin disk; c, intermediate space. C, D, portion of the same fibrillæ seen by moving the objective farther away and using a small diaphragm; n, thick; c, thin disk. × 2000 diam.—After Ranvier, from Perrier. E after Gehuchten, from Lang.

In describing the minute structure of the muscles of ants, wasps, and bees, C. Janet states that each consists of a group of fibres diverging from a tendon, which is an integumentary invagination (Fig. 236). Each fibre may be regarded as a multinucleate cell; 217the sarcolemma represents the cell-membrane. It forms a resistant and extremely elastic tube. The longitudinal (Fig. 236, E) and radiating filaments or reticulum (spongioplasm of Gehuchten) lie in a nutritive filling substance (the hyaloplasm of Gehuchten). The radiating filaments are formed of an exceedingly elastic substance, and serve to sustain the longitudinal filaments, to transmit the nervous stimulus to them, and to bring them back into position after contraction. Janet’s account agrees on the whole with that of Gehuchten.

Fig. 236.—Preparations from the adductor muscle of the mandible of Vespa crabro, worker, fixed by heat and alcohol several hours after leaving its cell. A to E × 425; F × 212: A, terminal cupule of the tendon of a fibre. B, C, union of the fibres with their tendon. D, branch of the tendon of a muscle sending out tendons of some of the fibres; this branch is accompanied with numerous nervous ramifications (N). E, fragment of a nerve which furnishes the ramifications of Fig. D. F, fragment of the tendon of the adductor muscle of the mandible; at the left are seen the terminal cupules of the fibres (td, c); on the right, on the body of the tendons, some sessile cupules, each of which forms the attachment of a fibre; td, b, tendons of the fibres.—After Janet.

The muscles of flight are said to be penetrated by fine tracheal branches, probably to supply a greater amount of oxygen, as the most energetic movements of the insect are made in moving the wings during flight; while the other muscles of the body are only surrounded by the air-tubes. (Sharp.)

Without entering into tedious details, the reader is referred to figures or references to the more important systems of muscles, such as those of the legs and other appendages, of the wings, of respiration, etc., to the sections treating of those organs or functions; also to Figs. 16, 17, 18, 22, 48, 74, 81, 83, 84, 115, 116, 172, 173, 174, etc.

Muscular power of insects.—The most detailed and careful experiments are those of Plateau. His experiments prove that even the 218weakest insects pull at least five times their own weight; many of them, however, get the better of a burden twelve to twenty fold as heavy as themselves, while a strong man or a draught horse, for example, is not even able to pull a burden which is equal to the weight of his body. Plateau came to the following results as to the relation of the weight of the body to the load drawn (1 and 2 are to be compared with each other, 1 being the larger, and 2 the smaller insect; it will be seen that the smaller insect is the stronger).

Fig. 237.Vespa crabro, worker, fixed by heat and alcohol some hours after leaving its cell. A × 425; B to D × 850 times: A, muscular fibre of the motor muscles of the mandibles treated, for ten minutes, by 1 per cent potassium to bring out the reticulum; the nodes of union of the rayed filaments with the longitudinal filaments are indicated by distinct granulations (l.d), and these longitudinal filaments present accessory thickenings (d.a); T, trachea; N, junction of a nervous filament with the muscular fibres. B, fibre of the same muscle, not treated with potassium, stained by hæmatoxylin; C, transverse section of a disk at the level of a layer of rayed filaments; Sarc, sarcolemma. D, transverse section of a disk at the level of the rods; nuc, nucleus.—After Janet.

1. Carabus auratus 17.4.
2. Nebria brevicollis 25.3.
1. Cetonia aurata 15.
2. Trichius fasciatus 41.3.
1. Melolontha vulgaris 14.3.
2. Anomala frischii 24.3.
1. Oryctes nasicornis 4.7.
2. Geotrupes stercorarius 9.8.
3. Onthophagus nuchicornis 14.4.
1. Necrophorus vespillo 15.1.
2. Silpha livida 24.4.
1. Ocypus morio 17.
2. Quedeus fulgidus 29.6.
1. Donacia nymphææ, 42.7.
2. Crioceris merdigera 39.2.
1. Bombus terrestris 16.1.
2. Bombus rupestris 14.5.
3. Apis mellifica 20.2.

As regards the pushing power, the relation of the load to the size of the body in different large beetles, gave the following figures:—

Oryctes nasicornis 3.2.
Geotrupes stercorarius 28.4.
Onthophagus nuchicornis 92.9.

219The leaping force of locusts was found by Straus-Dürckheim to be in Œdipoda grossa as 1.6, in Œ. parallela as 3.3 of their weight.

Fig. 238.Vespa crabro, fixed and stained as in the subjects of the other figures. I, N, P × 1700; H, J, M × 850; the others × 425 times: A-C, motor muscles of the antennal scape. D-P, motor muscles of the 3d coxa. A, B, the two ends, in very different states of contraction, of the same fibre; on one side the transverse striæ are near together, on the other very far apart. C, a crushed and split fibre showing a fibrous appearance, owing to the rupture of the radiated filaments, and the separation of the longitudinal filaments. D, muscular disk seen in section, with two rows of nuclei. E, a muscular fibre with three rows of nuclei. F, a nucleus, accompanied with coagulated protoplasm, oozing from a previous break of the muscular fibre. G, nerve-terminations very near each other on the same muscular fibre. H, longitudinal filaments, evenly covered with the coagulated substance, and forming, throughout the mass of the fibre, continuous filaments. I, filaments widely separated. J, longitudinal filaments showing the beginning of one of the transverse breaks which isolate some of the disks. K, oblique view of a disk obtained by such a break, and of a fibre in circular section, with an axial row of nuclei; this piece comprises three stages of radiated filaments. L, muscular fibre with a row of nuclei; at the lower part, the nuclei have issued from a longitudinal fissure in the fibre, and have remained attached in a chain. M, edge of fibre in which there is quite a large, clear space between the sarcolemma and the rods. N, passage of the trachea, with the spiral thread, into three capillaries with a smooth cuticula. O, elliptical disk from a fibre, with two rows of nuclei, and showing a layer of radiated filaments. P, fragment (highly magnified) of the edge of a disk seen in section.—After Janet.

A humble bee (Bombus terrestris) can carry while flying a load 0.63 of its own weight, and a honey bee 0.78; here, as usual, the smaller insect is the stronger.[39]

220

LITERATURE ON THE MUSCLES

a. General

Lyonet, P. Traité anatomique de la chenille. La Haye, 1762.

Cornalia, E. Monographia del Bombyce del gelso. (Mem. R. Instituto Lombardo Sc. Lett. ed Arte, 1856.)

Basch, S. Skelett und Muskeln des Kopfes von Termes. (Zeitschr. f. wissens. Zool., xv, 1865, pp. 55–75, 1 Taf.)

Lubbock, John. Arrangement of the cutaneous muscles of the larva of Pygæra bucephala. London, 1858. 2 Pls.

—— On some points in the anatomy of ants. (Month. Micr. Journ., xviii, pp. 121–142, 1877, 4 Pls.)

—— On the anatomy of ants. (Trans. Linn. Soc., Ser. 2; Zool., ii, 1879, pp. 141–154, 2 Pls.)

Poletajeff, N. Du développement des muscles d’ailes chez les Odonates. (Horæ Soc. Ent. Ross., xvi, 1879, pp. 10–37, 5 Pls.)

—— Die Flugmuskeln der Lepidopteren und Libelluliden. (Zool. Anzeiger, 1880, pp. 212, 213.)

—— Ueber die Flugmuskeln der Rhopaloceren. (Arbeiten d. Russ. Ent. Ges., 1881, xiii, p. 9, 1 Taf., in Russian.)

Lendenfeld, R. von. Der Flug der Libellen. (Sitzb. k. Akad. Wissens., 1 Abth. Wien, 1881, lxxxiii, pp. 289–376, 7 Taf.)

Luks, Constantine. Ueber die Brustmuskulature der Insekten. (Jena. Zeitschr. f. Naturwissen., xvi, N. Folge IX, 1883, pp. 520–552, 2 Taf.)

Carlet, G. Sur les muscles de l’abdomen de l’abeille. (Comptes rend., 1884, xcviii, pp. 758, 759.)

Janet, Charles. Sur les muscles des fourmis, des guêpes et des abeilles. (Comptes rend., cxxi, p. 610, 1 Fig., 1895.)

Also the writings of Straus-Durckheim, Newport, Graber, Burgess, Leydig, Dahl, Ockler, Dogiel, Dimmock, Kraepelin, Becher, Langer, Kolbe.

b. Histology

Aubert, H. Ueber die eigenthümliche Struktur der Thoraxmuskeln der Insekten. (Zeitschr. f. wissens. Zool., iv, 1853, pp. 388–399, 1 Taf.)

Verson, E. Zur Insertionsweise der Muskeln. (Sitzsb. Akad. d. wiss. math. naturw. Cl. Wien., lvii, 1 Abth., pp. 63–66, 1868.)

Künckel d’Herculais. Sur le développement des fibres musculaires striées chez les insectes. (Compt. rend. de l’Acad. Sc. Paris, lxxv, 1872.)

Grunmach, Emil. Ueber die Structur der quergestreiften Muskelfaser bei den Insekten. Berlin, 1872. pp. 47.

Fredericq, L. Note sur la contraction des muscles striés de l’Hydrophile. (Bull. Acad. Roy. Belgique, xli, p. 583, 2 Pls.)

Gehuchten, A. van. Étude sur la structure intime de la cellule musculaire striée. (La Cellule, ii, pp. 289, 293–453, 1886, 6 Pls.)

Janet, Charles. Études sur les fourmis, les guêpes et les abeilles. 12e note. (Structure des membranes articulaires des tendons et des muscles, Limoges, 1895, pp. 25, 11 Figs.)

Also the writings of Burmeister, Chabrier, Leydig, Meckel, Lebert, Wagner, Wagener, Amici, Krause, Heppner, Retzius, Rollet, G. Elias Müller, F. Merkel, Hensen, Kölliker, Dogiel, Dönitz, Hagen, Vosseler, Bütschli u. Schewiakoff, Lowne, Ciaccio, Biedermann, Cohnbeim, Brücke, Haycraft, Melland, Bowman.

221

c. Muscular power of insects

Plateau, Félix. Sur la force musculaire des insectes. (Bull. Acad. Roy. Belgique, 2 Sér. xx, 1865, pp. 732–757; xxii, 1866, pp. 283–308.)

—— Recherches sur la force absolue des muscles des invertébrés. 1884.

Radan, R. La force musculaire des insectes. (Revue de deux mondes, 2 Sér., lxiv, 1866, pp. 770–777.)

Bibiakoff, Paul von. Zur Muskelkraft der Insekten. (Natur, xvii, 1868, p. 399.)

Delbœuf. Nains et géants, Étude comparative de la force des petits et des grands animaux. Bruxelles. (Also in Kosmos, xiii, 1883, pp. 58–62.)

Camerano. Mem. Acc. Torino (2), xliii, 1893, p. 229.

Also Newport, Art. Insecta, p. 76. Kirby and Spence, Burmeister, Graber, Kolbe, pp. 375, 376.

222

THE NERVOUS SYSTEM

a. The nervous system as a whole

Fig. 239.—Central nervous system of Machilis maritima: au, eye; lo, optic tract; g, brain; an, antennal nerve; oe, œsophagus passing between the œsophageal commissures; usg, infraœsophageal ganglion; I-III, thoracic ganglia; 1–8, abdominal ganglia, the last (Sabc) consisting of three fused ganglia; s, sympathetic nervous system of the ventral cord.—After Oudemans, from Lang.

The nervous system of insects consists of a double series or chain of ganglia connected by nervous cords or commissures. The first of these is the brain or supraœsophageal ganglion; it is situated in the upper part of the head, above the gullet or œsophagus, while the rest of the system, called the ventral cord, lies on the floor of the body, under the digestive canal.

A ganglion or nerve-centre consists of a mass of ganglion-cells, from each of which a process or fibre passes off, uniting with others to form a nerve; by means of these nerves the ganglia are connected with other ganglia, and with the sensory cells and muscle-fibres. The ganglia may be simple, and arranged in pairs, corresponding to each segment of the body, or they may be compound, the result of the fusion of several pairs of ganglia, which in the early stages of the embryo are separate. Thus the brain of insects is a compound ganglion, or ganglionic mass.

The nerves are of two kinds: 1. Sensory, which transmit sensations from the peripheral sense-cells to the ganglion, or brain; 2. Motor, which send stimuli from the brain or any other ganglion to the muscles.

Of ganglion cells, some are tactile, and others give rise to nerves of special sense, being distributed to the eyes, or to the organs of hearing, smell, taste, or touch.

223

Fig. 240.—Nervous system of Melanoplus spretus: sp, supraœsophageal ganglion, sending off the large optic nerve (op) to the eyes, and an ocellar nerve to each ocellus (the dotted line oc stops short of the left ocellus); if, infraœsophageal ganglion; 1, 2, 3, thoracic ganglia; 1–5, five abdominal ganglia (the fifth the largest, and sending branches to the ovipositor, etc.) The sympathetic nerve and ganglia are represented by the two main nerves which arise from the medio-cephalic (as) resting on and above the œsophagus, and two ganglia (ps) on the under side of the crop. From each of these ganglia, two nerves are sent under the crop, and a larger nerve on each side to as far as the stomachal cæca, ending the figure at the dotted line 2, near the second thoracic ganglion. u, a round, shining body, connected by a nerve with the medio-cephalic ganglion, its nature unknown.

Fig. 241.—Section through the head of Machilis, showing the brain (br), and subœsophageal ganglion (soe. g); cl, clypeus; lbr, labrum; oc, ocellus.

While the supraœsophageal ganglion, or “brain,” of the insect is much more complex than any other ganglion, consisting more exclusively both of sensory as well as motor ganglia and their nerves, it should be borne in mind that the subœsophageal ganglion also receives nerves of special sense, situated on the palpi and on the tongue, as in the bee and other insects; hence this ganglion is probably complex, consisting of sensory and motor cells. The third thoracic ganglion is also, without doubt, a complex one, as in the locusts the auditory nerves pass into it from the ears, which are situated at the base of the abdomen, while in the green grasshoppers, such as the katydids and their allies, whose ears are situated in their fore legs, the first thoracic ganglion is a complex one. In the cockroach and in Leptis (Chrysopila), a common fly, the caudal appendages bear what are probably olfactory organs, and as these parts are undoubtedly supplied from the last abdominal ganglion, this is probably composed of sensory and motor ganglia; so that we have in the ganglionated cord of insects a series of brains, as it were, running from head to tail, and thus in a still stronger sense 224than in vertebrates the entire nervous system, and not the brain alone, is the organ of the mind of insects.

The simplest, most primitive form of the nervous system of insects is seen in that of the Thysanura. That of Campodea has not yet been fully examined, but in that of the more complicated genus, Machilis (Fig. 239), we see that there is a pair of ganglia to nearly each segment, while the brain (Fig. 241) is composed of three lobes, viz. the optic, the cerebral (Fig. 239, g), behind which is the antennal lobe, from which the antennal nerve takes its origin. Behind the opening for the throat (oe) is situated the first ganglion of the ventral cord, the subœsophageal ganglion, which gives rise to the nerves supplying the jaws and other mouth-parts.

Fig. 242, A-D.—The nervous systems of 4 genera of Diptera, to demonstrate their various degrees of fusion of ganglia: A, non-concentrated more primitive nervous system of Chironomus plumosus, with 3 thoracic and 6 abdominal ganglionic masses. B, nervous system of Empis stercorea, with 2 thoracic and 5 abdominal ganglionic masses. C, nervous system of Tabanus bovinus, with 1 thoracic ganglionic mass, and the abdominal ganglia closely approximated. D, highly modified nervous system of Sarcophaga carnaria, in which all the ganglia of the ventral cord behind the subœsophageal ganglion are fused into a single ganglionic mass.—After Brandt, from Lang.

In the Collembola, which are retrograde Thysanura, there are from one (Smynthurus), to three or four ventral ganglia.

In the winged insects, where the ganglia are more or less fused, the fusion taking place in the head and at the end of the abdomen; there are in the more simple and generalized forms, such as Ephemera, the grasshopper, locusts (Fig. 240), etc., thirteen ganglia besides the two pairs of compound ganglia in the head, three pairs of thoracic 225ganglia, and usually from five to eight pairs of ganglia in the abdomen.

Fig. 243.—Nervous system of the May beetle, Lachnosterna fusca: w1, nerve to 1st,—w2, nerve to 2d, pair of wings; ig, infraœsophageal ganglion.

Fig. 244.—The same of the stag-beetle, Lucanus dama, where there are 3 thoracic, and 3 separate abdominal ganglia.

In certain winged insects the process of fusion or degeneration is carried to such an extreme that there are either no abdominal ganglia (Fig. 242, D), or their vestiges are situated in the thorax and partially fused with the thoracic ones, as in the May beetle, in which the prothoracic pair of ganglia is separate, while the two other thoracic ganglia are fused with the abdominal, the latter being situated in the thorax; this fusion is carried to a further extent than in any other Coleoptera yet examined. In many Diptera and Hemiptera the abdominal ganglia are either absent or the vestiges are fused with the thoracic ganglia.

Rhizotrogus, which is allied to our May beetle, as also Hydrometra and the Stylopidæ are said to lack the subœsophageal ganglion (Brandt).

In numerous Coleoptera (Acilius, Gyrinus, Necrophorus, Melolontha, Bostrichus, Rhynchænus); in many Diptera (Culex, Tipula, 226Asilus, Xylophaga, and Phora); and in the higher Hymenoptera (Crabronidæ, Vespidæ, and Apidæ), as well as in many Lepidoptera (Vanessa, Argynnis, and Pontia), two of the thoracic ganglia are fused together, while all three are partially fused into a single mass in many brachycerous Diptera (Conops, Syrphus, Pangonia, and the Muscidæ); in certain Hemiptera (Pentatoma, Nepa, and Acanthia); also in a beetle (Serica brunnea). Sometimes the subœsophageal ganglion is fused with the first thoracic, as in Acanthia, Nepa, and Notonecta. The greatest amount of variation is seen in the number of abdominal ganglia, all being fused into a single one or from one to eight. The fusion is usually greatest where the abdomen is shortened, due to the partial atrophy and modification of the terminal segments which bear the ovipositor, where present, and the genital armature.

There is only one pair of abdominal ganglia in Gyrinus and in certain flies (Conops, Trypeta, Ortalis, and Phora); two in Rhynchænus, a weevil, and in the flies, Syrphus and Volucella; three in Crabro and Eucera; four in Sargus, Stratiomys and in butterflies, five in the beetle, Silpha, and in the fly, Sciara, and the moth, Hepialus.

The nervous system in the larvæ of the metabolous orders is not concentrated, though in that of the neuropterous Myrmeleo it has undergone fusion from adaptation to the short compressed form of this insect.

b. The brain

The brain of insects appears to be nearly, if not quite, as complex as that of the lower vertebrates. As in the latter, the pair of supraœsophageal ganglia, or brain, is the principal seat of the senses, the chief organ of the insect’s mind.

It is composed of a larger number of pairs of primitive ganglia than any of the succeeding nerve-centres, and is, structurally, entirely different from and far more complicated than the other ganglia of the nervous system. It possesses a central body in each hemisphere, a “mushroom body,” optic lobes and optic ganglia and olfactory lobe, with their connecting and commissural nerve-fibres, and a number of other parts not found in the other ganglia.

In the succeeding ganglia the lobes are in general motor; the fibres composing the œsophageal commissures, and which arise from the œsophageal commissural lobes, extend not only to the subœsophageal ganglion, but pass along through the succeeding ganglia to the last pair of abdominal nerve-centres.[40] Since, then, there is a 227direct continuity in the fibres forming the two main longitudinal commissures of the nervous cord, and which originate in the brain, it seems to follow that the movements of the body are in large part directed or coördinated by the brain.[41] Still, however, a second brain, so to speak, is found in the third thoracic ganglion of the locust, which receives the auditory nerves from the ears situated in the base of the abdomen; or in the first thoracic ganglion of the green grasshoppers (katydids, etc.), whose ears are situated in their fore legs; while even the last pair of abdominal ganglia in the cockroach and mole cricket, is, so to speak, a secondary brain, since it distributes sensory nerves to the caudal stylets, which are provided with organs probably olfactory in nature.

It is impossible to understand the morphology of the brain unless we examine the mode of origin of the nervous system in the early life of the embryo. The head of an embryo insect consists of six segments, i.e. the ocular, antennal, premandibular, mandibular, and the 1st and 2d maxillary segments, so named from the appendages they bear. Of these the first three in the larva and adult are preoral, and the last three are postoral. The antennal segment was probably either postoral in the progenitors of insects, or the antennæ were inserted on the side of the mouth, the latter finally moving back.[42]

The nervous system in the early embryonic condition, as shown by Wheeler (Fig. 245), at first consists of nineteen pairs of primitive ganglia, called neuromeres. Those of the head, which later in embryonic life fuse together to form the brain, are the first three, corresponding to the protocerebrum, deutocerebrum, and tritocerebrum 228of Viallanes. The first pair of primitive ganglia, and which is situated in front of the mouth, is divided into three lobes.

Fig. 245, A-D.—Diagrams of four consecutive stages in the development of the brain and nerve-chain of the embryo of Xiphidium: I, cephalic,—II, thoracic,—III, abdominal, region; st, stomodæum or primitive mouth; an, anus; e, optic plate; pc(og), 1st protocerebral lobe, or optic ganglion; pc2, pc3, 2d and 3d protocerebral lobes; dc, deutocerebrum; tc, tritocerebrum; 1–16, the 16 postoral ganglia; po. c, postoral commissure; fp, furcal pit; ac, anterior,—pc, posterior, ganglionic commissure; ag, anterior,—pg, posterior,—cg, central,—lg, lateral gangliomeres.—After Wheeler.

The first or outermost lobe, according to Wheeler, forms the optic ganglion of the larva and imago, while the second and third lobes. (pc2, pc3) ultimately form the bulk of the brain proper, or the protocerebral lobes. The second (primitively postoral) brain-segment or pair of ganglia gives origin to the antennæ, while the third brain, or premandibular (intercalary) segment, gives origin to a temporary embryonic pair of appendages found in Anurida and Campodea (the premandibular ganglia), and also to the nerves supplying the labrum. These three pairs of ganglia later on in embryonic life become preoral, the mouth moving backwards. The three pairs of primitive ganglia, behind, i.e. the mandibular and 1st and 2d maxillary ganglia, become fused together to form the subœsophageal ganglion, and which in larval and adult life is postoral.

229If the tongue (ligula, or hypopharynx) represents a distinct pair of appendages, then there are seven segments in the head.

Fig. 246.—Section through head of a carabid, Anopthalmus telkampfii: br, brain; fg, frontal ganglion; soe, subœsophageal ganglion; co, commissure; n. l, nerve sending branches to the lingua (l); mn, maxillary nerve; mx, 1st maxilla; mm, maxillary muscle; mx′ 2d maxilla; mt, muscle of mentum; le, elevator muscle of the œsophagus; l′ of the clypeus, and a third beyond raising the labrum (lbr); eph, epipharynx; g. g′, salivary glands above; g2, lingual gland below the œsophagus (œ); m, mouth; pv, proventriculus; md, mandible.

The brain, then, supplies nerves to the compound and simple eyes, and to the antennæ, and gives origin to the sympathetic nerves; it is thus the seat of the senses, also of the insect’s mind, and coördinates the general movements of the body.

Fig. 247.—Median longitudinal section through the head of Blatta orientalis. The nervous system of the head is drawn entire. hyp, hypopharynx; os. oral cavity; lbr, upper lip; gf, frontal ganglion; g, brain; na, root of the antennal nerve; no, root of the optic nerve; ga, anterior,—gp, posterior ganglion of the paired visceral nervous system; œ, œsophagus; c, œsophageal commissure; usg, infraœsophageal ganglia; cc, longitudinal commissure between this and the first thoracic ganglion; sg, common duct of the salivary glands; lb, labium (2d maxillæ); nr, recurrent nerve; d, nerve uniting the frontal ganglion with the œsophageal commissure; e, nerve from this commissure to the labrum; f, nerve from the infraœsophageal ganglion to the mandible, —g, to the 1st maxillæ, —h, to the lower lip (2d maxillæ).—After Hofer, from Lang.

231

Fig. 248.—1, front view of the brain of Melanoplus femur-rubrum: opt. gang, optic ganglion; oc, ocelli and nerves leading to them from the two hemispheres, each ocellar nerve arising from the region containing the calices; m. oc, median ocellar nerve; opt. l, optic lobe sending off the optic nerve to the optic ganglion; ant. l, antennal or olfactory lobe; ant. n, antennal nerve; f. g, frontal ganglion of sympathetic nerve; lbr. n, nerve to labrum; x, cross-nerve or commissure between the two hemispheres; œ. c, œsophageal commissure to subœsophageal ganglion. 2, side view of the brain and subœsophageal ganglion (lettering of brain as in 1): s. g, stomatogastric or sympathetic nerve; a. s. g, anterior, and p. s. g, posterior, sympathetic ganglia; g2, subœsophageal ganglion; md, nerve to mandible; mx, maxillary nerve; ln, labial nerve; nl, unknown nerve,—perhaps salivary. 3, interior view of the right half of the head, showing the brain in its natural position: an, antenna; cl, clypeus; lbr, labrum; m, mouth-cavity; md, mandible; t, tongue; œ, œsophagus; c, crop; en, right half of the endocranium or X-shaped bone, through the anterior angle of which the œsophagus passes, while the great mandibular muscles play in the lateral angles. The moon-shaped edge is that made by the knife passing through the centre of the X. 4, view of brain from above (letters as before). 5, subœsophageal ganglion from above: t. c, commissure to the succeeding thoracic ganglion (other letters as before). Fig. 3 is enlarged 8 times; all the rest 25 times.—Drawn from original dissections, by Mr. Edward Burgess, for the Second Report of the U. S. Entomological Commission.

The pair of subœsophageal ganglia distributes nerves to the mandibles, to the 1st and 2d maxillæ, and to the salivary glands (Fig. 248).

Its general shape and relations to the walls and to the outer organs of the head is seen in Figs. 247, 248. In all the winged insects (Pterygota) its plane is situated more or less at right angles to the horizontal plane of the ventral cord. On the dorsal and anterior sides are situated the ocular lobes, and below these the antennal lobes.

Viallanes first, independently of embryonic data, divided the brain of adult insects into three regions or segments; i.e. the “protocerebron,” “deutocerebron” and “tritocerebron,” which he afterwards found to correspond with the three primitive elements (neuromeres) of the brain and with the segments of the head of the embryo.

The brain of the locusts (Melanoplus and Œdipoda) being best known will serve as the basis of the following description, taken mainly from Viallanes, with minor changes in the name of the three segments, and other modifications.

I. The optic or procerebral segment is composed of a median portion, i.e. two fused procerebral lobes (median protocerebrum), and of two lateral masses, the optic ganglia (protocerebrum), and comprises the following regions fused together and forming the median procerebral mass (Viallanes):—

1. Procerebral lobes.
2. Optic ganglia.
3. Layer of postretinal fibres.
4. Ganglionic plate. (Periopticon of Hickson.)
5. External chiasma.
6. External medullary mass. (Epiopticon of Hickson.)
7. Internal chiasma.
2328. Internal medullary mass. (Opticon of Hickson.)
9. Optic ganglia and nerves.
10. Pedunculated or stalked body. (Mushroom body of Dujardin.)
11. Bridge of the procerebral lobes.
12. Central body.

Fig. 249.—Diagram of an insect’s brain: cc, central body; cg, ganglionic cells; che, external, chi, internal chiasma; , œsophageal commissure; cp, mushroom body; ctc, tritocerebral commissure; fpr, postretinal fibres; goc, ocellar ganglion; goc1, œsophageal ganglion, the dotted ring the œsophagus; gv1, gc2, gv3, 1st, 2d, 3d, unpaired visceral ganglion; gvl, lateral visceral ganglion; ld, dorsal lobe of the deutocerebrum; lg, ganglionic plate; lo, olfactory lobe; lpc, protocerebral lobe; me, external, mi, internal medullary mass; na, olfactory or antennal nerve; nl, nerve to labrum; no, ocular nerve; nt, tegumentary nerve; œ, œsophagus; plp, bridge of the protocerebral lobes; rvd, visceral root arising from the deutocerebrum; rvt, visceral root arising from the tritocerebrum; tr, tritocerebrum; to, optic nerve or tract.—After Viallanes.

Optic ganglia.—Each of the two optic ganglia is formed of a series of three ganglionic masses situated between the compound eyes and the median procerebral mass, i.e. the ganglionic plate (Fig. 249, lg), the external medullary mass (me), and the internal medullary mass (mi).

The postretinal fibres (fpr) arising from the facets or single eyes of the compound eye (ommatidia) pass into the ganglionic plate (lg), which is united within by the chiasmatic fibres (che, external chiasma) of the external medullary mass (me). The last is attached to the internal medullary mass (mi) by fibres (chi), some of which are chiasmatic, and others direct. Finally, the internal medullary mass connects with the median part of the protocerebrum by direct fibres forming the optic nerve or tract (to).

Procerebral lobes.—The median procerebral lobes are fused together on the median line, forming a single central mass. From each side 233or lobe arises the mushroom or stalked body. In the middle of the mass is the central body, and directly in front is the procerebral bridge (plp). The latter is a band uniting the two halves of the brain.

The procerebral lobes also give origin to the nerves to the ocelli (no).

Fig. 250.—Transverse section through the brain of the locust (Œdipoda and Caloptenus): c′, lower part of the wall of c, calyx;—st, stalk of the same; bpcl, bridge of the protocerebral lobes; mo, nerve of median ocellus; ch, transverse fascia of the optico-olfactory chiasma; fcb, fibrous region of the central body; lcb, tubercle of the central body; fch, descending fascia of the optico-olfactory chiasma; choo, superior fascia of the optico-olfactory chiasma; pt, protocerebral lobes; ld, dorsal lobe of the deutocerebrum; lt, tritocerebral lobe; gcld, gc, ganglion cells.—After Viallanes.

The mushroom or stalked bodies.—These remarkable organs were first discovered by Dujardin, who compared them to mushrooms, and observed that they were more highly developed in ants, wasps, and bees than in the lower insects, and thus inferred that the higher intelligence of these insects was in direct relation to the development of these bodies. We will call them the mushroom bodies.

These two bodies consist of a rounded lobular mass (the trabecula) of the procerebral lobe, from which arises a double stalk (Fig. 253), the larger called the cauliculus, the smaller the peduncle (or pedicel); these support the cap or calyx. The calices of the bee were compared by Dujardin to a pair of disks on each side of the brain as seen from above, “each disk being folded together and bent downwards before and behind, its border being thickened, and the inner 234portion radiated.” In the locust there are but two divisions of the calyx; in the cockroach, ants, wasps, and bees, four.

The shape and relation of the mushroom bodies are represented in Figs. 252 and 253. The bodies are connected by commissural fibres, and are connected with the optic ganglion of the same side, and with the central body; while they are connected with the antennal lobes by the optico-olfactory chiasma.

Fig. 251.—Sagittal section through the brain of the locust: l. oc. n, lateral ocellus nerve; a. t, anterior tubercle of the mushroom body; i. t, internal tubercle of the mushroom body; c. l, cerebral lobes; l. l, lateral lobe of the middle protocerebrum; com, commissural cord; c. mol, central mass of the olfactory lobe; ac. an. l, fibres uniting the median lobe of the middle protocerebrum with dorsal lobes of the deutocerebrum; gc. trit. l, ganglionated cortex of the tritocerebral lobe; c. an. l, cortex of antennal (olfactory) lobe; lab. fr, labrofrontal nerve; oe. com, œsophageal commissure; tr. com, transverse commissure of œsophageal ring; other letters as in Fig. 250.—After Viallanes.

The stalked bodies are enveloped by the cortical layers of ganglion-cells, those filling the hollow of the calyx having little or no protoplasm around the nucleus.

Structure of the mushroom bodies.—By staining the brain of the honey bee with bichromate of silver, Kenyon has worked out the structure of the mushroom bodies, with their cells. The cup-shaped bodies or calyces are composed of fibrillar substance (punktsubstanz). Each of these cups, he says, is “filled to overflowing with cells having large nuclei and very little cytoplasm.” From the under surface of each of these cups there descends into the general fibrillar substance of the brain “a column of fibrillar substance, which unites with its fellow of the same side to send a large branch obliquely downward to the median line of the brain, and an equally large or larger branch straight forwards to the anterior cerebral surface.”

The cells of the mushroom bodies, observes Kenyon, “stand out in sharp contrast to all other nerve cells known, though they recall to some extent the cells of Purkinje in the higher mammals. Each of the cells contained within the fibrillar cup sends a nerve-process into the latter, where it breaks up into a profusely arborescent system of branchlets, which often appear with fine, short, lateral processes, such as are characteristic of the dendrites of some mammalian nerve-cells.” Just before entering the fibrillar substance, a fine branch is given off that travels along the inner surface of the cup along with others of the same nature, forming a small bundle to the stalk of the mushroom body, down which it continues until it reaches the origin 235of the anterior and the inner roots above mentioned. “Here it branches, one branch continuing straight on to the end of the anterior root, while the other passes to the end of the inner root. Throughout its whole course the fibre and its two branches are very fine. Nearly the whole stalk and nearly the whole of each root is made up of these straight, parallel fibres coming from the cells within the cup of the mushroom bodies. What other fibres there are enter these bodies from the side, and branch between the straight fibres very much as the dendrites of the cells of Purkinje branch among the parallel fine fibres from the cells of the granular layer in the mammalian cerebellum. These fibres are of the nature of association fibres.”

Viallanes showed that from the olfactory or antennal lobes, as well as from the optic ganglia, there are tracts of fibres which finally enter the cups of the mushroom bodies, and Kenyon has confirmed this observation. Kenyon has also, by the Golgi method, detected another tract, before unknown, “passing down the hinder side of the brain, from the cups to the region above the œsophagus, where it bends forward and comes in contact with fibres from the ventral cord, which exists, although Binet was unable to discover any growth of fibres connecting the cord with the brain.

“The fibres entering the cups from the antennal lobe, the optic ganglia, and the ventral region, spread out and branch among the arborescent endings of the mushroom-body cells. The fibres branching among the parallel fibres of the roots and the stalk lead off to lower parts of the brain, connecting with efferent or motor-fibres, or with secondary association fibres, that in their turn make such connections. This portion of the circuit has not been perfectly made out, though there seems to be sufficient data to warrant the assumption just made.

Fig. 252.—Section 17, showing the central body (centr. b) and mushroom body, optic and antennal lobes (a. l), and procerebral lobes (pc. l); o. cal, outer division of the calyx; op. n, optic nerve; trab, trabeculum; tc. n, transverse nerve.

“Such fibres existing as described, there is then a complete circuit for sensory stimuli from the various parts of the body to the cells of the mushroom bodies. The dendritic or arborescent branches of these cells take them up and pass them on out along the parallel fibres or neurites in the roots of the mushroom bodies as motor or other efferent impulses.

“This, however, is not all. For there are numerous fibres evident in my preparations, the full courses of which I have not been thus far able to determine, but which are so situated as to warrant the inference that they may act as 236association fibres between the afferent fibres from the antennæ, optic ganglia, and ventral system, and the efferent fibres. There is then a possibility of a stimulus entering the brain and passing out as a motor impulse without going into the circuit of the fibres of the mushroom bodies; or, in other words, a possibility of what may be compared to reflex action in higher animals.”

Fig. 253.—Enlarged view of the trabeculum (the dotted lines tcn and obt. n pass through it) and its nerves, of the mushroom body,—its calices and stalk, and the origin of the optic nerve × 225 diameters: atn, ascending trabecular nerve; obt. n, oblique trabecular nerve; tcn, transverse nerve; lat. n, lateral nerve; cent. n, central nerve.

The mushroom bodies have not yet been found to be present in the Synaptera, but occur in the larvæ, at least of those of most metamorphic insects (Lepidoptera and Hymenoptera), though not yet found in the larvæ of Diptera. The writer has found these bodies in the nymphs of the locust (Melanoplus spretus), but not in the embryo just before hatching. They occur in the third larval or nymph stage of this insect. It is evident that by the end of the first larval stage the brain attains the development seen in the third larval state of the two-banded species (C. bivittatus).

237

Fig. 254.—Section through the brain of Caloptenus bivittatus in the third larval stage, showing the two hemispheres or sides of the brain, and the ocelli and ocellar nerves, which are seen to arise from the top of the hemispheres directly over the calices (compare Fig. 251): o. cal, outer division of calyx of left mushroom body.

The result of our studies on the brain of the embryo locust was that from the embryonic cerebral lobes are eventually developed the central body and the two mushroom bodies. Fig. 254 shows the early condition of the mushroom bodies and their undoubted origin from the cerebral ganglia. Hence these bodies appear to be differentiations of the cerebral ganglia or lobes, having no connection with the optic or antennal lobes.

The central body (Fig. 252, centr. b).—This is the only single or unpaired organ in the brain. Dietl characterizes it as a median commissural system. Viallanes describes it as formed entirely of a very fine and close fibrillar web, like a thick hemispherical skull-cap, situated on the median line and united with the cerebral lobes. “It is like a central post towards which converge fibres passing from all points of the brain; being bound to the cerebral lobes, to the stalked bodies, to the optic ganglia, and to the olfactory lobes by distinct fibrous bundles.”

The antennal or olfactory lobes (Deutocerebrum).—This portion of the brain consists of two hemispherical lobes, highly differentiated for special sensorial perceptions, and connected by a slightly differentiated medullary mass, the dorsal lobe (Figs. 248, 249 lo), from which arise the motor fibres and those of general sensibility. The antennal lobes are in part attached to the optic ganglia, and partly to the stalked body on the same side, by the optic olfactory chiasma (Fig. 250 fch, choo), a system of fibres partially intercrossed on the median line.

The œsophageal lobes (Tritocerebrum) (Figs. 249, 250).—From this region the labrum and viscera are innervated, the nerves to the latter being called the visceral, sympathetic, or stomatogastric system. As Viallanes remarks, though plainly situated in front of the mouth, they are in fact post-œsophageal centres. The two lobes are situated far apart, and are connected by a bundle of fibres passing behind the œsophagus, called the transverse commissure of the œsophageal ring (Lienard). The œsophageal ganglia, besides giving 238rise to the labral nerves, also give origin to the root of the frontal ganglion.

c. Histological elements of the brain

The brain and other ganglia are composed of two kinds of tissue.

1. The outer slightly darker, usually pale grayish white portion consists of cortical or ganglion-cells differing in size. This portion is stained red by carmine, the cells composing it readily taking the stain.

The large ganglion cells (represented in Figs. 252 and 253) are oval, and send off usually a single nerve-fibre; they have a thin fibrous cell-wall, and the contents are finely granular. The nucleus is very large, often one-half the diameter of the entire cell, and is composed of large round refractive granules, usually concealing the nucleolus.

2. The medullary or inner part of the brain consists of matter which remains white or unstained after the preparation has remained thoroughly exposed to the action of the carmine. It consists of minute granules and interlacing fibres. The latter often forms a fine irregular network inclosing masses of finely granulated nerve matter.

This is called by Dietl “marksubstanz.” Leydig, in his Vom Bau des thierischen Körpers, p. 89, thus refers to it:—

“In the brain and ventral ganglia of the leech, of insects, and in the brain of the gastropods (Schnecken) I observe that the stalks (stiele) of the ganglion-cells in nowise immediately arise as nerve-fibres, but are planted in a molecular mass or punktsubstanz, situated in the centre of the ganglion, and merged with this substance. It follows, from what I have seen, that there is no doubt that the origin of the nerve-fibres first takes place from this central punktsubstanz.”

“This relation is the rule. But there also occur in the nerve-centres of the invertebrates single, definitely situated ganglion-cells, whose continuations become nerve-fibres without the intervention of a superadded punktsubstanz.” We may, with Kenyon, call it the fibrillar substance.

Leydig subsequently (p. 91) further describes this fibrillar substance, stating that the granules composing it form a reticulated mass of fibrillæ, or, in other words, a tangled web of very fine fibres:—

“We at present consider that by the passage of the continuation of the ganglion-cells into the punktsubstanz this continuation becomes lost in the fine threads, and on the other side of the punktsubstanz the similar fibrillar substance forms the origin of the axis-cylinders arranged parallel to one another; so it is quite certain that the single axis-cylinder derives its fibrillar substance as a mixture from the most diverse ganglion-cells.”

d. The visceral (sympathetic or stomatogastric) system

This system in insects is composed (1) of a series of three unpaired ganglia (Fig. 249, gv1, gv2, gv3), situated over the dorso-median line of the œsophagus, and connected by a median nervous cord or recurrent 239nerve (nr, vagus of Newport). The first of these ganglia is the frontal ganglion, which is connected with the œsophageal ganglia by a pair of roots (rvt), which have an origin primitively common with that of the labral nerves (Fig. 248, fg and lbr).

Fig. 255.—Anterior portion of the paired and unpaired visceral nervous system of Blatta orientalis seen from above. The outlines of the brain (g) and the roots of the antennal nerve (na), which cover a portion of the sympathetic nervous system, are given by dotted lines. Lettering as in Fig. 247. nsd, nerve to salivary gland. The nervus recurrens (nr) enters an unpaired stomach ganglion farther back.—After Hofer, from Lang.

2. Of two pairs of lateral ganglia (Fig. 255, ga, gp) situated two on each side of the œsophagus. They are connected both with the antennal lobes by a nerve (rvd), and to the chain of unpaired ganglia by a special connective. The first pair of these ganglia sends nerves to the heart and aorta; the second pair to the tracheæ of the head.

The unpaired median or recurrent nerve (nr) extends back from under the brain along the upper side of the œsophagus, and (in Blatta), behind the origin of the nerves to the salivary glands, enters an unpaired ganglion, called the stomachic ganglion (ganglion ventriculare), situated in front of the proventriculus. The number of these stomachic ganglia varies in different orders of insects.

In Blatta, Küpffer and also Hofer have shown (Fig. 255) (Müller, Brandt, ex Kolbe) that the nerve to each salivary gland arises from three different centres: the anterior end situated under the œsophagus is innervated by the paired visceral nerves from the hinder paired ganglia; the remaining part by nerves arising from each side of the recurrent nerve; and thirdly by a pair of nerves arising from the subœsophageal ganglion which accompanies the common salivary duct, and ends in branches which partly innervate the salivary glands and in part their muscles.

Hofer considers that the function of this complex system of paired and unpaired ganglia, with their nerves, is a double one, viz. serving both as a centre for the peristaltic action of the œsophagus, and as innervating the salivary glands.

Besides these a second portion of the visceral system arises from the thoracic and abdominal ventral cord. It may be seen in the 240simplest condition yet known in the nervous system of Machilis (Fig. 239 s). It consists of a fine, slender nerve, which extends along the surface of the ventral chain of ganglia, and sending off a pair of branches (accessory transverse nerves) in front of each ganglion. These accessory nerves receive nerve-twigs from the upper cord of the ventral chain, dilating near their origins into a minute elongated ganglion, and then passing partly outwards to the branches of the tracheæ and the muscles of the spiracles, uniting in the middle line of each segment of the body behind the head, i.e. of those segments containing a pair of ganglia.

e. The supraspinal cord

In the adult Lepidoptera has been detected, continuous with and on the upper side of the abdominal portions of the ventral cord, a longitudinal cord of connective tissue forming a white or yellowish band, and which seems to be an outgrowth of the dorsal portion of the neurilemma of the ventral cord. Muscles pass from it to the neighboring ventral portions of the integument. Its use is unknown, and attention was first called to it by Treviranus, who called it “an unknown ventral vessel” (Bauchgefäss). Afterwards it was re-discovered by Newport, who described it as “a distinct vascular canal.” But Burger has proved by cross-sections that it is not tubular, but a comparatively solid cord composed, however, of loose connective tissue. Newport found it in the larva of Sphinx ligustri, but Cattie states that it is not present in that of Acherontia atropos. It has not yet been observed in insects of other orders, but its homologue exists in the scorpion and in the centipede, and it may prove to correspond with the far more complete arterial coat which, with the exception of the brain, envelops the nervous system of Limulus.

f. Modifications of the brain in different orders of insects

There are different grades of cerebral development in insects, and Viallanes claimed that it was no exaggeration to say that the brain of the locust (Melanoplus) differs as much from that of the wasp as that of the frog differs from that of man. He insists that the physiological conditions which determine the anatomical modifications of the brain are correlated with 1, the food; 2, the perfection of the senses; and 3, with the perfection of the psychic faculties. For example, in those which feed on solid food and whose œsophagus is large (Orthoptera and Coleoptera), the connectives are elongated, the subœsophageal commissure free in all 241its extent, and the tritocerebrum is situated quite far from the preceding segment of the brain.

On the other hand, in insects which feed on fluid food (Hymenoptera, Lepidoptera, Diptera, Hemiptera), the œsophagus is slender and the nervous centres which surround them are very much condensed; the connectives are short, and the tritocerebrum is closely fused, partly to a portion of the antennal lobes (deutocerebrum) and partly to the mandibular ganglion.

As regards the perfection of the senses, where, as in dragon-flies, the eyes are very large, the optic ganglia are correspondingly so, and in the same insects the antennæ being very small, the antennal lobes are almost rudimentary. The ants exhibit inverse conditions; in their brain the antennal lobes are well developed, while the optic ganglia are reduced, and where, as in Typhlopone, the eyes are wanting, they are completely atrophied.

Fig. 256.—Head of Anophthalmus tellkampfii, showing the brain,—the optic ganglia, nerves, and eyes totally atrophied.

Fig. 257.—Head of another Carabid, with the brain and eyes normal: op, optic ganglion; pcl, brain.

In certain cave insects where the eyes are wanting, the optic ganglia are also absent. In the eyeless cave species of Anophthalmus the optic ganglia and nerves are entirely atrophied, as they are in Adelops, which, however, has vestiges of the facets (ommatidia). Fig. 257 represents the brain of Chlænius pennsylvanicus, a Carabid beetle, with its eyes and optic ganglia (op) which may be compared with Anopthalmus, in which these parts are totally atrophied.

Dujardin claimed that the degree of complication of the stalked body of the Hymenoptera was in direct relation with their mental powers. This has been proved by Forel, who has shown that in the honey bee and ants the mushroom bodies are much more developed in the workers than in the males or females and Viallanes adds that these bodies are almost rudimentary in the dragon-flies, whose eyes are so large; while on the contrary in the blind ants (Typhlopone), these bodies are as perfect and voluminous as in the ants with eyes.

242

Fig. 258.—Diagrammatic outlines of sections of the upper part of the brain of a cockroach. Only one side of the brain is here represented. The numbers indicate the position in the series of 34 sections into which this brain was cut. mb, mushroom bodies, with their cellular covering (c) and their stems (st); a, anterior nervous mass; m, median nervous mass.—After Newton.

243Within the limits of the same order the stalked bodies are most perfect in the most intelligent forms. Thus in the Orthoptera, says Viallanes, the Blattæ, Forficulæ, and the crickets, the mushroom bodies are more perfect than in the locusts, which have simpler herbivorous habits. This perfection of the mushroom bodies is seen not only in the increase in size, but also in the complication of its structures. Thus in the groups with lower instincts (Tabanus, Æschna) the stalk does not end in a calyx projecting from the surface of the brain, but its end, simply truncated, is indicated externally only by an accumulation of the ganglionic nuclei which cover it.[43]

In types which Viallanes regards as more advanced, i.e. Œdipoda and Melanoplus, the end of the stalk projects and is folded into a calyx.

The brain of the cockroach (Periplaneta, Fig. 258) is a step higher than that of the locusts, each calyx being divided into two adjacent calices, although the cockroaches are an older and more generalized type than locusts.

The stalked bodies of cockroaches are thus complex, like those of the higher Hymenoptera, the calices in Xylocopa, Bombus, and Apis being double and so large as to cover almost the entire surface of the brain.

Finally, in what Viallanes regards as the most perfect type (Vespa), the sides of the calices are folded and become sinuous, so as to increase the surface, thus assuming an appearance which, he claims, strongly recalls that of the convolutions of the brain of the mammals.

Cheshire also calls attention to a progression in the size of these appendages, as well as in mental powers as we rise from the cockchafer (Melolontha vulgaris) to the cricket, up to the ichneumon, then to the carpenter bee, and finally to the social hive bee, “where the pedunculated bodies form the ⅕ part of the volume of the cerebral mass, and the 1
870
of the volume of the entire creature, while in the cockchafer they are less than 1
2300
the part. The size of the brain is also a gauge of intelligence. In the worker bee the brain is 1
174
of the body; in the red ant, 1
296
; in the Melolontha, 1
3500
; in the Dyticus beetle, 1
4400
.” (Bees and bee-keeping, p. 54.)

g. Functions of the nerve-centres and nerves

As we have seen, the central seat of the functions of the nervous system is not the brain alone (supraœsophageal ganglion), but each ganglion is more or less the seat of vital movements, those of the 244abdomen being each a distinct motor and respiratory centre. The two halves of a ganglion are independent of each other.

According to Faivre, the brain is the seat of the will and of the power of coördinating the movements of the body, while the infraœsophageal ganglion is the seat of the motive power and also of the will.

The physiological experiments of Binet, which are in the line of those of Faivre, but more thorough, demonstrate that an insect may live for months without a brain, if the subœsophageal ganglion is left intact, just as a vertebrate may exist without its cerebrum. As Kenyon says: “Faivre long ago showed that the subœsophageal ganglion is the seat of the power of coördination of the muscular movements of the body. Binet has shown that the brain is the seat of the power directing these movements. ‘A debrained hexapod will eat when food is placed beneath its palpi, but it cannot go to its food even though the latter be but a very small space removed from its course or position. Whether the insect would be able to do so if the mushroom bodies only were destroyed, and the antennal lobes, optic lobes, and the rest of the brain were left intact, is a question that yet remains to be answered’” (Kenyon).

In insects which are beheaded, however readily they respond to stimulation of the nerves, they are almost completely wanting in will power. Yet insects which have been decapitated can still walk and fly. Hymenoptera will live one or two days after decapitation, beetles from one to three days, and moths (Agrotis) will show signs of life five days after the loss of their head.

That the loss of will power is gradual was proved by decapitating Polistes pallipes. A day after the operation she was standing on her legs and opening and closing her wings; 41 hours after the operation she was still alive, moving her legs, and thrusting out her sting when irritated. Ichneumon otiosus, after the removal of its head, remained very lively, and cleaned its wings and legs, the power of coördination in its wings and legs remaining. A horse-fly, a day after decapitation, was lively and flew about in a natural manner.[44]

When the abdomen is cut off, respiration in that region is not at first interrupted. The seat of respiratory movements was referred by Faivre to the hinder thoracic ganglion, but Plateau says that this view must be entirely abandoned, remarking: “All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and 245in insects a respiratory centre, for the somite to which it belongs” (Miall and Denny’s The Cockroach, p. 164).

The last pair of abdominal ganglia serve as the nervous centre of the nerves sent to the genital organs.

The recurrent or stomatogastric nerve, which, through the medium of the frontal ganglion, regulates digestion, has only a slight degree of sensibility; the insect remains quiet even when a powerful allurement is presented to the digestive tract (Kolbe).

Faivre states that the destruction of the frontal ganglion, or a section of the commissures connecting it with the brain, puts an end to swallowing movements; on the other hand, stimulation results in energetic movements of this nature.

Yersin, by cutting through the commissure in different places, and thus isolating the ganglia of the nervous cord of Gryllus campestris, arrived at the following results:—

1. The section of a nerve near its origin rendered the organ supplied by this nerve incapable of performing its functions.

2. If the connectives between two ganglia, i.e. the second and third thoracic ganglia, are cut through, the fore as well as hinder parts of the body retain their power of motion and sensation; but a stimulus applied to the anterior part of the body does not pass to the hinder portion.

3. Insects with an incomplete metamorphosis after section of the connectives are not in every case unable to moult and to farther develop.

4. If only one of the two connectives be cut through, the appendages of the side cut through which take their origin between the place injured and the hinder end of the body, often lose sensation and freedom of motion, or the power of coördination of movements becomes irregular. Sometimes this is shown by an unsteadiness in the gait, so that the insect walks around in a circle; after a while these irregularities cease, and the movements of the limbs on the injured side are only slightly restrained. By a section of both connectives in any one place the power of coördination of movements is not injured.

5. The section of the connectives appear to have no influence on nutrition, but affects reproduction, the attempt at fertilization on the part of the male producing no result, and the impregnated female laying no eggs.

6. Injury to the brain, or to the subœsophageal, or one of the thoracic ganglia, is followed by a momentary enfeeblement of the ganglion affected. Afterwards there results a convulsive trembling, 246which either pervades the whole body or only the appendages innervated by the injured ganglion.

7. As a result of an injury to the brain there is such a lack of steadiness in the movements that the insect walks or flies in a circle; for instance, a fly or dragon-fly thus injured in flying describes a circle or spiral. Steiner, in making this experiment, observed that the insect circled on its uninjured side. The brain is thus a motor centre.

8. By injuring a thoracic ganglion, one or all the organs which receive nerves from the ganglion are momentarily weakened. Afterwards the functions become restored. Sometimes, however, the insect walks in a circle. Faivre observed that after the destruction of the metathoracic ganglion of Dyticus marginalis the hind wings and hind legs were partially paralyzed (Kolbe, ex Yersin).

LITERATURE ON THE NERVOUS SYSTEM

a. General

Newport, George. On the nervous system of the Sphinx ligustri L., and on the changes which it undergoes during a part of the metamorphoses of the insect. (Phil. Trans. Roy. Soc., London, 1832, pp. 383–398; 1834, pp. 389–423, Pls.)

Helmholtz, H. L. F. De fabrica systematis nervosi evertebratorum. Diss. in aug. Berolini, 1842.

Blanchard, E. Recherches anatomiques et zoologiques sur le système nerveux des animaux sans vertèbres. Du système nerveux des insectes. (Annales des Sciences nat., Sér. 3, v, 1846, pp. 273–379, 8 Pls.)

—— Du système nerveux chez les invertèbres dans ses rapports avec la classification de ces animaux. Paris, 1849.

—— in Cuvier’s Règne animal. (Edition accompagnée de planches gravées. Insectes. Pl. 3, 3a, and 4.)

Leidy, Joseph. History and anatomy of the hemipterous genus Belostoma. (Memoirs Amer. Acad. Arts and Sc., N. S. iv, 1849, pp. 57–67, 1 Pl.)

Scheiber, S. H. Vergleichende Anatomie und Physiologie der Œstridenlarven. (Sitzungsb. k. Akad. wiss. Wien. Math.-Naturwiss. Cl., xli, 1860, pp. 439–496; xlv, 1862, pp. 7–68; 5 Taf.)

Tullberg, Tycho. Sveriges Podurider. (K. Svenska vet. Akad. Handl. x, 1872, pp. 1–70, 12 Taf.)

Berlese, A. Osservazione sulla anatomia descrittiva del Gryllus campestris L. (Atti della soc. Veneto-Trentina, 1880, vii, pp. 200–299.)

Baudelot, E. Contributions à la physiologie der système nerveux des insectes. (Revue d. sc. nat., i, pp. 269–280, 1872.)

Studer, Th. Ueber Nervenendigung bei Insekten. Kleine Beiträge zur Histologie der Insekten. (Mitt. Naturf. Ges., Bern, 1874, pp. 97–104, 1 Taf.)

Brandt, E. Recherches anatomiques et morphologiques sur le système nerveux des insectes Hyménoptères. (Compt. rendus de l’Acad. Sc., Paris, 1875.)

—— Ueber das Nervensystem der Apiden. (Sitzungsb. d. naturf. Ges., in Petersbourg, vii, 1876.)

—— Ueber das Nervensystem der Schmetterlingsraupen. (Verhandl. der Russ. 247Ent. Gesellsch., x, 1877. Also 16 other articles with plates, in Horæ Soc. Ent. Ross., 1878–1882.)

Mark, E. L. The nervous system of Phylloxera. (Psyche, ii, pp. 201–207, 1879.)

Riley, Charles Valentine. The nervous system and salivary glands of Phylloxera. (Psyche, ii, pp. 225, 226, 1879.)

Cholodkowsky, N. Zur Frage über den Baue und über die Innervation der Speicheldrüsen der Blattiden. (Horæ Soc. Ent. Ross., 1881, xvi, pp. 6–9, 2 Taf.)

Liénard, V. Constitution de l’anneau œsophagien. (Archives de Biologie, i, pp. 381–391, 1880, 1 Taf.)

Michaels, H. Nervensystem von Oryctes nasicornis im Larven-, Puppen-, und Käferzustande. (Zeits. f. wissens. Zool., xxxiv, 1880, pp. 641–702, 4 Taf.)

Rossi, A. Sul modo di terminare dei nervi nei muscoli dell’ organo sonoro della Cicala commune (Cicada plebeja). (Mem. accad. sc. Bologna, 1880, 4 Ser., i, pp. 661–665.)

Foettinger, A. Sur le termination des nerfs dans les muscles des insectes. (Archiv de Biologie, i, 1880.)

Binet. Contribution à l’étude der system nerveux sous intestinal des insectes. (Journ. l’anat. et phys., xxx, pp. 449–580, 1894.)

Paulowa. Zum Bau des Eingeweide Nervensystems der Insekten. (Zool. Anzeiger., xviii, Feb. 25, 1895, pp. 85–87.)

Also the writings of Lyonet, Cuvier, Rolando, Straus-Durckheim, Leydig, Newport, Graber, Viallanes, Grassi, Oudemans.

b. The brain

Dujardin, F. Mémoires sur le système nerveux des insectes. (Annales des Sciences nat, Sér. 3, 1850, xiv, pp. 195–206, Pl. 1, 1850.)

Rabl-Rückhard. Studien über Insectengehirne. (Archiv für Anatomie, Physiologie, etc., herausg. von Reichert u. R. du Bois-Raymond, 1876, p. 480, Taf. i.)

Dietl, M. J. Die Organization des Arthropodengehirns. (Zeitschr. wissens. Zool., xxvii, 1876, p. 488, Taf. xxxvi.-xxxviii.)

Flogel, T. H. L. Ueber den einheitlichen Bau des Gehirns in den verschiedenen Insectenordnungen. (Zeitschr. wissens. Zool., xxx, Suppl., 1878, p. 556, Taf. xxiii, xxiv.)

Newton, E. T. On a new method of constructing models of the brains of insects, etc. (Journ. Quekett Microscopical Club, pp. 150–158, 1879.)

—— On the brain of the cockroach, Blatta orientalis. (Quart. Journ. Microscopical Science, July, 1879, p. 340, Pl. xv, xvi.)

Packard, A. S. The brain of the locust. (Chapter xi, Second Report of the U. S. Entomological Commission, pp. 223–242, Pls. ix-xv, 1880.)

Cuccati, Giovanni. Sulla stuttura del ganglio sopraesofageo di alcuni ortotteri. (Acrydium lineola, Locusta viridissima, Locusta (species?), Gryllotalpa vulgaris, Bologna, 1887, 4º, pp. 1–27, Pl. i-iv.)

—— Intorno alla struttura del cervello della Sonomya erythrocephala, nota preventiva. Bologna, 1887.

—— Ueber die Organization des Gehirns des Sonomya erythrocephala. (Zeitschr. f. wissens. Zool., 1888, xlvi, pp. 240–269, 2 Taf.)

Viallanes, H. Études histologiques et organologiques sur les centres nerveux et les organes des sens des animaux articulés.

1. Mémoire. Le ganglion optique de la langouste (Palinurus vulgaris). (Annal. d. Sc. Nat. Zool., 1884, 6e Sér., xvii, Art. 3, pp. 1–74, 5 Pls.)

2482. Mémoire. Le ganglion optique de la Libellule (Æschna maculatissima). (Ibid., 1885, 6e Sér., xviii, Art. 4, pp. 1–34, 3 Pls.)

3. Mémoire. Le ganglion optique de quelques larves de Diptères (Musca, Eristalis, Stratiomys). (Ibid., 1886, 6e Sér., xix, Art. M. 4, pp. 34, 2 Pls.)

4. Mémoire. Le cerveau de la guêpe (Vespa crabro et vulgaris). (Ibid., 1887, 7e Sér., ii, pp. 5–100, 6 Pls.)

5. Mémoire. 1. Le cerveau du criquet (Œdipoda cœrulescens et Caloptenus italicus). 2. Comparaison du cerveau des Crustacés et des Insectes. 3. Le cerveau et la morphologie du squelette céphalique. (Ibid., 1888, 7e Sér., iv, pp. 1–120, 6 Pls.)

—— Sur la structure interne du ganglion optique de quelques larves de Diptères. (Bull. Soc. Phil., Paris, 1885, 7e Sér., ix, pp. 75–78.)

—— La structure du cerveau des Hyménoptères. (Bull. Soc. Philomat., Paris, 1886, 7e Sér., x, pp. 82, 83.)

—— La structure du cerveau des Orthoptères. (Bull. Soc. Philomat., Paris, 1886, 7e Sér., xi, pp. 119–126.)

—— Sur la morphologie comparée du cerveau des Insectes et des Crustacés. (Compt. rend. Acad. Sc. Paris, 1887, civ, pp. 444–447.)

Kenyon, F. C. The meaning and structure of the so-called “mushroom bodies” of the hexapod brain. (Amer. Naturalist, xxx, 1896, pp. 643–650, 1 fig.)

—— The brain of the bee. (Journ. Comp. Neurology, vi, fasc. 3, 1896, pp. 133–210.)

—— The optic lobes of the bee’s brain in the light of recent neurological methods. (Amer. Nat., xxxi, 1897, pp. 369–376, 1 Pl.)

With the embryological works of Graber, Heider, Korscheldt, Patten, Wheeler, etc.

c. Histology of the nervous System

Helmholtz. De fabrica systematis nervosi evertebratorum. Diss. Berolini, 1842.

Remak. Ueber d. Inhalt d. Nervenprimitivröhren. (Archiv f. Anat. u. Phys., 1843.)

Leydig. Lehrbuch der Histologie der Menschen und der Thiere. 1857.

—— Vom Bau des thierischen Körpers. i. 1864.

—— Tafeln zur vergleichenden Anatomie. i. Tübingen, 1864.

—— Zelle und Gewebe, neue Beiträge zur Histologie des Tier-Körpers. Bonn, 1885, pp. 219, 6 Taf.

Walter. Mikroscopische Studien über das Centralnervensystem wirbelloser Thiere. 1863.

Dietl, M. J. Die Gewebselemente des Centralnervensystems bei wirbellosen Thieren. (Aus den Berichten des naturw.-medic. Vereins in Innsbruck.) Innsbruck, 1878.

Berger. Untersuchungen über den Bau des Gehirns und der Retina der Arthropoden. (Arbeiten des zool. Instituts zu Wien, Heft 2, p. 173, 1878.)

—— Nachtrag zu den Untersuchungen über den Bau des Gehirns und der Retina der Arthropoden. (Ibid., Heft 3.)

Viallanes, H. Recherches sur l’histologie des insectes, etc. Paris, 1882. (Annales des Sciences nat., pp. 1–348, Pls. 1–18.)

—— Sur la structure de la substance ponctuée des insectes. Paris, 1885.

Haller, B. Ueber die sogenannte Leydig’sche Punktsubstantz im Centralnervensystem. (Morp. Jahrb., xi, 1886.)

Nansen, F. The structure and combination of the histological elements of the central nervous system. (Bergen’s Museum Aarsberetning for 1886. Bergen, 1887.)

Also the writings of Benedicenti, Holmgren.

249

THE SENSORY ORGANS

a. The eyes and insect vision

Fig. 259.—Different forms of compound eyes. A, a bug (Pyrrhocoris). B, worker bee. C, drone. D, male Bibio, a holoptic insect.—From Judeich and Nitsche.

Of the eyes of insects there are two kinds, the simple and the compound. Of the former there are usually three, arranged in a triangle near the top of the head, between the compound eyes (Fig. 259, B). The compound or facetted eyes, which are usually round and prominent, differ much in size and in the number of facets.

The number of facets varies from 12 in Lepisma,—though in a Brazilian beetle (Lathridius) there are only seven unequal facets,—to 50 in the ant, and up to 4000 in the house-fly, 12,000 in Acherontia atropos, 17,000 in Papilio, 20,000 in the dragon-fly (Æschna), 25,000 in a beetle (Mordella), while in Sphinx convolvuli, the number reaches 27,000. The size of the facets seems to bear some relation to that of the insect, but even in the smallest species none have been observed less than 1
2000
of an inch in diameter. Day-flying Lepidoptera have smaller facets than moths (Lubbock).

Fig. 260.—Section through the ocellus of a young Dyticus larva: ct, cuticula; l, corneal lens; gh, cells of the vitreous body, being modified hypodermal cells (hy); st, rods; re, retinal cells; no, optic nerve.—After Grenacher, from Lang.

The simple, or single-lensed eye (ocellus).—Morphologically the simple eye is a modified portion of the ectoderm, the pigment enclosing the retinal cells arising from specialized hypodermal cells, and covered by a specialized transparent portion of the cuticula, forming the corneal lens. The apparatus is supplied with a nerve, the fibres of which end in a rod or solid nerve-ending, as in other sensory organs.

As seen in the ocellus of Dyticus (Fig. 260), under the corneal lens the hypodermis forms a sort of pit, and the cells are modified 250to form the vitreous body (vitrella) and retina. Each retinal cell (re) is connected with a fibre from the optic nerve, contains pigment, and ends in a rod directed outwards towards the lens. The cells at the end of the pit or depression are, next to the lens, without pigment, and, growing in between the retina and the lens, fill it up, and thus form a sort of vitreous body.

The ocellus appears to be a direct heirloom from the eyes of worms, while the many-facetted compound eye of the crustaceans and of insects is peculiar to these classes. The compound eye of the myriopod Scutigera differs structurally in many respects from the compound eye of insects, and that of Limulus still more so.

It should be observed that in the young nymph of Ephemera, as well as in the semipupa of Bombus, each of the three ocelli are situated on separate sclerites. In Bombus the anterior ocellus has a double shape, being broad, transversely ovate, and not round like the two others, as if resulting from the fusion of what were originally two distinct ocelli.

The ocelli are not infrequently wanting, as in adult Dermaptera, in the Locustidæ, and in certain Hemiptera (Hydrocora). In Lepidoptera there are but two ocelli; in geometrid moths they are often atrophied, and they are absent in butterflies (except Pamphila).

The compound or facetted eye (ommateum).—The facetted arthropod eye is wonderfully complex and most delicately organized, being far more so than that of vertebrates or molluscs. The simplest or most primitive facetted eye appears to be that of Lepisma. As stated by Watase, the compound eye of arthropods is morphologically “a collection of ectodermic pits whose outer open ends face towards the sources of light, and whose inner ends are connected with the central nervous system by the optic nerve fibres.”

The facetted eye is composed of numerous simple eyes called ommatidia, each of which is complicated in structure. The elements which make up an ommatidium are the following: (1) The facet or cornea, which is a specialized portion of the cuticula; and (2), the crystalline lens or cone; (3), the nerve-ending or retinula, which is formed out of the retinula cells and the rhabdom or rod lying in its axis; and (4) of the pigment enclosing the lens and rod; the last three elements are derived from the hypodermis. The single eyes are separated from each other by pigment cells.

The facet or cornea.—This is biconvex, clear, transparent, usually hexagonal in outline, and refracts the light. The corneal lenses are cast in moulting.

The corneal lenses are circular in most cases where they are very convex, as in Lathridius and Batocera. The hexagonal ones are very irregular. When they are very convex the eye has a granular appearance, but when not greater than the convexity of the eye itself, the eye appears perfectly smooth (Bolbocerus, 251etc.). The facets in the lower part of the eye of Dineutes are a trifle larger than in the upper part (about nine to ten). In many insects the reverse is the case, the upper facets being larger than the lower, a notable instance being Anax. The intervening lines between the facets are often beset with hairs, sometimes very long and dense, as in the drone bee and Trichophthalmus; and the modifications of the hairs into scales which takes place on the body occurs on the eyes also, the scales on the eyes of some beetles of the family Colydiidæ being very large, arranged in lines over the eyes like tombstones (Trachypholis).[45]

Fig. 261.—Section through the eye of a fly (Musca vomitoria): c, cornea, or facet; pc, pseudocone; r, retinula; Rh, rhabdom; pg1, pg2, pg3, pigment cells; b.m, basilar membrane; T, Tt1, Tt2, trachea; tv, tracheal vesicle; t.a, terminal anastomosis; op, opticon; c.op, epiopticon; p.op, periopticon; n.c, nuclei; n.c.s, nerve-cell sheath; N.f, decussating nerve-fibres.—After Hickson, from Lubbock.

The crystalline lens or cone.—Behind or within the facets is a layer composed of the cones, behind which are the layers of retinulæ and rhabdoms, and which correspond to the layer of rods and cones, but not the retina as a whole, of vertebrate animals.

The crystalline lens is, when present, usually more or less conical, and consists of four or more hypodermis-cells.

The cones are of various shapes and sizes in insects of different groups, or are entirely wanting, and Grenacher has divided the eyes of insects into eucone, pseudocone, and acone. As the pseudocone seems, however, to be rather a modification of the eucone eye, the following division may be made:—

1. Eucone eyes, comprising those with a well-developed cone. They occur in Lepisma, Blatta (Fig. 262), and other Orthoptera, in Neuroptera, in Cicadidæ, in those Coleoptera with five tarsal 252joints, in the dipterous genus Corethra, and in the Lepidoptera and Hymenoptera (Fig. 263).

Fig. 262.—Ommatidium of cockroach (Periplaneta): lf, cornea; kk, crystalline cone; pg′ pigment cell; rl, retinula; rm, rhabdom.—After Grenacher, from Lubbock.

Fig. 263.—Two separate elements of the eucone eye of a bee; Lf, cornea; n, nucleus of Semper; Kk, crystalline cone; Pg, pigment cells; Rl, retinula; Rm, rhabdom.—After Grenacher, from Lubbock.

Fig. 264.—Three ommatidia of a pseudocone eye, diagrammatic: A, a separate ommatidium of Musca vomitoria, semi-diagrammatic: c, cornea; p.c, pseudocone; pg′, pigmented cells surrounding the pseudocone; p.g2, additional pigment cells; p.g3, basal pigment cells; n.p.c, nuclei of pseudocone; r, retinulæ; n.r, n.r′, nucleus of retinulæ; R, rhabdom; b.m, basal membrane; t.a, terminal anastomosis sending nerve-fibrils to the retinulæ. B, section through a retinula and rhabdom near the basal membrane, the six retinulæ (r) fused into a tube ensheathing the rhabdom (R).—After Hickson.

a. Pseudocone eyes; in which, instead of the crystalline lens or cone, there are four cells filled with a transparent fluid medium, and a smaller protoplasmic portion containing a nucleus (Muscidæ, Fig. 264, pc). Hickson states that the difference between the eucone and pseudocone eyes lies in the fact that in the pseudocone eye “the refracting body formed by the cone-cell lies behind the nuclei,” and in the eucone eye in front of it.

2. Acone eyes, where the cone or refracting body is wanting, but is represented by the four primitive cone-cells. Acone eyes occur in Forficulidæ, Hemiptera (except Cicadidæ), the nematocerous Diptera (Tipula, etc.), and those Coleoptera which have less than five tarsal joints.

253The retinula and rod.—The retinula is morphologically a nerve-end cell, situated at the end of a nerve-fibril arising from the optic nerve. The elements of the retinula of Musca are six in number and surround the rhabdom (Fig. 264), which consists of a bundle of six long, delicate chitinous rods, more or less firmly united together (Fig. 264, R).

The six elements of the retinula of Musca are in their outer or distal portion free from one another, but towards their base are fused into a sheath (Fig. 264, r). They are true nerve-end cells, as shown by Müller and by Max Schultze, their views having been confirmed by Grenacher and by Hickson. The relations of the nerves to the rods after passing through the basal membrane is seen in Fig. 266.

The pigment.—The cones or pseudocones are mostly buried in pigment, as well as the rods; and the pigment forms two layers. The outer of the two layers is called the iris pigment (Fig. 265, e, iris tapetum), and the inner (f) the retinal pigment.

Between the ommatidia internally there occur, according to Hickson, pigment cells (Fig. 264, p.g3), each of which stands on the basilar membrane and sends a fine process outwards towards the internal process of the external pigment-cell (p.g2). A long, slender tracheal vesicle also passes in between the retinulæ.

Fig. 265.—Two ommatidia from the eye of Colymbetes fuscus, × 160: a, cornea; b, cone; c, rhabdom; d, basal membrane, with nerve filaments below it: e, iris pigment; f, retina pigment.—After Exner, from Sharp.

The basilar membrane.—This is a thin fenestrate membrane (Fig. 261) separating the cones and rods from the optic tract (Fig. 264, b.m). It is perforated for the passage of tracheal diverticula and of the optic nerve fibrils. It separates the dioptric or instrumental portion of the eye from the percipient portion, i.e. the optic tract.

The optic tract.—This is the optic ganglion of earlier writers, and appears to be the percipient portion of the eye, as opposed to the dioptric portion. If the reader will examine Figs. 249 and 261, he will see that it consists of three distinct ganglionic swellings, i.e. the opticon, epiopticon, and periopticon, whose structure is very complicated. In Musca (Fig. 261) the first ganglionic swelling (opticon) is separated from the brain by a slight constriction, which Berger regards as the homologue of the optic nerve of the other arthropods. It consists of a very fine granular matrix traversed throughout by a fine meshwork 254of minute fibrillæ, the neurospongium of Hickson. In the young cockroach (Periplaneta) the optic nerve separating the cerebral ganglion from the opticon is much longer in proportion than it is in the adult blow-fly.

Fig. 266.—Periopticon and terminal anastomosis of Agrion, showing the character of the elements of the periopticon (p.op) and the structure of the terminal anastomosis (t.a). 1. The first layer of the terminal anastomosis, consisting of a plexus of fibrils and nerve-cells (n.c). 2. The second layer, in which the fibrils are collected together in bundles. 3. The final optic plexus and nerve-cells. 4. The layer in which the optic fibrils are collected in bundles to be distributed to the retinulæ (r); b.m, basal membrane.—After Hickson.

The second ganglionic swelling (epiopticon, Fig. 261, c.op) is separated from the opticon by a tract of fine nerve-fibrils, which partially decussate; at the decussation two or three larger nerve-cells may be seen. It also contains a few scattered nerve-cells (n.c). The third ganglionic swelling (periopticon, p.op) is separated from the others by a bundle of long optic nerve-fibrils, which cross one another. It is composed of a number of cylindrical masses of neurospongium arranged side by side (Fig. 261, p.op). Between these elements of the periopticon, which do not seem to bear any relation to the number of ommatidia, a single nerve-cell is very frequently seen. The periopticon does not occur in Periplaneta and Nepa (Hickson). The three optic ganglia thus described, together with the cerebral ganglia, are surrounded by a sheath of densely packed nerve-cells.

Bearing in mind the fact that the retinulæ are the nerve-end cells of the fibres passing through the periopticon, it will be well to read the following account, by Hickson, of the terminal anastomosis of the optic fibrils in the periopticon of Agrion bifurcatum, and to examine his sketch (Fig. 266):

“The terminal anastomosis of Agrion may be conveniently divided into four regions. First the region (1) lying nearest to the periopticon in which the nerve-cells are numerous, and the fibrils leaving the periopticon form a complicated plexus; the region (2) next to this, in which the fibrils have collected into bundles separated by spaces occupied by very thin-walled tracheæ in which there are no spiral markings, and lymph-spaces; next, the region (3) in which the fibrils form a final plexus, and in which there are again a considerable number of nerve-cells; and, lastly, the region (4) in which the fibrils are again collected into bundles, separated by spaces containing tracheæ, which perforate the basement membrane to supply the retinulæ.”

It would seem as if the decussation of the optic nerve-fibrils were a matter of 255primary importance, as it so generally occurs, but in the young of that most generalized of all pterygote insects, the cockroach (Periplaneta), Hickson states that the optic nerve-fibrils which leave the periopticon pass without decussating to the ommateum, and in the adult there is only a partial decussation. In Nepa there is no decussation, but the anastomosis is complicated by the presence of looped and transverse anastomoses.

Looking at the eye as a whole, Hickson regards all the nerve structure of the eye lying between the crystalline cone-layer and the true optic nerve to be analogous with the retina of other animals. With Ciaccio, Berger, and others, he does not regard the layer composed of the retinulæ and rhabdoms as the equivalent of the retina of vertebrates, etc.

Origin of the facetted eye.—The two kinds of eye, the simple and the compound, are supposed to have been derived from a primitive type, resembling the single eye (ommatidium) of the acone eye of Tipula. As stated by Lang, “an increase of the elements of this primitive eye led to the formation of the ocellus; an increase in number of the primitive eyes, and their approximation, led to the formation of the compound facet eye.” This view is suggested, he says, by the groups of closely contiguous single eyes of the myriopods, considered in connection with the compound eye of Scutigera. Grenacher looks upon simple (ocelli) and compound eyes as “sisters,” not derived from one another, but from a common parentage.

Immature insects rarely possess compound eyes; they are only known to occur in the nymphs of Odonata and Ephemeridæ, and in the larvæ and pupa of Corethra.

Mode of vision by single eyes or ocelli.—In their simplest condition, the eyes of worms and other of the lower invertebrates, probably only enable those animals to distinguish light from darkness. The ocelli of spiders and of many insects, however, probably enable them, as Lubbock remarks, to see as our eyes do. The simple lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. The ocelli of different arthropods differ, however, very much in degree of complexity.

Müller considered that the power of vision of ocelli “is probably confined to the perception of very near objects.”

“This may be inferred,” Müller states, “partly from their existing principally in larvæ and apterous insects, and partly from several observations which I have made relative to the position of these simple eyes. In the genus Empusa the head is so prolonged over the middle inferior eye that, in the locomotion of the animal, the nearest objects can only come within the range. In Locusta cornuta, also, the same eye lies beneath the prolongation of the head.... In 256the Orthoptera generally, also, the simple eyes are, in consequence of the depressed position of the head, directed downwards towards the surface upon which the insects are moving.”[46] Lowne considers that in the ocellus of Eristalis, the great convexity of the lens must give it a very short focus, and the comparatively small number of rods render the picture of even very near objects quite imperfect and practically useless for purposes of vision, and that the function of the ocelli is “the perception of the intensity and the direction of light, rather than of vision, in the ordinary acceptation of the term.”

Réaumur, Marcel de Serres, Dugès, and Forel have shown by experiment, that in insects which possess both ocelli and compound eyes, the former may be covered over without materially affecting the movements of the animals, while if the facetted eyes are covered, they act as if in the dark (Lubbock).

While Plateau regards the ocelli as of scarcely any use to the insect, and Forel claims that wasps, humble bees, ants, etc., walk or fly almost equally well without as with the aid of their ocelli, Lubbock demurs to this view, and says the same experiments of Forel’s might almost be quoted to prove the same with reference to the compound eyes. Indeed, the writer has observed that in caves, eyeless beetles apparently run about as freely and with as much purpose, as their eyed relatives in the open air.

Plateau has recently shown that caterpillars which have ocelli alone are very short-sighted, not seeing objects at a distance beyond one or two centimetres, and it has been fully proved by Plateau and others, that spiders, with their well-formed ocelli, are myopic, and have little power of making out distinctly the shape of the objects they see.

On the whole, we are rather inclined to agree with Lubbock and Forel, that the ocelli are useful in dark places and for near vision. They are, as Lubbock states, especially developed in insects, such as ants, bees, and wasps, which live partly in the open light and partly in the dark recesses of nests. Moreover, the night-flying moths nearly all possess ocelli, while with one known exception (Pamphila) they are wanting in butterflies.

Finally, remarks Lubbock, “Whatever the special function of ocelli may be, it seems clear that they must see in the same manner as our eyes do—that is to say, the image must be reversed. On the other hand, in the case of compound eyes, it seems probable that the vision is direct, and the difficulty of accounting for the existence in the same animal of two such different kinds of eyes is certainly enhanced by the fact that, as it would seem, the image given by the medial eyes is reversed, while that of the lateral ones is direct” (p. 181).

Mode of vision by facetted eyes.—The complexity of the facetted eyes of insects is amazing, and difficult to account for unless we accept the mosaic theory of Müller, who maintained that the distinctness of the image formed by such an eye will be greater in proportion to the number of separate cones. His famous theory is thus stated: “An image formed by several thousand separate points, of which each corresponds to a distinct field of vision in the external world, will resemble a piece of mosaic work, and a better idea cannot be conceived of the image of external objects which will be depicted on the retina of beings endowed with such organs of vision, than by comparing it with perfect work of that kind.”

257

Fig. 267.—From Lubbock.

How vision is effected by a many-facetted eye is thus explained by Lubbock: “Let a number of transparent tubes, or cones with opaque walls, be ranged side by side in front of the retina, and separated from one another by black pigment. In this case the only light which can reach the optic nerve will be that which falls on any given tube in the direction of its axis.” For instance, in Fig. 267, the light from a will pass to a′, that from b to b′, that from c to c′, and so on. The light from c, which falls on the other tubes, will not reach the nerve, but will impinge on the sides and be absorbed by the pigment. Thus, though the light from c will illuminate the whole surface of the eye, it will only affect the nerve at c′.

According to this view those rays of light only which pass directly through the crystalline cones, or are reflected from their sides, can reach the corresponding nerve-fibres. The others fall on, and are absorbed by, the pigment which separates the different facets. Hence each cone receives light only from a very small portion of the field of vision, and the rays so received are collected into one spot of light.

It follows from this theory that the larger and more convex the eye, the wider will be its field of vision, while the smaller and more numerous are the facets, the more distinct will be the vision (Lubbock).

The theory is certainly supported by the shape and size and the immense number of facets of the eye of the dragon-fly, which all concede to see better, and at a longer range, than probably any other insect.

Müller’s mosaic theory was generally received, until doubted and criticised by Gottsche (1852), Dor (1861), Plateau, and others. As Lubbock in his excellent summary states, Gottsche’s observation (previously made by Leeuwenhoek) that each separate cornea gives a separate and distinct image, was made on the eye of the blow-fly, which does not possess a true crystalline cone. Plateau’s objection loses its force, since he seems to have had in his mind, as Lubbock states, Gottsche’s, rather than Müller’s, theory.

Müller’s theory is supported by Boll, Grenacher, Lubbock, Watase, and especially by Exner, who has given much attention to the subject of the vision of insects, and is the weightiest authority on the subject.

Gottsche’s view that each of the facetted eyes makes a distinct image which partially overlaps and is combined with all the images made by the other facets, 258was shown by Grenacher to be untenable, after repeating Gottsche’s experiments with the eyes of moths, in which the crystalline cones are firm and attached to the cornea. He was thus able to remove the soft parts, and to look through the cones and the cornea. When the microscope was focussed at the inner end of the cone, a spot of light was visible, but no image. As the object-glass was moved forward, the image gradually came into view, and then disappeared again. Here, then, the image is formed in the interior of the cone itself.

Exner attempted to make this experiment with the eye of Hydrophilus, but in that insect the crystalline cones always came away from the cornea. “He, however, calculated the focal length, refraction, etc., of the cornea, and concluded that, even if, in spite of the crystalline cone, an image could be formed, it would fall much behind the retinula.”

“In these cases, then,” adds Lubbock, “an image is out of the question. Moreover, as the cone tapers to a point, there would, in fact, be no room for an image, which must be received on an appropriate surface. In many insect eyes, indeed, as in those of the cockchafer, the crystalline cone is drawn out into a thread, which expands again before reaching the retinula. Such an arrangement seems fatal to any idea of an image.”

Lubbock thus sums up the reasons which seem to favor Müller’s theory of mosaic vision, and to oppose Gottsche’s view: “(1) In certain cases, as in Hyperia, there are no lenses, and consequently there can be no image; (2) the image would generally be destroyed by the crystalline cone; (3) in some cases it would seem that the image would be formed completely behind the eye, while in others, again, it would be too near the cornea; (4) a pointed retina seems incompatible with a clear image; (5) any true projection of an image would in certain species be precluded by the presence of impenetrable pigment, which only leaves a minute central passage for the light-rays; (6) even the clearest image would be useless, from the absence of a suitable receptive surface, since both the small number and mode of combination of the elements composing that surface seem to preclude it from receiving more than a single impression; (7) no system of accommodation has yet been discovered; finally (8), a combination of many thousand relatively complete eyes seems quite useless and incomprehensible.”

In his most recent work (1890) on the eyes of crustacea and insects, Exner states that the numerous simple eyes which make up the compound eye have each a cornea, but it is more or less flat, and the crystalline part of the eye has not the shape of a lens, but of a “lens cylinder,” that is, of a cylinder which is composed of sheets of transparent tissue, the refracting powers of which decrease toward the periphery of the cylinder. If an eye of this kind is removed and freed of the pigment which surrounds it, objects may be looked at through it from behind; but its field of vision is very small, and the direct images received from each separate eye are either produced close to one another on the retina (or rather the retinulæ of all the eyes) or superposed. In this last case no less than thirty separate images may be superposed, which is supposed to be of great use to night-flying insects. Exner claims that many other advantages result from the compound nature of an insect’s eye. Thus the mobile pigment, which corresponds to our iris, can take different positions, either between the separate eyes or behind the lens cylinders, in which case it acts as so many screens to intercept the over-abundance of light. Exner finds that with its compound eyes the common glow-worm (Lampyris) is capable of distinguishing large signboard letters at a distance of ten or more feet, as well as extremely fine lines engraved one-hundredth of an inch apart, if they are at a distance of less than half an inch from the eye. Exner substantiates the truth of the results of Plateau’s 259experiments, and claims that while the compound eye is inferior to the vertebrate eye for making out the forms of objects, it is superior to the latter in distinguishing the smallest movements of objects in the total field of vision.

More recently Mallock has given some optical reasons to show that Müller’s view is the true one. He concludes, and thus agrees with Plateau, that insects do not see well, at any rate as regards their power of defining distant objects, and their behavior certainly favors this view. It might be asked, What advantage, then, have insects with compound eyes over those with simple eyes? Mallock answers, that the advantage over simple-eyed animals lies in the fact that there is hardly any practical limit to the nearness of the objects they can examine. “With the composite eye, indeed, the closer the object the better the sight, for the greater will be the number of lenses employed to produce the impression; whereas, in the simple eye the focal length of the lens limits the distance at which a distinct view can be obtained.” He gives a table containing measures of the diameters and angles between the axes of the lenses of various insect eyes, and states that the best of the eyes would give a picture about as good as if executed in rather coarse woodwork and viewed at a distance of a foot, “and although a distant landscape could only be indifferently represented on such a coarse-grained structure, it would do very well for things near enough to occupy a considerable part of the field of view.”

The principal use of the facetted eye to perceive the movements of animals.—Plateau adopts Exner’s views as to the use of the facetted eye in perceiving the movements of other animals. He therefore concludes that insects and other arthropods with compound eyes do not distinguish the form of objects; but with Exner he believes that their vision consists mainly in the perception of moving bodies.

Most animals seem but little impressed by the form of their enemies or of their victims, though their attention is immediately excited by the slightest displacement. Hunters, fishermen, and entomologists have made in confirmation of this view numerous and demonstrative observations.

Though the production of an image in the facetted eye of the insect seems impossible, we can easily conceive, says Plateau, how it can ascertain the existence of a movement. Indeed, if a luminous object is placed before a compound eye, it will illuminate a whole group of simple eyes or facets; moreover, the centre of this group will be clearer than the rest. Every movement of the luminous body will displace the centre of clearness; some of the facets not illuminated will first receive the light, and others will reënter into the shade; some nervous terminations will be excited anew, while those which were so formerly will cease to be. Hence the facetted eyes are not complete visual organs, but mainly organs of orientation.

Plateau experimented in the following way: In a darkened room, with two differently shaped but nearly equal light-openings, one square and open, the other subdivided into a number of small holes, and therefore of more difficult egress, he observed the choices of opening made by insects flying from the other end of the room. Careful practical provisions were made to eliminate error; the light-intensity of the two openings was as far as possible equalized or else noted, and no trees or other external objects were in view. The room was not darkened beyond the limit at which ordinary type ceases to be readable, otherwise the insects refused to fly (it is well known that during the passage of a thick cloud insects usually cease to fly). These observations were made on insects 260both with or without ocelli, in addition to the compound eyes, and with the same results.

From repeated experiments on flies, bees, etc., butterflies and moths, dragon-flies and beetles, Plateau concludes that insects with compound eyes do not notice differences in form of openings in a half-darkened room, but fly with equal readiness to the apparently easy and apparently difficult way of escape; that they are attracted to the more intensely lighted opening, or to one with apparently greater surface; hence he concludes that they cannot distinguish the form of objects, at least only to a very slight extent, though they readily perceive objects in motion.

One result of his experiments is that insects only utilize their eyes to choose between a white luminous orifice in a dark chamber, or another orifice, or group of orifices, equally white. They are guided neither by odorous emanations nor by differences of color. He thinks that bees have as bad sight and act almost exactly as flies.

From numerous experiments on Odonata, Coleoptera, Lepidoptera, Diptera, and Hymenoptera Plateau arrives provisionally at the following conclusions:

1. Diurnal insects have need of a quick strong light, and cannot direct their movements in partial obscurity.

2. Insects with compound eyes do not notice differences of form existing between two light orifices, and are deceived by an excess of luminous intensity as well as by the apparent excess of surface. In short, they do not distinguish the form of objects, or if they do, distinguish them very badly.

Lubbock, however, does not fully accept Plateau’s experiments with the windows, and thinks they discern the form of bodies better than Plateau supposes.

How far can insects see?—It is now supposed that no insects can perceive objects at a greater distance than about six feet. On an average Lepidoptera can see the movements of rather large bodies 1.50 meters, but Hymenoptera only 58 cm., and Diptera 68 cm.; while the firefly (Lampyris) can see tolerably well the form of large objects at a distance of over two meters.

Until further experiments are made, it seems probable, then, that few if any insects have acute sight, that they see objects best when moving, and on the whole—except dragon-flies and other predaceous, swiftly flying insects, such as certain flies, wasps, and bees, which have very large rounded eyes—insects are guided mainly rather by the sense of smell than of sight.

Relation of sight to the color of eyes.—It appears from the observations of Girschner that those Diptera with eyes of a uniform color see better than those with brightly banded or spotted eyes. Thus those flies (Asilidæ, Empidæ, Leptidæ, Dolichopidæ) whose predaceous habits requires good or quick sight have uniformly dark eyes, as have also such flies as live constantly on the wing, i.e., the holoptic Bombyliidæ, Syrphidæ, Pipunculidæ, etc., whose eyes are also very large.

Those flies whose larvæ are parasitic on other animals have eyes of a uniform color that they may readily detect the most suitable host for their young; such are the Bombyliidæ, Conopidæ, Pipunculidæ, and Tachinidæ.

Certain flies which live in the clear sunlight, as many Dolichopidæ, some Bombyliidæ, and certain Tabanidæ (Tabanus, Chrysops, Hæmatopota), and which are often easily caught with the hand, have eyes spotted or banded with bright or metallic colors. This is also a sexual trait, as the males of some horse-flies visiting flowers have eyes of a single color, the spots and bands surviving only on the lower and hinder parts of the eye, while their voracious blood-sucking females have the entire eye spotted or banded (Kolbe).

The color-sense of insects.—Insects, as Spengel first suggested, appear to be 261able to distinguish the color of objects. Lubbock has experimentally proved that bees, wasps, and ants have this power, blue being the favorite color of the honey-bee, and violet of ants, which are sensitive to ultra-violet rays.

It is well known that butterflies will descend from a position high in the air, mistaking white bits of paper for white flowers; while, as we have observed, white butterflies (Pieris) prefer white flowers, and yellow butterflies (Colias) appear to alight on yellow flowers in preference to white ones.

The late Mr. S. L. Elliott once informed us that on a red barn with white trimmings he observed that white moths (Spilosoma, Hyphantria, and Acronycta oblinita) rested on the white parts, while on the darker, reddish portions sat Catocalæ and other dark or reddish moths. Gross observed that house-flies would frequent a bluish green ring on the ceiling of his chamber; but if it were covered by white paper, the flies would leave the spot, though they would return as soon as the paper ring was removed (Kolbe). We have observed that house-flies prefer green paper to the yellowish wall of a kitchen, but were not attracted to sheets of a Prussian blue paper, attached to the same wall and ceiling.

It is generally supposed that the shape and high colors of flowers attract insects; but Plateau has made a number of ingenious experiments which tend to disprove this view. He used in his investigations the dahlia, with its central head of flowerets, which contrast so strongly with the corolla. He finds (1) that insects frequent flowers which have not undergone any mutilation, but whose form and colors are hidden by green leaves. (2) Neither the shape nor lively colors of the central head (capitulum) seem to attract them. (3) The gayly colored peripheral flowerets of simple dahlias and, consequently, of the heads of other composite flowers, do not play the rôle of signals, such as has been attributed to them. (4) The insects are evidently guided by another sense than that of sight, and this sense is probably that of smell.

LITERATURE ON THE EYES AND VISION

a. General

Serres, Marcel de. Mémoires sur les yeux composés et les yeux lisses des insectes. Montpellier, 1813.

Müller, Johannes. Zur vergleichenden Physiologie des Gesichtssinnes der Menschen und der Tiere. 8 Taf. Leipzig, 1826.

—— Ueber die Augen des Maikäfers. (Meckel’s Archiv f. Anat. u. Phys., 1829, pp. 177–181; Ann. d. Sc. nat., 1829, sér. 1, xviii, pp. 108–112.)

Dujardin, F. Sur les yeux simples ou stemmates des animaux articulés. (C. R. Acad. Sci., Paris, 1847, xxv, pp. 711–714.)

Gottsche, C. M. Beitrag zur Anatomie und Physiologie des Auges der Krebse und Fliegen. (Müller’s Archiv für Anat. u. Phys., 1852, pp. 483–492. Figs.)

Murray, Andrew. On insect vision and blind insects. (Edinburgh New Phil. Jour., new ser. vi, 1857, pp. 120–138.)

Claparède, Édouard. Zur Morphologie der zusammengesetzten Augen bei den Arthropoden. (Zeitschr. f. wissensch. Zool., 1859, x, pp. 191–214, 3 Taf.)

Dor, H. De la vision chez les Arthropodes. (Archives Sci. Phys, et Nat., 1861, xii, p. 22, 1 Pl.)

Landois, H. Die Raupenaugen (Ocelli compositi mihi). (Zeitschr. f. wissensch. Zool., xvi, 1866, pp. 27–44, 1 Taf.)

—— und W. Thelen. Zur Entwicklungsgeschichte der fasettierten Augen von Tenebrio molitor L. (Zeitschr. f. wissensch. Zool., xvii, 1867, pp. 34–43, 1 Taf.)

262Schultze, Max. Untersuchungen über die zusammengesetzten Augen der Krebsen und Insecten. Bonn, 1868.

Schmidt, Oscar. Die Form der Krystallkegel in Arthropodenauge. (Zeitschr. f. wissensch. Zool., xxx, Suppl., 1878, pp. 1–12, 1 Taf.)

Grenacher, H. Untersuchungen ueber das Sehorgan der Arthropoden, insbesondere Spinnen, Insecten und Crustaceen. (Göttingen, 1879, 4º, pp. 1–188, 11 Taf.)

Reichenbach, H. Wie die Insekten sehen. Fig. (Daheim, xvi Jahrg., 1880, pp. 284–286.)

Poletajew, N. Ueber die Ozellen und ihr Sehvermögen bei den Phryganiden. (Horæ Soc. Ent. Ross., 1884, xviii, p. 23, 1 Taf. In Russian.)

Hickson, S. J. The eye and optic tract of insects. (Quart. Journ. Micr. Sc., ser. 2, xxv, 1885, pp. 215–221, 3 Pls.)

Notthaft, Jul. Ueber die Gesichtswahrnehmungen vermittelst des Fazettenauges. (Abhandl. Senckenberg. naturf. Ges., xii., 1880, pp. 35–124, 5 Taf.)

—— Die physiologische Bedeutung des fazettierten Insektenauges. (Kosmos, 1886, xviii, pp. 442–450, Fig.)

Mark, E. L. Simple eyes in arthropods. (Bull. Mus. Comp. Zool., 1887, xiii, pp. 49–105, 5 Pls.)

Girschner, E. Einiges über die Färbung der Dipterenaugen. (Berlin. Ent. Zeitschr., 1888, xxxi, pp. 155–162, 1 Taf.)

Graber, V. Das unicorneale Tracheatenauge. (Archiv f. Mikroskop. Anat., xvii, 1879, pp. 58–93, 3 Taf.; Nachtrag, p. 94.)

—— Fundamentalversuche über die Helligkeits- und Farbenempfindlichkeit augenloser und geblendeter Tiere. (Sitzgs.-Ber. Akad. Wissensch., Wien, 1883, lxxxvii, pp. 201–236.)

Dahl, Fr. Die Insekten können Formen unterscheiden. (Zool. Anz., xii, 1889, pp. 243–247.)

Ciaccio, G. V. Figure dichiarative della minuta fabbrica degli occhi de’ Ditteri. Bologna, 1884, 12 Taf., 30 pp.

—— Della minuta fabbrica degli occhi de’ Ditteri. (Mem. Accad. Bologna, 1886, ser. 4, vi, pp. 605–660.)

—— Sur la forme et la structure des facettes de la cornée et sur les milieux refringents des yeux composés des Muscidés. (Journ. Micr., Paris, 1889, xiii Année, pp. 80–84.)

Carrière, J. On the eyes of some invertebrata. (Quart. Journ. Micr. Sc. 1884, ser. 2, xxiv, pp. 673–681, 1 Pl.)

—— Ueber die Arbeiten von Viallanes, Ciaccio und Hickson. (Biolog. Centralblatt, v, 1885, pp. 589–597.)

—— Die Sehorgane der Tiere vergleichend anatomisch dargestellt. München u. Leipzig, 1885, 205 pp., 147 Figs., 1 Taf.

—— Kurze Mitteilungen aus fortgesetzten Untersuchungen über die Sehorgane. (Zool. Anz., ix Jarhg., 1886, pp. 141–147, 479–481, 496–500.)

Forel, A. Les fourmis de la Suisse. (Neue Denkschriften der schweiz. naturforsch. Gesellsch. xxvi. 1874, pp. 480, 2 Pls.) Separate. pp. iv u. 457. Genève.

—— Beitrag zur Kenntnis der Sinnesempfindungen der Insekten. (Mitteil. d. Münchener Ent. Vereins, ii Jahrg., 1878, pp. 1–21.)

—— Sensations des insectes. (Recueil Zool. Suisse, iv, 1886 et 1887.)

Plateau, F. L’instinct chez les insectes mis en défaut par les fleurs artificielles? (Assoc. française avancement des sciences. Congrès de Clermont. Ferrand, 1876.)

263Plateau, F. Recherches expérimentales sur la vision chez les insectes. Les insectes distinguent-ils la forme des objets? (Bull. Acad. Belg. 3 Sér. x, 1885, pp. 231–250.)

—— Recherches expérimentales sur la vision chez les insectes.

1. Part, a. Résumé des travaux effectués jusqu’en 1887 sur la structure et le fonctionnement des yeux simples. b. Vision chez les Myriapodes. (Ibid. Sér. 3, xiv, 1887, pp. 407–448, 1 Pl.)

3. Part, a. Vision chez les chenilles, b. Rôle des ocelles frontaux chez les insectes parfaits. (Ibid. Sér. 3, xv, 1888, pp. 28–91.)

4. Part. Vision à l’aide des yeux composés. a. Résumé anatomo-physiologique. b. Expériences comparatives sur les insectes et sur les vertébrés. (Mém. cour. et autres Mém. Acad. Belg. 1888, xliii, pp. 1–91, 2 Pls.)

5. Part, a. Perception des mouvements chez les insectes. b. Addition aux recherches sur le vol des insectes avenglés. c. Résumé général. (Bull. Acad. Belg. 1888, sér. 3, xvi, pp. 395–457, 1 Pl.)

—— Recherches expérimentales sur la vision chez les Arthropodes, 2 Pls. (Mém. couronn. et autres Mém. publ. p. l’Acad. Roy. d. Sciences, etc., de Belgique, xliii, Bruxelles, 1889.)

Watase, S. On the morphology of the compound eyes in the Arthropoda. (Studies from biol. laborat. Johns-Hopkins Univ., 1890, pp. 287–334, 4 Pls.)

Stefanowska, M. La disposition histologique du pigment dans les yeux des Arthropodes. (Recueil Zool. Suisse, 1890, pp. 151–200, 2 Pls.)

Pankrath, O. Das Auge der Raupen und Phryganiden larven. (Zeitschr. f. wissensch. Zool., 1890, xlix, pp. 690–708, 2 Taf.)

Lowne, B. Th. On the modifications of the simple and compound eyes of insects. (Philos. Trans. Roy. Soc., London, clxix, 1878, pp. 577–602, 3 Pls.)

—— On the structure and functions of the eyes of Arthropoda. (Proc. Roy. Soc., London, 1883, xxxv, pp. 140–145.)

—— On the compound vision and the morphology of the eye in insects. (Trans. Linn. Soc., London, 1884, ii, pp. 389–420, 4 Pls.)

—— On the structure of the retina of the blow-fly (Calliphora erythrocephala). (Jour. Linn. Soc., London, 1890, xx, pp. 406–417, 1 Pl.)

Patten, W. Eyes of molluscs and arthropods. (Journal of Morphol., Boston, 1887, i, pp. 67–92, 1 Pl.; Mitteil. Zool. Stat. Neapel, vi, 1886, pp. 542–756, 5 Taf.)

—— Studies on the eyes of arthropods.—1. Development of the eyes of Vespa, with observations on the ocelli of some insects. (Ibid., pp. 193–226, 1 Pl.)—2. Eyes of Acilius. (Ibid., 1888, ii., pp. 190–97, 7 Pls.)

—— On the eyes of molluscs and arthropods. (Zool. Anzeiger, 1887, x Jahrg., pp. 256–261.)

—— Is the ommatidium a hair-bearing sense-bud? (Anatom. Anzeiger, 1890, v, pp. 353–359, 4 Figs.)

Exner, S. Ueber das Sehen von Bewegungen und die Theorie des zusammengesetzten Auges. (Sitzgsber. d. math. naturwiss. Cl. kais. Akad. d. Wissens. Wien, lxxii Jahrg., 1875, 3 Abt. Physiologie, pp. 156–190, 1 Taf.)

—— Die Frage von der Funktionsweise der Fazettenauges. (Biolog. Centralblatt, i, 1881, pp. 272–281.)

—— Das Netzhautbild des Insektenauges. (Sitzgsber. kais. Akad. d. Wissensch. Wien, 1889, xcviii, 3 Abt., pp. 13–65, 2 Taf. u. 7 Figs.)

—— Durch Licht bedingte Verschiebungen des Pigmentes im Insektenauge und deren physiologische Bedeutung. (Ibid., pp. 143–151, 1 Taf.)

264Exner, S. Die Physiologie der fazettierten Augen von Krebsen und Insekten, 7 Taf., 1, Lichtdruck u. 23 Holzschn. pp. 206. Wien, F. Deuticke, 1891.

Lubbock, John. On the senses, instincts, and intelligence of animals, with special reference to insects. London, 1888, pp. 292.

Mallock, A. Insect sight and the defining power of composite eyes. (Proc. Roy. Soc., London, 1894, lv, pp. 85–90, 3 Figs.)

b. The color-sense

Nussli, J. Ueber den Farbensinn der Bienen. (Schweiz. Bienenzeitung, N. F., ii Jahrg., 1879, pp. 238–240.)

Kramer. Der Farbensinn der Bienen. (Ibid., iii Jahrg., 1880, pp. 179–198.)

Gross, Wilhelm. Ueber den Farbensinn der Tiere, insbesondere der Insekten. (Isis v. Russ., v Jahrg., 1880, pp. 292–294, 300–302, 308–309.)

Lubbock, John. Ants, bees, and wasps. London, 1882, pp. 448. Also On the senses, etc., of animals, 1889.

Graber, Vitus. Grundlinien zur Erforschung des Helligkeits und Farbensinnes der Tiere. Prag u. Leipzig, 1884, pp. 322. (See also p. 262.)

Forel, Auguste. Les Fourmis perçoisent-elles l’ultra-violet avec leurs yeux ou avec leur peau? (Arch. Sci. Phys. Nat. Genève, 1886, 3 sér., xvi, pp. 346–350.)

Also the works of Darwin, Wallace, F. Müller, Grant Allen’s The Color Sense (1879), Beddard’s Animal Coloration, etc.

b. The organs of smell

The seat of the organs of smell is mainly in the antennæ, and they may be regarded as the principal olfactory organs. For our present knowledge of the anatomy and physiology of the olfactory organs of insects we are mainly indebted to the recent investigations of Hauser and of Kraepelin. The following historical and critical remarks are translated from Kraepelin’s able treatise:

Historical sketch of our knowledge of the organs of smell.—In the first half of the last century began the inquiries as to the seat of the sense of smell in the arthropods. Thus Réaumur, in his Mémoires (i, p. 283; ii, 224), expressed the view that in the antennæ was situated a special organ which might be an organ of smell.

Lesser, Roesel, Lyonet, Bonnet, and others expressed the same opinion. Before this Sulzer suggested that an “unknown sense” might exist in the antennæ; others regarded the stigmata as organs of smell, as these were considered the natural passages for the olfactory currents. Duméril, in two special treatises as well as in his Considérations générales, sought to prove the theory as to the seat of the organs of smell in the stigmata.

Against both of these leading views as to the seat of the sense of smell were expressed, in the last century, different opinions. Thus Comparetti thought that the sense of smell might be localized in very different points of the head, in the antennal club of lamellicorns, in the sucking-tube of Lepidoptera, in 265special frontal holes of flies and Orthoptera, etc., while Bonsdorf considered the palpi as organs of smell.

Thus four different views, confused, were held at the opening of this century; the Hamburg zoölogist, M. C. S. Lehrman, in three different treatises, brought together all the hitherto known observations and arguments, treated them critically, and completed them by his own extended studies. Lehrman adopted the opinions of Reimarus, Baster, Duméril, and Schelver, that the stigmata presented the most convenient place for the site of the organs of smell. Cuvier followed throughout the lead of Lehrman, but Latreille returned to the view of the perception of smell by the antennæ, while Treviranus considered the mouth of arthropods as the probable site of the sense of smell, an opinion which, before his time, Huber, in his experiments on bees, had thought to be correct. Marcel de Serres (1811) returned again to the palpi, and asserted—at least in the Orthoptera—their functions to be olfactory, while Blainville, ten years later, again expressed anew the old opinion that the antennæ, or at least their terminations, were organs of smell. Up to that date there was an uncertainty as to the seat of the organs both of smell and hearing. Fabricius, indeed, had already, in 1783, thought he had found an organ of hearing at the base of the outer antenna. In 1826 J. Müller mentioned an already well-known organ in the abdomen of crickets as an organ of hearing. Müller, however, was doubtful, from the fact that the nerve passing to this organ arose, not from the brain, but from the third thoracic ganglion; but, notwithstanding, he remarks: “Perhaps we have not found the organ of hearing in insects because we sought for it in the head.” This discovery was afterwards considerably broadened and extended by Siebold’s work, for the views of these naturalists on the seat of both organs had a definite influence, especially in Germany. For awhile, indeed, Müller’s hypothesis stood in complete contradiction, so that during the following decennial was presented anew the picture of opposing observations and opinions as to the nature of the organs of smell. While Robineau-Desvoidy, at the end of the twentieth year, and also later, in different writings, strove energetically for the olfactory nature of the antennæ, Straus-Dürckheim held fast to the view that the tracheæ possessed the function under discussion. At the same period Kirby and Spence, in their valuable Introduction to Entomology, maintained that “two white cushions on the under side of the upper lip” in the mouth of biting insects formed a nose or “rhinarium” peculiar to insects. This opinion was afterwards adopted by Lacordaire (Introduction à Entomologie), and also by Oken in his Lehrbuch der Naturphilosophie, while Burmeister, rejecting all the views previously held, believed that insects might perhaps smell “with the inner upper surface of the skin.” Müller’s locust’s ear he regarded as a vocal organ.

Besides these occasional expressions of opinion, the French literature of the thirtieth and fortieth years of this century recorded a long series of special works, with weighty experimental and physiological contents, on this subject. Thus Lefebre, in 1838, described the experiments which he made on bees, and which seemed to assign the seat of the sense of smell to the antennæ. Dugès reported similar researches on the Scolopendræ, and Pierret thought that the great development of the antennæ in the male Bombycidæ might be similarly interpreted. Driesch sought to give currency to the views of Bonsdorf, Lamarck, and Marcel de Serres, that the sense of smell was localized in the palpi, though Duponchel went back to the old assertion of æroscepsis of Lehrman, i.e. of the air-test through the antennæ, and Goureau again referred the seat of the sense of smell to the mouth. In England, Newport at this period put forth a work in which he considered the antennæ as organs of touch and hearing, and the palpi 266as organs of smell—a view which, as regards the antennæ, was opposed by Newman.

Thus the contention as to the use of the antennæ and the seat of the organs of smell and hearing fluctuated from one side to the other, and when in 1844 Küster, by reason of his experiments on numerous insects, again claimed that “the antennæ are the smelling organs of insects,” he argued on a scientific basis; yet v. Siebold and Stannius (1848), in their valuable Lehrbuch der vergleichenden Anatomie (p. 581), remarked that “organs of smell have not yet with certainty been discovered in these animals.”

The following decennial was of marked importance in the judgment of many disputed questions. Almost contemporaneously with Siebold and Stannius’ Lehrbuch appeared an opportune treatise by Erichson, in which this naturalist first brought forward certain anatomical data as to the structure of the antennæ of insects. In a great number of insects Erichson described on the upper surface of the antennæ peculiar minute pits, “pori,” which, according to him, were covered by a thin membrane, and to which he ascribed the perception of smell. A still more thorough work on this subject was published in the following year by Burmeister, who recognized in the pits of lamellicorns many small tubercles and hairs; and about the same time Slater, as also Pierret and Erichson before him had done, out of the differences of the antennal development in the males and females in flesh and plant-eating insects, brought together the proof of the olfactory function of the antennæ. But the most valuable work of this period is that of Perris, who, after a review of previous opinions, by exact observations and experiments, a model of their kind, sought to discover the seat of the sense of smell. He comes to the conclusion that the antennæ, and perhaps also the palpi, may claim this sense, and finds full confirmation of Dufour’s views, and adopts as new the physiological possibility expressed by Hill and Bonnet, that the antennæ might be the seat of both senses—those of smell and hearing.

The beautiful works of Erichson, Burmeister, and Perris could not remain long unnoticed. In 1857 Hicks published complete researches on the peculiar nerve-endings which he had found in the antennæ, also in the halteres of flies and the wings of all the other groups of insects, and which he judged to be for the perception of smell. But Erichson’s and Burmeister’s “pori” were by Lespès, in 1858, explained to be so many auditory vesicles with otoliths. This view was refuted by Claparède and Claus without their deciding on any definite sense. Leydig first made a decided step in advance. In different writings this naturalist had busied himself with the integumental structures of arthropods, and declared Erichson’s view as to the olfactory nature of the antennal pits as the truest, before he, in his careful work on the olfactory and auditory organs of crabs and insects, had given excellent representations of the numerous anatomical details which he had selected from his extensive researches in all groups of arthropods. Besides the pits which were found to exist in Crustacea, Scolopendræ, beetles, Hymenoptera, Diptera, Orthoptera, Neuroptera, and Hemiptera, and which had only thus far been regarded as sense-organs, Leydig first calls attention to the widely distributed pegs and teeth, also considering them as sense-organs. “Olfactory teeth,” occurring as pale rods, perforated at the end, on the surface of the antennæ of Crustacea, Myriopoda, Hymenoptera, Lepidoptera, Coleoptera, are easily distinguished, and besides the “olfactory pegs” of the palpi, may be claimed as organs of smell. The nerve-end apparatus first discovered by Hicks in the halteres and wings, Leydig thinks should be ranked as organs of hearing.

There was still some opposition to Leydig’s opinion that in the insects the 267sense of smell is localized in the antennæ (teeth and pits), and here the work of Hensen might be mentioned, which in 1860 had a decided influence upon the conclusion of some inquiries.

Thus Landois denied that the antennæ had the sense of smell, and declared that the pits in the antennæ of the stag beetle were auditory organs. So, also, Paasch rejected Leydig’s conclusion, while he sought to again reinstate the old opinion of Rosenthal as to the olfactory nature of the frontal cavity of the Diptera. In spite of the exact observations and interesting anatomical discoveries of Forel in ants, made in 1874, there appeared the great work of Wolff on the olfactory organs of bees, in which this observer, with much skill and acuteness, sought to give a basis for the hypothesis of Kirby and Spence that the seat of the sense of smell lay in the soft palatine skin of the labrum within the mouth (i.e. the epipharynx). Joseph, two years later, drew attention to the stigmata as olfactory organs, referring to the olfactory girdle, and Forel sought by an occasional criticism of Wolff’s conclusions to prove experimentally the olfactory function of the antenna; but Graber, in his widely read book on insects, defended the Wolffian “nose” in the most determined way, and denied to the antennæ their so often indicated faculty of smell. In 1879 Berté thought he had observed in the antenna of the flea a distinct auditory organ, and Lubbock considered the organs of Forel in the antennæ of ants as a “microscopic stethoscope.” In 1879 Graber described a new otocyst-like sense-organ in the antennæ of flies, which was accompanied by a complete list of all the conceivable forms of auditory organs in arthropods. In this work Graber described in Musca and other Diptera closed otocysts with otoliths and auditory hairs, as Lespès had previously done. But Paul Mayer, in two essays, refuted this view in a criticism of the opinion of Berté, referring the “otocysts with otoliths” to the well-known antennal pits into which tracheæ might pass. Mayer did not decide on the function of the hairs which extend to the bottom of the pits; while in the most recent research, that of Hauser, the author again energetically contended for the olfactory function of the antennæ. Both through physiological experiments and detailed anatomical investigations Hauser sought to prove his hypothesis, as Pierrot, Erichson, Slater, and others had done before him, besides working from an evolutional point of view. In a purely anatomical aspect, especially prominent are his discovery of the singularly formed nerve-rods in the pits and peg-like teeth of the Hymenoptera and their development, as well as the assertion that numerous hairs in the pits described by Leydig, Meyer, etc., should be considered as direct terminations of nervous fibres passing into the pits. In the pits he farther, with Erichson, notices a serous fluid, which may serve as a medium for the perception of smells. Among the latest articles on this subject are those of Künckel and Gazagnaire, which are entirely anatomical, while the latest treatise of Graber on the organs of hearing in insects opposes Hicks’s theory of the olfactory function of the nerve-end apparatus in the halteres, wings, etc., and argues for the auditory nature of these structures. Finally, according to Voges, the sense of smell is not localized, but spread over the whole body.

My own observations on different groups of insects agree, in general, with those of Perris, Forel, and Hauser, without being in a position to confirm or deny the varying relations of the Hemiptera. That irritating odorous substances (chloroform, acetic acid) cause the limbs to move in sympathy with the stimulus, I have seen several times in Acanthosoma; still it may be a gustatory rather than olfactory stimulus.

Turning now from speculation and simple observation to exact anatomical and histological data, the nerve-end apparatus seems to have a distinct reference to 268the perception of odors. It comprises a structure composed of nervous substances which are enclosed in a chitinous tube, and either only stand in relation to the surrounding bodies by the perforated point, or pass to the surface as free nerve-fibrillæ.

In insects there is a remarkable and fundamental difference in the structures of the parts supposed to be the organs of smell. Erichson was acquainted only with the “pori” covered by a thin membrane; but Burmeister, in his careful work on the antennæ of the lamellicorns, distinguished pits at the bottom of which hairs rise from a cup-like tubercle, from those which were free from hairs. Leydig afterwards was the first to regard as olfactory organs the so-called pegs (kegel), a short, thick, hair-like structure distinctly perforated at the tip, which had already, by Lespès in Cercopis, etc., been described as a kind of tactile papilla. Other very peculiar olfactory organs of different form, Forel (Fourmis de la Suisse) discovered in the antennæ of ants, which Lubbock incorrectly associated with the nerve-end apparatus found by Hicks in other insects.

As the final result of his researches Kraepelin states that the great variety of antennal structures previously described may be referred to a single common fundamental type of a more or less developed free or sunken hair-like body which stands in connection by means of a wide pore-canal with a many-nucleated ganglion-cell. The latter sends only a relatively slender nerve-fibre (axial cord) through the pore-canal into the hair; but the same is enclosed by epithelial cells which surround the pore-canal.

Hauser’s researches on the organs of smell in insects were so carefully made and conclusive that our readers will, we feel sure, be glad to have laid before them in detail the facts which prove so satisfactorily that the antennæ of most insects are olfactory rather than auditory in their functions.

Physiological experiments.—First of all one should observe as exactly as possible the normal animal in its relation to certain odorous substances, whose fumes possess no corrosive power or peculiarities interfering with respiration; then remove the antennæ and try after several days to ascertain what changes have taken place in the relation of the animal to the substance. In order to come to no false results it is often necessary to let the insects operated upon rest one or two days, for immediately after the operation they are generally so restless that a careful experiment is impossible.

The extirpation of the antennæ is borne by different insects in different ways; many bear it very easily, and can live for months after the operation, while others die in the course of a few days after the loss of these appendages. The animals seem to be least injured if the operation is performed at a time when they are hibernating. Pyrrhocoris apterus, and many other insects, afforded a very striking proof of this relation.

269Experiments made by placing the antennæ in liquid paraffine so as to cover them with a layer of paraffine, thus excluding the air, gave the same result as if the antennæ had been removed.

The experiments may be divided, according to their object, into three groups. Experiments of the first kind were made on insects in their relation to strong-smelling substances, as turpentine, carbolic acid, etc., before and after extirpation of the antennæ. The second group embraces experiments on the relation of animals as regards their search for food; and finally the third group embraces experiments on the relation of the sexes relative to reproduction before and after the extirpation of the antennæ.

Relation of insects to smelling substances before and after the loss of their antennæ.—Taking a glass rod dipped in carbolic acid and holding it within 10 cm. of Philonthus œneus, found under stones at the end of February, it was seen to raise its head, turn it in different directions, and to make lively movements with its antennæ. But scarcely had Hauser placed the rod close to it when it started back as if frightened, made a sudden turn, and rushed, extremely disturbed, in the opposite direction. When he removed the glass rod, the creature busied itself for some time with its antennæ, while it drew them, with the aid of its fore limbs, through its mouth, although they had not come into direct contact with the carbolic acid. There was the same reaction against oil of turpentine, and it was still more violent against acetic acid.

After having many times carefully tested the relations of the normal animal to the substances mentioned, the antennæ were removed from the socket-cavity.

On the second day after Hauser experimented with the insects, they exhibited no reaction either against the carbolic acid, the oil of turpentine, or even against the acetic acid, although he held the glass rod which had been dipped into it for one or two minutes before and over the head. The creatures remained completely quiet and immovable, at the most slightly moving the palpi. They showed otherwise no change in their mode of life and their demeanor; they ate with great eagerness flesh which had been placed before them, or dead insects, and some were as active as usual as late as May. These beetles had, as proved by the experiments, lost the sense of smell alone; how far the sense of touch was lost Hauser could not experimentally decide.

The same results followed experiments with species of the genus Ptinus, Tenebrio, Ichneumon, Formica, Vespa, Tenthredo, Saturnia, Vanessa, and Smerinthus; also many species of Diptera and Orthoptera, 270besides Julus and Lithobius, while many larvæ reacted in the same manner.

Less satisfactory were the experiments with Carabus, Melolontha, and Silpha; there is no doubt that the species of these genera, through the extirpation of their antennæ, become more or less injured as to the acuteness of their powers of smelling; but they never show themselves wholly unable to perceive strong-smelling substances.

The allurement of the substance acts for a longer time on those deprived of their antennæ, then they become restless, then they wander away from the glass tube held before them; still all their movements are but slightly energetic, and the entire reaction is indeterminate and enfeebled.

Experiments with the Hemiptera gave still more unfavorable results; after the loss of their antennæ they reacted to smells as eagerly as those did which were uninjured.

Experiments on the use of the antennæ in seeking for food.—Under this head experiments were made with Silpha, Sarcophaga, Calliphora, and Cynomyia.

Silpha and its larva were treated in the following manner: they were placed in large boxes whose bottoms were covered with moss, etc.; in a corner of the box was placed a bottle with a small opening, in which was placed strong-smelling meat. So long as the beetles were in possession of their antennæ they invariably after a while discovered the meat exposed in the bottle, while after the loss of their antennæ they did not come in contact with it.

In a similar way acted the species of Sarcophaga, Calliphora, and Cynomyia. Hauser, in experimenting with these, placed a dish with a large piece of decayed flesh on his writing-table. In a short time specimens of the flies referred to entered through the open window of the room. The oftener he drove them away from the meat would they swarm thickly upon it. Then closing the window and catching all the flies, he deprived them of their antennæ and again set them free. They flew about the room, but none settled upon the flesh nor tried to approach it. Where a fly had alighted on a curtain or other object, the decayed flesh was placed under it so that the full force of the effluvium should pass over it, but even then no fly would settle upon it.

Experiments testing the influence of the antennæ of the males in seeking the females.—For this purpose Hauser chose those kinds in which the male antennæ differ in secondary sexual characters from those of the female, and in which it is known that they readily couple in confinement, as Saturnia pavonia, Ocneria dispar, and Melolontha vulgaris. The two first-named insects did not couple after the extirpation of their antennæ. Of Melolontha vulgaris 271twenty pairs were placed in a moderately sized box. On the next morning twelve pairs of them were found coupling. Hauser then, after removing the first lot, placed a new set of thirty pairs in the same box, cut off all the antennæ of the males and those of a number of females. On the following morning only four pairs were found coupling, and at the end of three days five others were observed sexually united.

From these experiments Hauser inferred that those insects deprived of their antennæ were placed in the most favorable situation, such as they would not find in freedom; for the space in which the insects moved about was so limited that the males and females must of necessity meet. But at the same time the results of the experiments cannot absolutely be regarded as proving that the males, after the loss of their antennæ, were then not in condition to find the females, because in the case of the above-mentioned moths, under similar conditions, after the extirpation of the antennæ no sexual union took place. If, however, the experiments made do not all lead to the results desired, Hauser thinks that the results agree with those of his histological researches, that in the greater number of insects the sense of smell has its seat in the antennæ. His results also agree with those of Perris.

Structure of the organs of smell in insects.—The olfactory organs consist, in insects,—i.e., all Orthoptera, Termitidæ, Psocidæ, Diptera, and Hymenoptera, also in most Lepidoptera, Neuroptera, and Coleoptera,—

1. Of a thick nerve arising from the brain, which passes into the antennæ.

2. Of a sensitive apparatus at the end, which consists of staff-like cells, which are modified hypodermis cells, with which the fibres of the nerves connect.

3. Of a supporting and accessory apparatus, consisting of pits, or peg- or tooth-like projections filled with a serous fluid, and which may be regarded as invaginations and outgrowths of the epidermis.

Hauser adds a remark on the distribution of the pits and teeth in the larvæ of insects, saying that his observations are incomplete, but that it appears that in the larvæ the teeth are most generally distributed, and that they occur not on the antennæ alone, but on the palpi; but in very many larvæ neither pits nor teeth[47] occurred. In the Myriopoda teeth-like projections occur on the ends of the antennæ. In Lithobius they form very small, almost cylindrical, pale organs.

272

Fig. 268.—Olfactory organ of Caloptenus.

Fig. 269.—Olfactory pits of the antenna of Stenobothrus. This and Fig. 268 after Hauser.

Lettering for Figs. 268, 269, 273, 275, 276, 278–281.a, a, circular thickening of the skin surrounding the opening of the olfactory pit; ax, thread-like continuation of the nerve-cell; b, vesicle-like bottom of the olfactory pit, through which the olfactory style passes; br bristle in Fig. 283, stout, and protecting the olfactory pit; bs, bent bristle or seta; ch, chitinous integument of the antennæ; d, seen in section; f, invaginated pit; Fv, Forel’s flask-shaped organ; Fvo, its opening seen from the surface; gl, gland-like mass of cells; hyc, hypodermic cells; i, entrance into the canal belonging to the pit; m, olfactory membrane; m′, m″, mc, membrane-forming cell; n, nerve of special sense; nc, nucleus of the sense- or ganglion-cell; o, opening into the olfactory pit; p, olfactory pit; cp, compound pits; pw, wall of the pit; s, a large seta; sc, sense- or ganglion-cell; st, olfactory or sense-style, sometimes peg-shaped; tb, tactile bristle.

Fig. 270.A, b, sense-organ on the abdominal appendages of a fly (Chrysopila); c, sense-organ on the terminal joint of palpus of Perla.

Fig. 271.—Longitudinal section of part of cercus of Acheta domestica: ch, cuticula; hyp, hypodermis; n. nerve; h′1, integumental hairs, not sensory; h2, ordinary hair; h3, sensory hair; h4, bladder-like hair; sz, sense-cell.—After Vom Rath, from Sharp.

In the course of a special description of these sense-organs in the Orthoptera, Hauser describes at length those of Œdipoda cœrulescens and Caloptenus italicus. On one antennal joint of Caloptenus (Fig. 268) was often counted 50 pits; on the anterior joints the number diminishes to about 30. Hauser thinks that in all Orthoptera whose antennæ are like those of Caloptenus occur similar pits, as he found them in Stenobothrus (Fig. 269) as well as in Œdipoda. Gryllotalpa possesses similar pits,—four to six on each antennal joint, making between 300 and 400 pits on each antenna.[48] In Mantis 273religiosa the pits were not detected, but on each joint, except the eighth basal, there are about 200 small, hollow, curved teeth with a fine opening in front.

In the Neuroptera (Chrysopa) there occur on the antennæ, besides numerous very long tactile bristles, small pale, transparent teeth. No pits could be detected.

In the Hemiptera (two species of Pyrrhocoris only were examined) only two kinds of tactile bristles occurred, but Hauser detected no pits, though Lespès states that they are present.

Fig. 272.—Longitudinal section of apex of palpus of Pieris brassicæ: sch, scales; ch, cuticula; hyp, hypodermis; n, nerve; sz, sense-cells; sh, sense-hairs.—After Vom Rath, from Sharp.

Of the Diptera, Hauser examined more than 60 species. The pits in the Diptera brachycera (Muscidæ, etc.) are unexceptionally confined to the third antennal joint. Their number varies extraordinarily in the different species. Helophilus florens has on each antennal disk only a single pit, while Echinomyia grossa possesses 200 of them. In flies of certain families the pits are compound, and contain 10, 20, and often 100 olfactory hairs, partly arising from the coalescence of several pits. Such pits are usually divided by lateral walls into several chambers, whose connection is only indicated by their common outlet. Simple olfactory pits with a single olfactory style were observed only in the Tabanidæ, Asilidæ, Bombylidæ, Leptidæ, Dolichopidæ, Stratiomyidæ, and Tipulidæ. In the last the compound forms do not occur at all, but in the other families mentioned also occur compound pits, receiving from two to ten nerve-terminations.

The antennal pits of flies are always sac-like invaginations of the external chitinous integument, of manifold shapes, opening externally and never closed by a membrane. The pits differ but slightly in the different species, and that of Cyrtoneura stabulans (Fig. 273) is described at length as typical of those of brachycerous flies in general.

The olfactory pits of the Tipulidæ seem to have a somewhat different structure, since the external passage is closed. It is circular, surrounded with a slight chitinous wall, and not covered with bristles. Such pits in their external appearance are like 274those of the locust (Caloptenus) and many Hymenoptera. They are situated usually on the third antennal joint. Pachyrhina pratensis L. has about 60 of them, as have Tipula oleracea L. and Ctenophora.

In the Lepidoptera, olfactory pits are much like those of flies. Hauser describes in detail those of Vanessa io. Those of the moths were not examined, but they can be readily and satisfactorily proved to be the site of the olfactory sense.

Fig. 273.—Longitudinal section through the third antennal joint of a fly (Cyrtoneura stabulans), showing the compound pits from above and in section.—After Hauser.

Fig. 274.—Antenna of Adelops, showing the olfactory organs (p) in the five last joints.

Historical researches in respect to the Coleoptera generally gave a very unfavorable result, contrary to Lespès’s views. That author states that in the Carabidæ the pits are found on the four first joints, but Hauser could discover them in none which he examined. Usually only tactile bristles occur, so also in the Cerambycidæ, Curculionidæ, Chrysomelidæ, and Cantharidæ. In a blind silphid beetle 275(Adelops hirtus) of Mammoth Cave we have found well-marked olfactory organs (Fig. 274). Similar organs occur in the antennæ of the Panorpidæ.

Olfactory pits, however, without doubt occur in Silpha, Necrophorus, Staphylinus, Philonthus, and Tenebrio. The openings of the pits are small and surrounded with a small chitinous ring; in Silpha, Necrophorus, and Tenebrio they cannot easily be distinguished from the insertion-cavities of the bristles, but in Philonthus and Staphylinus they are less like them, being distinguished by their somewhat larger size and their often more oval form. In Philonthus æneus about 100 such small pits occur irregularly on the terminal joints; besides, in this species on each side of the terminal joint is an apparatus which is like the compound pit generally occurring in the Diptera.

Fig. 275.—Olfactory pits of the antenna of Melolontha vulgaris.—After Kraepelin.

Fig. 276.—Antennal pit of Melolontha vulgaris, seen in vertical section.—After Hauser.

Very remarkable pits occur in the antennal lamellæ of Melolontha vulgaris (Fig. 275) and other lamellicorns. On the outer surface of the first and seventh (in the female the sixth) antennal leaf, as also on the edges of the other leaves, only arise scattered bristles; on the inner surface of the first and seventh leaves, as also on both surfaces of the second to sixth leaves, are close rows of rather shallow depressions of irregular form, some circular, others regularly hexagonal. Their number is enormous: in the males 39,000, in the females about 35,000, occur on each antenna.

276

Fig. 277.—Organ of smell of Anophthalmus.—After Hauser. A, a, b, the same in A. tenuis, B in A. tellkampfii.

Fig. 278.—Section through antennal joint of Vespa crabro, showing the great number of olfactory pits, olfactory and tactile bristles. A, section through an olfactory pit of Vespa crabro.—After Hauser.

The antennal pits and teeth of Dyticus marginalis are morphologically and physiologically identical with those of bees and wasps. In Anophthalmus bilimekii, Hauser found on the last antennal joints about 60 teeth, which essentially differ in form from those previously described; they are very pale, transparent, cylindrical, elongated, and bent elbow-shaped on the first third, so that the last two-thirds run parallel with the antenna. The length of these remarkable teeth is 0.035 mm., their breadth 0.005 mm. He only found them in Anophthalmus, and in no other species of Carabidæ; they must resemble the teeth described in Chrysopa. Our species possesses similar processes (Fig. 277). Similar teeth occur on the maxillary and labial palpi of beetles. Dyticus marginalis possesses at the end of each terminal palpal joint a group of very small teeth, which were also detected in Anophthalmus bilimekii, Melolontha vulgaris, etc. In 277Carabus violascens were detected on the maxillary palpi large, plainly microscopical, white disks, which are surrounded with a great number of extremely small teeth.

Whether the above-described organs on the palpi of beetles should be considered as olfactory or gustatory in their nature can only be determined by means of physiological experiments; they probably receive taste-nerve terminations.

Fig. 279.—Olfactory pits of the antenna of Vespa vulgaris.—After Kraepelin.

The Hymenoptera furnished very good material for histological purposes, so that Hauser could not only study the terminal apparatus of the olfactory nerves in the perfect insect, but also in three different stages of the pupa. These are described at length, as regards the distribution of the pits and teeth, in Vespa crabro; each joint of the antenna (flagellum) possesses between 1300 and 1400 pits, nearly 60 teeth, and about 70 tactile hairs; on the terminal joint there are more than 200 teeth, so that each antenna has between 13,000 and 14,000 olfactory pits and about 700 teeth (Kegeln). Fig. 278 represents a cross-section through the penultimate antennal joint of Vespa crabro; we can see how thick are the series of openings on the surface of the antennæ, and how regular is the distribution of the teeth.

The distribution of the olfactory pits and olfactory teeth is thus seen to be very general; the deviations are so insignificant that there is no reason for the establishment of more than one type.

Antennal pits with a small crevice-like opening occur in genera nearly allied to Vespa and also in most Ichneumonidæ, Braconidæ, and Cynipidæ. But the crevice-like openings in these families are considerably longer and often are of a somewhat twisted shape. In all the species with translucent antennæ we can recognize the inner mouth of the pit as a round or nearly round disk situated usually under the middle of the opening. The antennal pits of Apis mellifica, as well as those of Bombus (Fig. 280) and allied genera, differ 278from those of the Ichneumonidæ in being not like crevices, but circular openings.

Fig. 280.—Olfactory pits of the antenna of Bombus.—After Kraepelin.

Fig. 281.—Olfactory pits of the antenna of Formica: Fv, Hicks’ “bottle,” Forel’s flask-shaped organ, Fvo, its opening.—After Kraepelin.

Fig. 282.—Supposed olfactory organs at end of antenna of Campodea: A, C. staphylinus. B, C. cookei, from Mammoth Cave.

Fig. 283.—Vertical section through a single olfactory pit in the antenna of the horse-fly (Tabanus bovinus). For lettering see p. 272.—After Hauser.

The distribution of the olfactory peg or tooth-like projections seems to be much more limited than that of the pits in the Ichneumonidæ. Hauser could not find any. Apis mellifica possesses on each antennal joint only about twenty slender pale teeth, scarcely a third as many as in Vespa crabro; on the other hand, Formica, of which genus several species were examined, seems to have far more teeth than pits; they are relatively long, pale, transparent, and somewhat clavate; they are not unlike those of Chrysopa; on the terminal joint only occur the round openings (Fvo), which lead into a bottle-shaped invagination of the integument (Fv) and contain an olfactory style (Fig. 281). In the Tenthredinidæ only teeth and no pits were to be detected. Sirex has on the under side of the nine last joints of each antenna a group of from 200 to 300 small teeth, which resemble those of Vespa crabro; Lyda has on the terminal joints about 100 279teeth. We may add that supposed organs of smell occur on the antennæ of Campodea (Fig. 282).

Kraepelin also thus briefly summarizes Hauser’s statements as to the forms of the different organs of smell.

The manifold nature of the antennal organs has, by Hauser, from thorough studies of the nerve-elements belonging to them, been not simplified but rendered more complicated. According to this naturalist we may distinguish the following forms which the olfactory organs may assume: 1. “Pale, tooth-like chitinous hairs on the outer surface of the antennæ, which are perforated at the end; nothing is known as to the relation of the nerve passing into it (Chrysopa, Anophthalmus). 2. In pit-like depressions of the antennæ arise nerve-rods (without a chitinous case) which stand in direct relation with a ganglion-cell lying under it. These pits are either simple, viz. with only an ‘olfactory rod’ (Tabanus, Fig. 283, and other Diptera, Vanessa), or compound (Muscidæ, and most other Diptera, and Philonthus). It is important that these pits are partly open (in the above-named groups of insects), and partly closed and covered with a thin membrane, under whose concavity the olfactory rods end (Orthoptera, Melolontha, and other lamellicorns). 3. Short, thick pits sunken slightly into the surface of the antennæ, and over this a chitinous peg perforated at the end, in whose base, from the interior, projects a very singular nerve-peg, which is situated over an olfactory ganglion-cell, and provided with a slender crown of little rods, and flanked on each side by a flagellum-cell (Hymenoptera). 4. Round or crevice-like pits covered over by a perforated chitinous membrane with nerve-rods like those in 3, but in place of the flagellum-cell with ‘membrane-forming’ cells spread before it. Hauser finally mentions further differences in the ganglion-cells sent out into the nerve-end apparatus. These exhibit in Diptera and Melolontha only one nucleus, in Hymenoptera a single very large one (with many nucleoli) and three small ones, in Vanessa six, in Orthoptera a very large number of nuclei, etc.”

LITERATURE OF THE ORGANS OF SMELL

Réaumur, Réné Ant. de. Mémoires pour servir à l’histoire des insectes. Paris, 1734–42. (i, 283; ii, 224).

Lesser, F. C. Insecto-theologia, 1740, p. 262.

Roesel, A. J. Insektenbelustigungen, 1746, ii, p. 51.

Reimarus, H. S. Allgemeine Betrachtung ueber die Triebe der Thiere hauptsächlich ihre Kunsttriebe (Instinct). Hamburg, 1760, p. 355.

Sulzer, J. H. Die Kenntzeichen der Insecten. Zürich, 1761.

Lyonet, P. Traité anatomique de la chenille, 1762, pp. 42, 96, 195.

Comparetti, A. De aure interna comparata. Patavii, 1769.

Bonnet, C. Œuvres complètes, 1779–1783, ii, p. 36.

—— Contemplation de la nature, Ch. iii, p. 18.

Scarpa, Ant. Anatomicæ disquisitiones de auditu et olfactu. Ticini, 1789.

Huber, F. Nouvelles observations sur les abeilles, 1792, ii, p. 475.

Lehrmann, M. C. G. De antennis insectorum. Londini, Hamburgi, 1799, p. 48, Diss. posterior; Hamburg and London, 1800, p. 80 (especial sense, ærocepsis).

Latreille, P. A. Histoire naturelle des crustacés et des insectes, 1806–1809, ii, 50.

Blainville, M. H. D. Principes d’anatomie comparée, 1822, i, p. 339.

Dugès, A. L. Traité de physiologie comparée, 1838, i, p. 161.

280Newport, G. On the use of the antennæ of insects. (Trans. Ent. Soc., London, ii, 1840, pp. 229–248.)

Robineau-Desvoidy, A. J. B. Sur l’usage réel des antennes chez les insectes. (Ann. Soc. Ent. France, 1842, xi Bull., pp. 23–27.)

Erichson, W. F. De fabrica et usu antennarum in insectis. Berlin, 1847, 1 Tab., p. 13.

Perris, E. Mémoire sur le siège de l’odorat dans les articulés. (Ann. sc. nat., Sér. 3, 1850, xiv, pp. 159–178.)

Dufour, L. Quelques mots sur l’organe de l’odorat et sur celui de l’ouie dans les insectes. (Actes d. l. Soc. Linn., Bordeaux, 1850, xvii, Ann. sc. nat., Sér. 3, Zool., xiv, 1850, pp. 179–184.)

Leydig, F. Zum feineren Bau der Arthropoden. (Müller’s Archiv, 1855, pp. 376–480); Lehrbuch der Histologie, 1857, p. 220; Zur Anatomie der Insekten (Archiv für Anatomie, 1859, pp. 35–89 and 149–183).

—— Ueber Geruchs- und Gehörorgane der Krebse und Insekten. (Archiv f. Anat. u. Phys., 1860.)

—— Die Hautsinnesorgane der Arthropoden. (Zool. Anzeiger, 1886, pp. 284–291, 308–314, 265–314.)

Landois, H. Das Gehörorgan des Hirschkäfers. (Archiv f. mikrosp. Anat., 1868, iv, pp. 88–95.)

Troschel, H. Ueber das Geruchsorgan der Gliedertiere. (Verhandl. d. naturhist. Vereins d. preuss. Rheinlande u. Westfal., xxvii Jahrg., 1870, pp. 160–161.)

Packard, A. S. The caudal styles of insects sense-organs, i.e., abdominal antennæ. (Amer. Naturalist, 1870, pp. 620, 621. Also Proc. Bost. Soc. Nat. Hist., 1868, xi, p. 398.)

Paasch, A. Von den Sinnesorganen der Insekten im Allgemeinen, von Gehörund Geruchsorganen im Besondern. (Archiv für Naturgesch., xxxix Jahrg., i, 1873, pp. 248–275.)

Chadima, Jos. Ueber die von Leydig als Geruchsorgane bezeichneten Bildungen bei den Arthropoden. (Mitteil. d. naturwiss. Ver. f. Steiermark, 1873, pp. 36–44.)

Forel, A. Les Fourmis de la Suisse. (Neue Denkschr. Allg. Schweiz. Gesellsch. f. d. ges. Naturw., xxvi, 1874, pp. 118, 144.)

—— Études myrmécologiques en 1884, avec une description des organes sensoriels des antennes. (Bull. Soc. Vaud. sc. nat., 1885, Sér. 2, xx, pp. 316–380.)

Graber, V. Die Insekten. München, 1877.

—— Neue Versuche über die Functionen der Insektenfühler. (Biol. Centralb., vii, 1887, pp. 13–19.)

Trouvelot, L. The use of the antennæ in insects. (Amer. Naturalist, xi, 1877, pp. 193–196.)

Mayer, P. Sopra certi organi di senso nelle antenne dei Ditteri. (Atti R. Accad. d. Lincei, Roma, Ser. 3, Mem. Cl. sc. fis., mat. e natur., iii, 1879, p. 11.)

Hauser, Gustav. Physiologische und histiologische Untersuchungen über das Geruchsorgan der Insekten. (Zeitschr. f. wissens. Zool., xxxiv, 1880, pp. 367–403, 3 Taf.)

Kraepelin, Karl. Ueber die Geruchsorgane der Gliedertiere (Oster-Programm der Realschule des Johanneums. Hamburg, 1883, pp. 48, 3 Taf.)

Schiemenz, Paulus. Ueber das Herkommen des Futtersaftes und die Speicheldrüsen der Bienen nebst einem Anhange über das Riechorgan. (Zeitschr. f. wissens. Zool., xxxviii, pp. 71–135, 3 Taf.)

Plateau, F. Expériences sur le rôle des palpes chez les arthropodes maxillés, I., Palpes des insectes broyeurs. (Bull. Soc. Zool. France, x, 1885, pp. 67–90.)

281Plateau, F. Une expérience sur la fonction des antennes chez la Blatte (Periplaneta orientalis). (Comptes rend. Soc. Ent. de Belgique, 1886, pp. 118–122, 1 Fig.)

Lubbock, J. On some points in the anatomy of ants. (Monthly Micr. Journ., 1887, pp. 121–142.)

Ruland, Franz. Beiträge zur Kenntniss der antennalen Sinnesorgane der Insekten. Diss. Marburg, 1888, p. 31, 1 Taf.

Wasmann, E. Die Fühler der Insekten. (Stimmen aus Maria-Laach. Freiburg i. B., 1891, p. 37.)

Sergi, G. Ricerche su alcuni organi di senso nelle antenne delle formiche. (Riv. Filos. Sci. Milano, 1891, p. 10, 3 Figs.)

Nagel, Wilibald. Die niederen Sinne der Insekten, 19 Figs., Tübingen, 1892, pp. 67.

With the writings of Baster, Lamarck, Cuvier, Treviranus, Oken, Lefebure, Duméril, Schelver, Bousdorf, Rosenthal, Burmeister, Slater, Balbiani, Marcel de Serres, Garnier, Berté, Porter, Sazepin, Reuter, Pierret, Duponchel, Driesch, Küster, Peckham, Lubbock, A. Dohrn, Lespès.

c. The organs of taste

The gustatory organs of insects are microscopic pits or setae, either hair-like or resembling short pegs, which form the ends of ganglionated nerves. They are difficult to distinguish morphologically from certain olfactory structures, and it is owing to their position at or very near the mouth that they are supposed to be gustatory in nature.

Meinert was the first (1860) to suggest that organs of taste occurred in ants. He observed in the maxillæ, and tongue of these insects a series of canals in the cuticula of these organs connected with ganglion-cells, and through them with the nerves, and queried whether they were not organs of taste. Forel afterwards (1874) confirmed these observations. Wolff in an elaborate work (1875) described a group of minute pits (Fig. 284) at the base of the tongue of the honey bee, which he supposed to possess the sense of smell, but Forel and also Lubbock attributed to these sensory pits the function of taste. Ten years afterward Will showed conclusively, both by anatomical studies and by experiments, that Diptera and Hymenoptera possess gustatory organs. He, however, denied that the organs of Wolff were gustatory, and maintained that organs of smell were confined to the maxillæ, paraglossæ, and tongue. As we shall see, however, what appear to be with little doubt taste-pits, with hairs or pegs arising from them, are most numerous on the epipharynx of nearly all insects, and situated at a point where they 282necessarily must come in contact with the food as it enters the mouth and passes down the throat.

Fig. 284.—Taste-pits on the epipharynx (C) of the honey-bee: B, horny ridge; R, R, taste-pits; L, A, A, muscular fibres; S, S′, a b c d e f, section of skin of œsophagus.—After Wolff.

Fig. 285.—Tip of the proboscis of the honey bee, × 140: L, terminal button or ladle: Gs taste-hairs; Sh, guard-hairs; Hb, hooked hairs.—After Will.

Kraepelin (1883) discovered taste-organs on the proboscis of the fly, and taste-hairs at the end of the tongue of the humble-bee (Fig. 285), and afterwards Lubbock critically discussed the subject, and concluded that the organs of taste in insects are situated “either in the mouth itself, or on the organs immediately surrounding it.”

Structure of the taste-organs.—The organs have been best studied by Will, who, besides describing and figuring the chitinous structures, such as the pits or cups, hairs and the pegs, showed that they were the terminations of ganglionated nerves.

Figure 286 represents the taste-cups on the maxilla of a wasp, and Fig. 287 the taste-cone or peg projecting from the cup or pit. The cell out of which the pit and projecting hair or peg are formed is a modified hypodermis cell; and the seta is apparently a modification of a tactile hair, situated at the end of a nerve, which just beneath the chitinous structures passes into a ganglion-cell, which sends off a nerve-fibre to the main nerve.

Will detected on the tongue of the yellow ant (Lasius flavus) from 20 to 24, and in Atta from 40 to 52, of these structures. The number of pits on the maxillæ vary much, not always being the same on the two sides of the same insect. We have observed these taste-cups in the honey and humble bee, not only at the base of the second maxillæ (Fig. 288, g), but also on the paraglossæ (pg).

Distribution in other orders of insects.—The writer has detected these taste-cups in other orders than Diptera and Hymenoptera. They very generally occur in mandibulate insects on the more exposed surface of the epipharynx 283(compare pp. 43–46). We have not observed them in the Synaptera (Lepisma and Machilis).

In the Dermaptera the taste-cups appear to be undeveloped in the nymph, while in the adult they are fewer in number than in any other pterygote order yet investigated.

In a species of Forficula from Cordova, Mexico, the taste-pits are few in number, there being only about a dozen on each side in all; most of them being situated on the anterior half, and a few near the base. The taste-pits are provided each with a short fine seta, as usual arising from the centre.

Fig. 286.—Under side of left maxilla of Vespa: Gm, taste-cups; Shm, protecting hairs; Tb, tactile hairs; Mt, base of maxillary palpus.—After Will.

In the order Platyptera (including Perla, Pteronarcys, Psocus, Termes, Eutermes, and Termopsis) we have been unable to detect any organs of taste.

Fig. 287.—Section through a taste-cup: SK, supporting cone; N, nerve; SZ, sense-cell.—After Will. This and Figs. 284–286 from Lubbock.

Fig. 288.—Tongue of worker honey-bee: pg, paraglossæ; B, the same enlarged, showing the taste-papillæ; C, D, base of a labial palpus (mx.′p) with the taste-papillæ; E, taste-cups on paraglossæ of Bombus; F, group of same on left, G, on right side, at base of labial palpus.

In the Odonata, however, they are fairly well developed; in Calopteryx, about 50 taste-cups were discovered; in a species of Diplax about 28, there being a group of 14 at the base of the epipharynx on each side of the median line, while in Æschna heros there are two groups of from 25 to 30 taste-cups, situated as in the two aforenamed genera.

In the Orthoptera the gustatory cups are numerous, well developed, and present in all the families except the Phasmidæ, where, however, they may yet be found to occur.

284In a large cockroach (Blabera) from Cuba they are well developed. On each side of the middle of the epipharynx is a curved row of stiff, defensive spines, and at the distal end of each row is a sensory field, containing 20 taste-cups on one side and 23 on the other. Near the front edge of the clypeal region are two more sensory fields, situated on each side of the median line, there being 35 taste-cups in each field. The taste-cups in this form are rather smaller than usual in the order.

In the Acrydiidæ they are more numerous than in the Blattidæ. For example, in Camnula pellucida, near what corresponds to the front edge of the clypeus are two gustatory fields, each bearing about 35 taste-pits. Just in front, under the clypeo-labral suture, are two similar fields, each containing from 40 to 42 taste-pits. There are none in front of these. There are thus about 140–150 sense-cups in all.

The members of the Locustidæ (Fig. 26) appear to be better provided with the organs of taste than any other Orthoptera, those of the katydid numbering from 170 to 180. There are from 50 to 60 taste-cups in the front region; behind the middle a group of 25 on each side, and over an area corresponding to the base of the labrum and front edge of the clypeus is a sensory field with about 70 taste-cups on each side. They are true cups or beaker-like papillæ, some with a fine, others with a short, stout, conical seta.

The gustatory organs in the cave cricket (Hadenœcus subterraneus, Fig. 27), from Mammoth Cave, are highly developed, being rounded papillæ with the nucleus at the top or end. They are grouped on each side of the middle near the front edge, there being 25 on each side. An irregular row of these beaker-like organs extends along each side; some occur under the base of the labrum, but they are most numerous in a field corresponding to the front edge of the clypeus, there being 50 on each side, or 100 in all, where in Ceuthophilus there are only 9 or 10. It would thus appear as if the sense of taste were much more acute in the cave-dweller than in the out-of-doors form.

In the Coleoptera taste-cups and setæ are very generally distributed, though we were unable to detect them in Dendroctonus or in Lucanus dama. As seen in Fig. 57, we have observed numerous taste-pegs along the maxilla of Nemognatha lurida, but otherwise taste-organs have only been detected in the epipharynx. They not only occur in the adult beetles, but we have found them in the larvæ of cerambycid, scarabæid, and other beetles. In the adults taste-cups appear to be about as well developed in the carnivorous forms (Carabidæ) as in the phytophagous or lignivorous groups.

In Chlænius tomentosus there are about half as many of these organs as in Harpalus, while in Calosoma there are 90 taste-cups, 45 on each side, under the base of the labrum. The cups are papilliform, being rather high, with a seta arising from each.

In the Cicindelidæ, the epipharynx bears a sensory field quite different from that of the Carabidæ. There are no normal taste-cups, except a few situated on two large, round, raised areas which are guarded in front by a few very long setæ. On the surface of each area are numerous very long setæ which may, if not tactile, have some other sense, as they arise from cup-like bases or cells. Those on the outside are like true taste-cups, with a bristle but little larger than normal in taste-cups generally. We are disposed to regard this sensory field as a highly specialized gustatory apparatus.

In the Dyticidæ the taste-cups are nearly as described in the Carabidæ.

The Staphylinidæ are not well provided with taste-organs. Under the clypeus of Staphylinus violaceus, on each side near the middle, is a bare rounded area, in which are situated 4 or 5 papilliform taste-cups, and at the base behind them 285is another linear group of about 7 slenderer, somewhat curved, taste-cups. In the Elateridæ these organs are scantily developed.

In the Buprestidæ (Buprestis maculiventris alone examined) no true taste-cups were detected. On the other hand, the Lampyridæ are well supplied with them. Under the clypeus is situated a sensory field bearing 26 taste-cups, which are rather smaller than usual. Over the epipharyngeal surface are scattered a few taste-cups, but they are small and perhaps not gustatory. Under the clypeus of Lucidota punctata Lec. is a group of 12 taste-cups, and in the middle region of the epipharynx, situated in a field extending from near the base to near the front edge, are about 40 taste-cups, which, however, are not, as is usual, arranged on each side of the median line, but are scattered among the hairs of the pilose surface of the epipharynx. In the Cleridæ the taste-cups are few in number.

In the great family of Scarabæidæ, the presence of gustatory organs is variable. None occur in Lucanus dama, though in the June beetle (Lachnosterna fusca Fröhl.) they are abundantly developed. The epipharynx bears on each side outside of a spiny area a group of about 50 taste-cups, each bearing a long seta, those on the outside of the area passing into a few high, rather slender, papillæ, without a seta. On the under side of the clypeus is a median group of 10 taste-cups of singular form, the cups being large, with broad bases, which posteriorly bear three spines, of which the median one is the largest.

Taste-cups occur without any known exception in the longicorn beetles. In Leptura canadensis they are numerous; in Euryptera lateralis they are abundant along and near the middle of the anterior half of the labral region, and in Cyllene robiniæ Forst. (or pictus Drury) they are more numerous than usual, extending in an unbroken sensory field from near the front margin of the clypeal region to near the front edge of the epipharynx. The cups vary much in size, some being one-half as large as others; and those on the sides of the sensory field bear short, and a few others rather long, bristles, showing that the taste-cups are modified tactile bristles.

The Tenebrionidæ are fairly well endowed with taste-cups, their number in Eleodes obsoleta Say amounting to 30 or 40.

Those of the Meloidæ especially are unusual in size and number.

Fig. 289.—Epipharynx (ep) of Nemognatha: cl, clypeus; gh, gathering hairs; tc, triangular sensory field dotted with taste cups; A, the field enlarged.

In Nemognatha lurida Lec. (Fig. 289) the front edge of the epipharynx contains about 80 remarkably small taste-cups, arranged irregularly in a triangular sensory space, and not more than ¼ to ⅙, as large as those on the maxillæ of the same beetle. Unless the former structures are gustatory it is difficult to account for their presence here, and it will be observed that the taste-cups in Epicauta are unusually abundant. Thus in the middle and near the front of the epipharynx of the blister-beetle over 100 gustatory cups were counted. They are conical, papilliform, and truncated at the end as if open, the edge of the opening being ragged, though bearing no bristle, except in a few cases. Around the edge of the sinus, on the under side of the labrum, is a regular marginal row of large, longer, more distinctly chitinized taste-cups, whose walls are streaked up and down by chitinous thickenings. In E. callosa Lec. there are about 55 taste-cups under 286the labrum, besides about 10 cells, which may be gustatory structures, situated on either side of a median setose ridge which passes back under the clypeal region.

The taste-cups of the leaf-beetles are fairly numerous, judging from an examination of Diabrotica vittata. The surface of the epipharynx is pilose, but the median region is naked, and on the anterior half bears from 11 to 12 taste-cups, arranged each side of the median line in a rude Y. On each side, at the base of the labial region, are two sensitive fields, each bearing about 25 to 26 taste-cups. More were seen under the clypeus.

In the Neuroptera unmistakable taste-cups are not always present. In Sialis infumata along the median line of the epipharynx and near the front are about 20 scattered gustatory pegs, which are minute, but longer and more acute than usual. In Chauliodes maculatus there are one or two taste-cups under the front edge of the clypeus; others are scattered along the middle from the base of the labrum to the front, but are not arranged in definite order. In Corydalis cornutus no sense-cups, pits, or rods are present. In Chrysopa there are scattered cups armed with a short acute bristle, which are possibly gustatory in function. In Myrmeleon diversum also the presence of sense-pits or of taste-cups is doubtful, though a group of about 12 pits on each side of the clypeal region of the epipharynx, and a few situated at the base of the labral region, may be endowed with the sense of taste. In Mantispa brunnea, however, along the middle of the epipharynx are scattered about 30 unmistakable taste-cups, each bearing a short, fine hair.

In the Mecoptera (Panorpa debilis?) taste-cups, giving rise to a minute hair, occur on the labium in two regions, and also on the maxillæ situated on the stipes near the base of the palpi, and on the lacinia and galea. They are also to be found on the maxillæ of Boreus californicus, but were not detected on the labium.

They were first detected by Reuter in various microlepidoptera, and occur on the “basal spot” of the palpi of many butterflies. In a Tineid moth (Coleophora coruscipennella) we have detected what we suppose to be a group of four taste-pits on the inner side of the basal joint of the labial palpi.

Experimental proof.—No one, says Lubbock, who has ever watched a bee or wasp can entertain the slightest doubt as to their possession of the sense of taste. “Forel mixed morphine and strychnine with some honey, which he offered to his ants. Their antennæ gave them no warning. The smell of the honey attracted them, and they began to feed; but the moment the honey touched their lips they perceived the fraud.”

Will at first fed wasps with sugar, so that they frequently visited it; afterwards he substituted alum for the sugar. Eagerly flying to it, they had scarcely touched it when they drew back from the distasteful substance with the most comical gestures, and cleaned their tongues by frequently running them in and out, repeatedly stroking them with their fore feet. He noticed a great repugnance to quinine in nearly all the insects experimented on. Bees and wasps were observed to have a more delicate gustatory sense than flies, etc., which are more omnivorous in their tastes.

287

LITERATURE ON THE ORGANS OF TASTE

Meinert, F. Bidrag til de danske myrers naturhistorie. (Kgl. Dansk Vidensk. selsk. skrifter. Kjoebenhavn. Raekke 5 Naturvid. og math. Afd., v, 1861, pp. 273–340.)

Wolff, O. J. B. Das Riechorgan der Biene. (Nova acta d. K. Leop.-Carol. Akad., xxxviii, 1875, pp. 1–251, 8 Taf.)

Joseph, G. Zur Morphologie des Geschmacksorganes bei Insekten. (Amtlicher Bericht der 50. Versammlung deutscher Naturforscher und Aerzte in München, 1877, pp. 227–228.)

Künckel et Gazagnaire. Du siège de la gustation chez les insectes Diptères. Constitution anatomique et physiologique de l’epipharynx et l’hypopharynx. (Comptes-rend. Acad. Sc., Paris, 1881, xcv, pp. 347–350.)

—— Recherches sur l’organisation et le développement des Diptères et en particulaire des Volucelles, i, 1875, ii, 1881, 26 Pls.

Kraepelin, K. Zur Kenntniss der Anatomie und Physiologie des Rüssels von Musca. (Zeitschr. f. wissens. Zool., xxxix, 1883, pp. 683–719, 2 Taf.)

Kirbach, P. Ueber die Mundwerkzeuge der Schmetterlinge. (Zool. Anzeiger, 1883, pp. 553–558, 2 Figs.)

Will, F. Das Geschmacksorgan der Insekten. (Zeitschr. f. wissens. Zool., 1885, xlii, pp. 674–707, 1 Taf.)

Gazagnaire, J. Du siège de la gustation chez les insectes Coléoptères. (Comptes-rend. Acad. Sc., Paris, 1886, cii, pp. 629–632; Ann. Soc. Ent. France, Sér. 6, Bull., pp. 79–80.)

Forel, A. Expériences et remarques critiques sur les sensations des insectes. 2me Part. (Recueil Zool. Suisse, iv, 1887, pp. 161–240.)

Reuter, Enzio. Ueber den “Basalfleck” auf den Palpen der Schmetterlinge. (Zool. Anzeiger, 1888, pp. 500–503.)

—— Ueber die Palpen der Rhopaloceren, etc., 6 Taf. Helsingfors, 1896, pp. 1–578. (Acta Soc. Sc. Fennicæ, xxii, 1896.)

Packard, A. S. On the occurrence of organs, probably of taste, in the epipharynx of the Mecoptera (Panorpa and Boreus). (Psyche, 1889, v, pp. 159–164.)

—— Notes on the epipharynx, and the epipharyngeal organs of taste in mandibulate insects. (Psyche, 1889, v, pp. 193–199, 222–228.)

Also Lubbock’s Senses, etc., of animals, and the writings of Briant, Breithaupt (titles on p. 85).

d. The organs of hearing

Although it has been denied by Forel that insects have the sense of hearing, yet the majority of writers and experimenters agree that insects are not deaf. On general grounds if, as we know, many insects produce sounds, it must follow that they have ears to hear, for there is every reason to suppose that the sounds thus made are, as in other animals, either for attracting the sexes, for a means of communication, or to express the emotions. We will begin by briefly describing the structures now generally supposed to be auditory in 288function, and about which there can be no reasonable doubt, and then consider the more problematical organs, closing with an account of the extremely various means of producing sounds and cries.

The ears or tympanal and chordotonal sense-organs of Orthoptera and other insects.—The ears or tympana of locusts (Acrydiidæ) are situated one on each side, on the basal joint of the abdomen, just behind the first abdominal spiracle. That this is a true ear was first suggested by J. Müller, and his opinion was confirmed by Siebold, Leydig, Hensen, Graber, Schmidt, Lubbock, etc.[49]

Fig. 290.—Ear of a locust (Caloptenus italicus), seen from the inner side: T, tympanum; TR, its border; o, u, two horn-like processes; bi, pear-shaped vesicle; n, auditory nerve; ga, terminal ganglion; st, stigma; m, opening, and m′, closing, muscle of the same; M, tensor muscle of the tympanum membrane.—After Graber.

The apparatus consists of a tense membrane, the tympanum, surrounded by a horny ring (Fig. 290). “On the internal surface of this membrane are two horn-like processes (o, u), to which is attached an extremely delicate vesicle (bi) filled with a transparent fluid, and representing a membranous labyrinth. This vesicle is in connection with an auditory nerve (n) which arises from the third thoracic 289ganglion, forms a ganglion (ga) upon the tympanum, and terminates in the immediate neighborhood of the labyrinth by a collection of cuneiform, staff-like bodies, with very finely pointed extremities (primitive nerve-fibres?), which are surrounded by loosely aggregated ganglionic globules” (Siebold’s Anatomy of the Invertebrates).

Fig. 291.—Fore tibia of Locusta viridissima. td, cover of the drum; tr, fissure between the drum and its cover.—After Graber, from Lang.

In the green grasshoppers, katydids, and their allies, the ears are situated on the fore tibiæ, where these organs can be found after a careful search (Figs. 291, 292).

The presence of the structure is indicated by the oval disc, the drum, which is a thin tense membrane covering the auditory apparatus of nerves, ganglion cells, and auditory rods beneath.

The tympana, or drums, are not present in all Locustidæ and Gryllidæ, and, as Lubbock states, it is an additional reason for regarding them as auditory organs, that in those species which possess no stridulating organs the tympana are also wanting. In many of the Locustidæ the tympana are covered or protected by a fold of the skin projecting over them. These covered ones are, Graber thinks, derived from the open ones.

Fig. 292.A, fore tibia of a European grasshopper (Meconema), containing the ear: Ty, tympanum or outer membrane; Tr 1, Tr 2, tracheæ. B, diagrammatic cross-section through the tibia and ear of the same; Ty, tympanum; Ct, cuticula; CM, hypodermis: A, the auditory organ connecting with the tympanum; B, supra-tympanal auditory organ; GZ, the ganglion-cell belonging to them; Hst, the auditory rod connecting with the ganglion-cells.—After Graber, from Judeich and Nitsche.

On examining the apparatus within the leg under the drum, it is seen to consist of the trachea, the auditory vesicles and rods, ganglion cells, and acoustic nerve. The trachea is greatly modified (Fig. 292, Tr 1). On passing into the tibia the trachea enlarges and divides into two branches, which reunite lower down. The spiracles supplying the air to this enlarged trachea are considerably enlarged, while in the dumb species it is of the normal size. The enlarged 290trachea passes close to the tympanum, which thus has air on both sides of it: the open air on the outer, the air of the trachea on its inner surface. In fact, as Lubbock states, “the trachea acts like the Eustachian tube in our own ear; it maintains an equilibrium of pressure on each side of the tympanum, and enables it freely to transmit the atmospheric vibrations.”

Fig. 293.—The auditory apparatus in the tibia of a grasshopper, showing the tympanal nerve-endings in situ: EBI, terminal vesicles of Siebold’s organ; SN, nerve of the organ of Siebold; Gr, group of vesicles of same; SO, nerve-endings of the same; vT, front tympanum; vTr, front branch of the trachea; hT, hinder tympanum; hTr, hinder branch of the trachea; Sp, space between the tracheæ; go, supra tympanal ganglion; rN, connecting nerve-fibrils between the ganglion cells and the terminal vesicles; R, upper, n-S, lower, root of the transparent covering membrane. (Other lettering not explained by author.)—After Graber.

Fig. 294.—Auditory rod of Gryllus viridissimus: fd, auditory rod; ko, terminal piece.—After Graber, from Lubbock.

These tracheæ, says Graber, though formed on a similar plan, present many variations, corresponding to those of the tympana, and showing that the tympana and the tracheæ stand in intimate connection with one another. For instance, in those species where the tympana are equal, the tracheæ are so likewise; in Gryllotalpa, where the front tympanum only is developed, though both tracheal branches are present, the front one is much larger than the other; and where there is no tympanum, the trachea remains comparatively small, and even in some cases undivided (Lubbock, ex Graber).

The acoustic nerve, which next to the optic is the thickest in the body, divides soon after entering the tibia into two branches, one almost immediately forming a ganglion, the supra-tympanal ganglion, the other passing down to the tympanum, where it expands into an elongated flat ganglion, the organ of Siebold (Fig. 293), and closely applied to the anterior tracheæ.

At the upper part of the ganglion is a group terminating below in a single row of vesicles, the first few of which are approximately equal, but which subsequently diminish regularly in size. Each 291of these vesicles is connected with the nerve by a fibril (Fig. 293, vN), and contains an auditory rod (Fig. 294). They are said by Graber to be brightly refractive, hollow (thus differing from the retinal rods, which are solid), and terminate in a separate end-piece (ko). The rods were first discovered by Siebold, and, as Lubbock remarks, may be regarded as specially characteristic of the acoustic organs of insects.

Fig. 295.—Chordotonal organ in nymph of a white ant.—After Müller, from Sharp.

Fig. 296.—Right half of 8th body-segment of Corethra plumicornis: g, ganglion of ventral cord; lm, longitudinal muscle; cn, chordotonal nerve; cl, chordotonal ligament; cg, chordotonal ganglion; cs, rod of chordotonal organ; cst, terminal cord; tb, tactile setæ; hn, out-going fibres of the integumental nerves.—After Graber, from Lang.

As will be seen in Fig. 293, at the upper part of the tibial organ of Ephippigera there is a group of cells, and below them a single row of cells gradually diminishing in size from above downwards. “One cannot but ask oneself,” says Lubbock, “whether the gradually diminishing size of the cells in the organ of Siebold may not have reference to the perception of different notes, as is the case with the series of diminishing arches in the organ of Corti of our own ears.”

These organs were supposed to be restricted to the Orthoptera, but in 1877 Lubbock discovered what seems to resemble the supra-tympanal auditory organ of Orthoptera in the tibia of the yellow ant (Lasius flavus). Graber confirmed Lubbock’s account, and also discovered these organs in the tibia of a Perlid (Isopteryx apicalis), and Fritz Müller has detected them in the fore tibiæ of the nymph of Calotermes rugosus (Fig. 295). To these structures Graber gave the name of chordotonal organs.

He has also detected these organs in all the legs of other insects (Trichoptera, Pediculidæ), and auditory rods have been discovered in the antennæ of Dyticus and of Telephorus by Hicks, Leydig, and Graber. Graber classifies the chordotonal organs into truncal and membral. In Coleoptera and Trichoptera they may occur on several joints of the leg; others are more localized,—thus he distinguishes femoral (Pediculidæ), tibial (Orthoptera, Perlidæ, Formicidæ), and tarsal organs (Coleoptera).

A type of chordotonal organ, observed in the body-segments of the larvæ of several insects by Leydig, Weismann, Graber, Grobben, and Bolles Lee, is to be seen in the transparent larva of Corethra (Fig. 296), where the auditory organ extends to the skin. It contains at the point cs two or three auditory 292rods. In the opposite direction a fine ligament (cl) passes from cg to the skin; in this way the auditory organ is suspended in a certain state of tension, and is favorably situated to receive even very fine vibrations. A similar apparatus has been detected in the larva of Ptychoptera.

Antennal auditory hairs.—It is not at all improbable that the antennæ of different insects contain auditory as well as olfactory structures. Lubbock has suggested that the singular organs which have only been found in the antennæ of ants and certain bees, and to which he gives the name of “Hicks’ bottles” (Fig. 281), may act as microscopic stethoscopes, while Leydig also regards them as chordotonal organs.

That, however, some of the antennal hairs of the mosquito, as first suggested by Johnson and afterwards proved experimentally by Mayer, are auditory, seems well established. Fastening a male mosquito down on a glass slide, Mayer then sounded a series of tuning-forks. With an Ut4 fork of 512 vibrations per second, some of the hairs were seen to vibrate vigorously, while others remained comparatively at rest. The lower (Ut3) and higher (Ut5) harmonics of Ut4 also caused more vibration than any intermediate notes. These hairs, then, are specially tuned so as to respond to vibrations numbering 512 per second. Other hairs vibrated to other notes, extending through the middle and next higher octave of the piano.

Mayer then made large wooden models of these hairs, the one corresponding to the Ut3 hair being about a metre in length, and on counting the number of vibrations they made when they were clamped at one end and then drawn on one side, he found that it “coincided with the ratio existing between the numbers of vibrations of the forks to which covibrated the fibrils,” or hairs. It should be observed that the song of the female mosquito corresponds nearly to this note, and would consequently set the hairs in vibration. Mayer observed that the song of the female vibrates the hairs of one of the antennæ more forcibly than those of the other. Those auditory hairs are most affected which are at right angles to the direction from which the sound comes. Hence from the position of the antennæ and the hairs a sound will be loudest or most intense if it is directly in front of the head. If, then, the song of the female affects one antenna more than another, the male turns his head until the two antennæ are equally affected, and is thus able to fly straight towards the female. From his experiments Mayer found that the male can thus guide himself to within 5° of the direction of the female. Hence he concludes that “these insects must have the faculty 293of the perception of the direction of sound more highly developed than in any other class of animals.” (Also see Child’s work.)

Special sense-organs in the wings and halteres.—Organs of a special sense, which Hicks supposed to be those of smell, were found by him near or at the base of the wings of Diptera, Coleoptera, and less perfect ones in Lepidoptera, Neuroptera, and Orthoptera, with a trace of them in Hemiptera; but these were considered by Leydig to be auditory organs, since he found the nerves to end in club-shaped rods, like those of Orthoptera.

Hicks found, as to the halteres and their sense-organs, that the nerve in the halter is the largest in the insect, except the optic nerve; and that at the base of the halteres is a number of vesicles arranged in four groups, to each of which the nerve sends a branch. Afterwards Bolles Lee discovered that the vesicles, undoubtedly perforated, contain a minute hair, those of the upper groups being protected by hoods of chitin. He regarded them as olfactory organs, while Lubbock seems inclined to consider them as auditory structures. Graber also regards the vesicles of Hicks as chordotonal organs.

In his elaborate account of the balancers, Weinland concludes that the organs of sense of varying structure occurring at the base of these appendages allow the perception of movements which the halteres perform and which enable the fly to steer or direct its course. The halteres can thus cause differences in the direction of the flight of a fly in the vertical plane. If the balancers act unequally, there is a change in direction.

e. The sounds of insects

Insects have no true voice; but sounds of different intensity, shrill cries, and other noises are produced mechanically by insects, either being love-songs to attract the sexes, to give signals, to communicate intelligence, or perhaps to express the emotions. The loud, shrill cry of the Cicada, or chirp of the cricket, is evidently a love-call, and results in the mating of individuals of separate broods more or less widely scattered, thus preventing too close interbreeding.

The simplest means of making a noise is that of the death-watch (Anobium), which strikes or taps on the wall with its head or abdomen. Longicorn beetles make a sharp sound by the friction of the mesoscutellum against the edge of the prothoracic cavity, the head being alternately raised and lowered, Burying-beetles (Necrophorus) rub the abdomen against the hinder edges of the elytra. Weevils make a loud noise by rapidly rubbing the tips of the abdomen on the ends of the elytra.

Landois offers the following summary of the kinds of noises produced by beetles:

1.
Tapping sounds (Bostrycinæ, Anobium).
2.
Grating sounds (Elateridæ).
294
3.
Friction without special rasping organs (Euchirus longimanus).
4.
Rasping sounds produced by friction:
 
a. Rubbing of the pronotum on the mesonotum (Cerambycidæ except Spondyli and Prionus).
 
b. Friction of the prosternum on the mesosternum (Omaloplia brunnea).
 
c. Elytra with a rasp at the end (Curculionidæ, Dyticidæ, Pelobius).
 
d. With a coxal rasp (Geotrupes, Ceratophyus). The male of Ateuchus stridulates to encourage the female in her work, and from distress when she is removed. (Darwin.)
 
e. Friction of the edge of the elytra against the femur (Chiasognathus grantii).
 
f. Pygidium with two rasps in the middle (Crioceris, Lema, Copris, Oryctes, Necrophorus, Tenebrionidæ).
 
g. Abdomen with a grating ridge and four grating plates (Trox).
 
h. Abdomen with two toothed ridges rubbing on a rasp on edge of wing-cover (Elaphus, Blethisa, Cychrus).
 
i. Rubbing the elytra on a rasp on the hind wings (Pelobius hermanni).
 
j. Friction of the wing against the abdominal segments (Melolontha fullo).

Mutilla makes a rather sharp noise by rubbing one abdominal segment against another. Ants (Ponera) have a stridulating apparatus, and other genera numerous (20) ridges between the segments.

Even certain moths and butterflies emit a rasping or crackling noise. The death’s-head moth and other sphinges cause it by rubbing the palpi against the base of the proboscis. These and certain butterflies are provided with parallel ridges forming a rasp on the “basal spot” of the inner side of the basal joint of each palpus (Reuter). A South American butterfly (Ageronia feronia) can be heard for several yards as it flies with a crackling sound. Hampson finds that the cause of the clicking sound is due to a pair of strong chitinous hooks attached to the thorax, against which play the spatulate ends of a pair of hooks attached to the fore wings. An Australian moth (Hecatesia) flies with a whizzing sound; Vanessa is said to be sonorous.

The males of Orthoptera produce their shrill cries or chirping noises, 1, by rubbing the thighs against the sides of the body (Acrydiidæ); 2, by the friction of the base of the fore wings on each other (Locustidæ); 3, by rubbing the base of the upper on the base of the hinder or under pair (Gryllidæ), in the two last there being a shrilling apparatus consisting of a file on the hind wings, which rubs on a resonant surface on the fore wings. The females are not invariably dumb, both sexes of the European Ephippigera being able to faintly stridulate. Corixa also produces shrill chirping notes. (Carpenter.)

Certain insects also hum, and have what may perhaps be called a voice. The cockchafer, besides humming with the wings, produces a sound almost like a voice. In the large trachea, just behind each spiracle, is a chitinous process, which is thrown into vibrations by the air during respiration, and thus produces a humming noise. (Lubbock.) Such is also the case with flies, the mosquito, dragon-flies, and bees. In flies and dragon-flies the “voice” is caused by the air issuing from the thoracic spiracles; while in the humble-bee the abdominal spiracles are also musical. The sound made by the spiracles bears no relation to that caused by the wings. Landois 295tells us that the wing-tone of the honey-bee is A′; its voice, however, is an octave higher, and often goes to B″ and C″.

The sounds produced by the wings are constant in each species, except where, as in Bombus, there are individuals of different sizes; in these the larger ones generally give a higher note. Thus the comparatively small male of Bombus terrestris hums on A′, while the large female hums an entire octave higher.

From the note produced the rapidity of the vibrations can be calculated. For example, the house-fly, which produces the sound of F, vibrates its wings 21,120 times in a minute, or 335 times in a second; and the bee, which makes a sound of A′, as many as 26,400 times, or 440 times in a second. On the contrary, a tired bee hums on E′, and therefore, according to theory, vibrates its wings only 330 times in a second. Marey has confirmed these numbers graphically, and found by experiment that the fly actually makes 330 strokes in a second. (Lubbock.)

A different kind of musical apparatus is that of the cicada, which has been elaborately described by Graber. The shrill, piercing notes issue from a pair of organs on the under side of the base of the abdomen of the male, these acting somewhat as two kettle-drums, the membrane covering the depressions being rapidly vibrated.

LITERATURE ON THE ORGANS OF HEARING

a. The auditory organs

Siebold, C. Th. E. von. Ueber das Stimm- und Gehörorgan der Orthopteren. (Archiv f. Naturgesch., 1844, x, pp. 52–81.)

Johnston, Christopher. Auditory apparatus of the culex mosquito. (Quart. Journ. Micr. Soc., 1855, iii, pp. 97–102, 1 Fig.)

Hicks, Braxton. On a new organ in insects. (Journ. Linn. Soc. Zool., London, 1857, pp. 130–140, 1 Pl.)

—— Further remarks on the organ found on the bases of the halteres and wings of insects. (Trans. Linn. Soc., London, 1857, xxii, pp. 141–145, 2 Pls.)

Hensen, V. Ueber das Gehörorgan von Locusta. (Zeitschr. f. wissens. Zool., xvi, 1866, pp. 190–207.)

Graber, V. Bemerkungen über die Gehör- und Stimmorgane der Heuschrecken und Cicaden (Wiener Sitzungsber. Math.-natur-wiss. Cl., lxvi, 1 Abt., 1872, pp. 205–213, 2 Figs.)

—— Die tympanalen Sinnesapparate, der Orthopteren. (Denkschr. d. k. Akad. d. wissens. Wien, xxxvi, 1876, 2 Abt., pp. 1–140, 10 Taf.)

—— Die abdominalen Tympanalorgane der Cicaden und Gryllodeen. (Ibid., 1870, xxxvi, pp. 273–290, 2 Taf.)

—— Ueber neue, otocystenartige Sinnesorgane der Insekten. (Archiv f. mikroskop. Anat., 1878, pp. 35–57, 2 Taf.)

—— Die chordotonalen Sinnesorgane und das Gehör der Insekten. (Archiv f. mikroskop. Anat., 1882, xx, pp. 506–640; 1883, xxi, pp. 65–145, Taf.)

296Mayer, Alfred Marshall. Researches in Acoustics No. 5. 3. Experiments on the supposed auditory apparatus of the culex mosquito. (Amer. Jour. Sc. and Arts, Ser. 3, viii, 1874, pp. 81–103; also Amer. Naturalist, viii, pp. 577–592.)

Schmidt, Oscar. Die Gehörorgane der Heuschrecken. (Archiv f. mikroskop. Anat., xi, 1875, pp. 195–215, 3 Taf.)

Ranke, J. Beiträge zu der Lehre von den Uebergangssinnesorganen, das Gehörorgan der Acridier und das Sehorgan der Hirudineen. (Zeitschr. f. wissens. Zool., xxv, 1875, pp. 143–164, 1 Taf.)

Lee, A. Bolles. Les balanciers des Diptères, leurs organes sensifères et leur histologie. (Recueil Zool. Suisse, ii, 1885, pp. 363–392, 1 Pl.)

—— Bemerkungen über der feineren Bau der Chordotonalorgane. (Archiv f. mikroskop. Anat., 1883, xxiii, pp. 133–140, 1 Taf.)

—— Les organes chordotonaux des Diptères et la méthode du chlorure d’or. (Observations critiques.) (Recueil Zool. Suisse, 1884, ii, pp. 685–689, 1 Pl.)

Weinland, E. Ueber die Schwinger (Halteren) der Dipteren (Zeitschr. f. wissens. Zool., 1890, li, pp. 55–166, 5 Taf.)

Adelung, N. v. Beiträge zur Kenntnis des tibialen Gehörapparates der Locustiden. Inaug. Diss., Leipzig, 1892, 2 Taf.

Child, Ch. M. Ein bisher wenig beachtetes antennales Sinnesorgan der Insekten, mit besonderer Berücksichtigung der Culiciden und Chironomiden. (Zeitschr. f. wissens. Zool., lviii, 1894, pp. 475–528, 2 Taf.; also, Zool. Anzeiger, xvii Jahrg., pp. 35–38, and in Annals and Mag. Nat. Hist., 1894 (6), xiii, pp. 372–374).

Also the writings of J. Müller, Kirby and Spence, Burmeister, Gilbert White, Westwood, Guilding, Meinert, Paasch, Leydig, Viallanes, Minot, Forel, Mayer, Darwin (Descent of Man, i, ch. x.), F. Müller, Lubbock (Senses of animals), Westring, Köppen, Bates, Vom Rath, Peckham, Jourdan, Nagel, etc.

b. The sounds made by insects

Scudder, S. H. Notes on the stridulation of grasshoppers. (Proc. Bost. Soc. Nat. Hist., xi, 1868, pp. 306–313 and 316.)

—— The songs of the grasshoppers. (Amer. Naturalist, ii, 1868, pp. 113–120, 5 Figs.)

Riley, C. V. The song notes of the periodical Cicada. (Proc. Amer. Assoc. Adv. Science, xxxiv, 1885, pp. 330–332; also in Kansas City Rev., October, 1885, pp. 173–175.)

Swinton, A. H. (Ent. Month. Mag., 1877.) Sound produced in Ageronia by a modification of the hook and bristle of the wings.

Hampson, G. F. On stridulation in certain Lepidoptera, etc. (Proc. Zool. Soc. London, 1892, ii, pp. 188–193, Fig; also Psyche, vi, p. 491, 1 Fig.)

With the writings of Landois, Lubbock, Graber, Kolbe, Carpenter (Nat. Science), Bruyant, and others.

297

THE DIGESTIVE CANAL AND ITS APPENDAGES

Fig. 297.—Transverse section through an abdominal segment of larva of Megalopyge crispata, showing the relations of the digestive canal to the other organs: int, hind-intestine, with its mucous or epithelial layer (ep), and ml its outer or muscular layer; ng, ventral ganglion; ht, heart; mp, urinary tubes; f, fat-body; sc, thickened portion of the hypodermis (hy) containing the setigenous cells; m, muscles; m′, a pair of retractor muscles inserted near the base of the lateral glandular process (lgp); cut, cuticula; l, legs. Also compare Figs. 142–144 and 234.

298

Fig. 298.299
The alimentary or digestive canal of insects is a more or less
straight tube, which connects the mouth and anus, the latter invariably
situated in the last segment of the body, under the last
tergite or suranal plate. It lies directly over the ventral nervous
cord and under the dorsal vessel, passing through the middle of the
body (Fig. 297). It is loosely held in place by delicate retractor
muscles (retractores ventriculi, found by Lyonet in the larvæ of
Lepidoptera, and occurring in those of Diptera), but is principally
supported by exceedingly numerous branches of the main tracheæ.
Fig. 298.—Internal anatomy of Melanoplus femur-rubrum: at, antenna and nerve leading
to it from the “brain” or supraœsophageal ganglion (sp); oc, ocelli, anterior and vertical ones,
with ocellar nerves leading to them from the brain; œ, œsophagus; m, mouth; lb, labium or under
lip; if, infraœsophageal ganglion, sending three pairs of nerves to the mandibles, maxillæ, and
labium respectively (not clearly shown in the engraving); sm, sympathetic or vagus nerve, starting
from a ganglion resting above the œsophagus, and connecting with another ganglion (sg) near the
hinder end of the crop; sal, salivary glands (the termination of the salivary duct not clearly
shown by the engraver); nv, nervous cord and ganglia; ov, ovary; ur, origin of urinary tubes;
ovt, oviduct; sb, sebaceous gland; bc, bursa copulatrix; ovt, site of opening of the oviduct (the
left oviduct cut away); 1–10, abdominal segments. The other organs labelled in full.—Drawn from
his original dissections by Mr. Edward Burgess.

Fig. 299.—Digestive canal of Anabrus: m, mouth: œ, œsophagus; sm, the sympathetic nerve passing along the crop; t, tongue; fg, frontal ganglion; br, brain, the nervous cord passing backward from it; sr, salivary reservoir; sg, salivary gland; pv, proventriculus; ur, origin of urinary tubes; sb, sebaceous gland; 1–10, the ten abdominal segments.—Burgess del.

It is in the higher adult insects differentiated into the mouth and pharynx, the œsophagus or gullet, supplementary to which is the crop (ingluvies) or “sucking stomach” of Lepidoptera, Diptera, and Hymenoptera; the proventriculus or gizzard; the ventriculus, “chyle-stomach,” or, more properly, mid-intestine, and the hind-intestine, which is divided into the ileum, or short intestine, the long intestine, often slender and coiled, with the colon and the rectum. Morphologically, however, the digestive or enteric canal is divided into three primary divisions, which are indicated in the embryo insect; i.e., the fore-intestine (stomodæum of the embryo), mid-intestine or “chyle-stomach,” and hind-intestine or proctodæum (Fig. 300). The three primary regions, with their differentiations, may be tabulated thus:—

Fore-intestine (Stomodæum). Mouth and pharynx.
  Pumping apparatus of Hemiptera, Lepidoptera, and Diptera.
  Œsophagus.
  Crop or ingluvies, food reservoir, or “sucking stomach.”
  Proventriculus.
 
Mid-intestine (Mesenteron). Mid-intestine, “chylific stomach,” or ventriculus (with cœcal glands).
 
Hind-intestine (Proctodæum). Ileum, or short intestine (with the urinary tubes).
  Long intestine.
  Colon.
  Rectum (with rectal glands).
  Anus (with anal glands).

Fig. 300.—The three primary divisions of the alimentary canal of an embryonic orthopterous insect: br, brain; sbg, subœsophageal ganglion; ng, nervous cord; st, stomodæum; pr, proctodæum; mv, malphigian tubes; mesen, mid-intestine; ht, heart; md, mandibles; mx, mx′, 1st and 2d maxillæ.—After Ayers, with some changes.

The appendages of the alimentary canal are: (1) the salivary and poison glands, which arise from the stomodæum in embryonic life; 300(2) while to the chylific stomach a single pair of cœcal appendages (Orthoptera and larval Diptera, e.g. Sciara), or many cœca may be appended; (3) the urinary tubes, also the rectal glands and the paired anal glands. In a Hemipter (Pyrrhocoris apterus) appendages arise from the intestine in front of the origin of the urinary tubes. In certain insects a single cœcal appendage (Nepa, Dyticus, Silpha, Necrophorus, and the Lepidoptera) arises from the proctodæum.

Fig. 301—Larva of honey-bee: g, brain; bm, ventral nervous cord; œ, œsophagus; sd, spinning-gland; cd, mid-intestine or chyle-stomach; ed, hind-intestine, not yet connected with the mid-intestine; vm, urinary tube; an, anus; st, stigmata.—After Leuckart, from Lang.

In certain larval insects, as those of the Proctotrypidæ (first larval stage), the higher Hymenoptera (ichneumons, ants, wasps, and bees, Fig. 301), in the Campodea-like larvæ of the Meloidæ and Stylopidæ, the larva of the ant-lion (Myrmecoleo), and those of Diptera pupipara (Melophagus), the embryonic condition of the separation of the proctodæum and mid-gut (mesenteron) persists, the stomach ending in a blind sac; in such cases the intestine, together with the urinary tubes, is entirely secretory.

The anus is wanting in the larva of the ant-lion, as also in the wasps (in which there is a rudimentary colon) and in freshly hatched bees, though it becomes perfectly formed in the fully grown larvæ (Newport, art. Insecta, p. 967, and H. Müller).

In the larvæ of lamellicorn Coleoptera (Melolontha vulgaris) the digestive tube is nearly as simple as in bees, though there is a large colon, which at its beginning forms an immense cœcum, and has also one anal aperture (Newport).

The length and shape of the digestive canal is dependent on the nature of the food and also on the mode of life, especially the ease or difficulty with which the food is digested.

301Newport, while stating that the length of the alimentary canal in larvæ is not in general indicatory of the habits of the species, makes this qualification after describing the digestive canal of Calandra as compared with that of Calosoma: “The length and complication of the intestines, therefore, appear to have some reference to the quality of the food to be digested, since it is well known that the food of these latter insects (weevils) is of difficult assimilation, being as it is chiefly the hard ligneous fibres of vegetable matter; but they cannot be received as always indicatory of a carnivorous [or] vegetable feeder, since, as above remarked, the length of the canal is considerable in one entirely carnivorous larva, while it is much shorter in some herbivorous, and particularly in pollenivorous larvæ, as in the Melolontha and the apodal Hymenoptera.”

Fig. 302.—Digestive canal of a carabid beetle: b, œsophagus; c, crop; d, proventriculus; f, mid-intestine, or “chyle-stomach,” with its cœeca; g, posterior division of the stomach; i, the two pairs of urinary tubes; h, intestine; k, rectum; l, anal glands.—After Dufour, from Judeich and Nitsche.

Newport also contends that the length of the alimentary canal is not more indicative in the perfect insect of the carnivorous or phytophagous habits of the species than in the larva. It is nearly as long (being from two to three times the length of the whole body), and is more complicated, in the rapacious Carabidæ (Fig. 302) than in the honey-sipping Lepidoptera, whose food is entirely liquid. Referring to the digestive canal of Cicindelidæ, which is scarcely longer than the body, he claims that “we cannot admit that the length of the digestive organs, and the existence of a gizzard and gastric vessels, are indicatory of predacity of habits in the insect, because a similar conformation of parts exists often in strictly vegetable feeders. The existence and length of these parts seem rather to refer to the comparative digestibility of the food than to its animal or vegetable nature.” Newport then refers to the digestive canal of Forficulidæ (in which the gizzard is present, the canal, however, passing in an almost direct line through the body, making but one slight convolution), “a farther proof that the length of the canal must not be taken as a criterion whereby to judge of the habits of a species.” He adds this will apply equally well to the omnivorous Gryllidæ, in which there exists a short alimentary canal, but a gizzard of more complicated structure than that of the Dytiscidæ.

In larval insects and others (Synaptera, Orthoptera, etc.), in which the digestive canal is simplest, it is scarcely longer than the body, and passes through it as a straight tube.

In the caterpillar, which is a voracious and constant feeder, the digestive canal is a large straight tube, not clearly differentiated into fore-stomach, stomach, and intestine; but in the imago, which only 302takes a little liquid food, it is slender, delicate, and highly differentiated. In the larva the mid-gut forms the largest part of the canal; in the imago, the intestine becomes very long and coiled into numerous turns; at the same time the food-reservoir (the “sucking stomach”) develops, and the excretory tubes are longer.

a. The digestive canal

Fig. 303.—Interior view of the bottom of the head of Danais archippus, the top having been cut away, showing, in the middle, the pharyngeal sac with its five muscles: the frontal (f.m), dorsal pair (d.m), and the lateral pair (l.m); cl, clypeus; cor, cornea; œ, œsophagus; p.m, one of the large muscles which move the labial palp.—After Burgess.

It will greatly simplify our conception of the anatomy of the digestive canal if we take into account its mode of origin in the embryo, bearing in mind the fact that during the gastrula condition the ectoderm is invaginated at each pole to form the primitive mouth and fore-gut (stomodæum) and hind-gut (proctodæum). The cells of the ectoderm secrete a chitinous lining (intima), which forms the continuation of the outer chitinous crust, and thus the lining of each end of the digestive canal is cast whenever the insect molts; while the mid-intestine (mesenteron), arising independently of the rest of the canal much later in embryonic life from the mesoderm, is not the result of any invagination, being directly derived from the mesoderm, and is not lined with chitin.

The mouth, or oral cavity, and pharynx.—This is the beginning of the alimentary bounded above by the clypeus, and labrum, with the epipharynx, and below by the hypopharynx, or tongue, as well as the labium. Into it pour the secretion of the salivary glands, which passes out through an opening at the base of the tongue or hypopharynx. On each side of the mouth are the mandibles and first maxillæ.

The sucking or pharyngeal pump.—This organ has been observed by Graber in flies and Hemiptera, but the fullest account is that by Burgess, who was the first to discover it in Lepidoptera. In the milk-weed butterfly (Danais archippus) the canal traversing the proboscis 303opens into a pharynx enclosed in a muscular sac (Figs. 303, 304, and 310).

The pharyngeal sac, says Burgess, serves as a pumping organ to suck the liquid food through the proboscis and to force it backwards into the digestive canal.

Fig. 304.—Longitudinal section through the head of Danais, showing the interior of the left half: mx, left maxilla, whose canal leads into the pharynx; hph, floor of the latter, showing some of the taste-papillæ; oe, œsophagus; ep, epipharyngeal valve; sd, salivary duct; d.m, f.m, and cl, as in Fig. 302.—After Burgess.

Meinert (“Trophi Dipterorum”) has made elaborate dissections of the mouth and its armature, including the pharynx of several types of Diptera, with its musculature. He describes the pharynx as the principal, and in most Diptera, as the only part of the pump (antlia), and says: “By the muscles of the pump (musculis antliæ) the superior lamina of the pharynx is varied that the space between the two laminæ may be increased, and the liquid is thus led through the siphon formed by the mouth-parts into the mouth” (Fig. 81).

The œsophagus.—This is a simple tube, largest in those insects feeding on solid, usually vegetable, food, and smallest in those living on liquid food. It usually curves upwards and backwards, passing directly under the brain, and merges into the crop or proventriculus either at the back part of the head or in the thorax, its length being very variable. Its inner walls longitudinally are folded and lined with chitin.

According to Newport, in the œsophagus of the Gryllidæ, of the two layers of the mucous lining the second is distinctly glandular and secretory, and in it there are many thousands of very minute granular glandular bodies, which probably secrete the “molasses” or repellent fluid often ejected by these and other insects when captured.

The crop or ingluvies.—This, when present, is an enlargement of the end of the œsophagus, and lined internally with a muscular coat. It is very large in locusts (Fig. 298), Anabrus (Fig. 299), and other Orthoptera (the Phasmidæ excepted), in the Dermaptera, and most 304adult Coleoptera. A crop-like dilatation in front of a spherical gizzard is also present in the Synaptera (Poduridæ and Lepismidæ), as well as in the Mallophaga (Nirmidæ).

Fig. 305.—Digestive canal of Calandra: H, pear-shaped œsophagus; I, crop; K, gastric cœca L, ilium; MN, colon; P, urinary tubes.—After Newport.

Fig. 306.—Section of the crop (H), gizzard (I), and stomach (K) of Athalia.-After Newport.

Fig. 307.—Upper side of head and digestive canal of Myrmeleon larva: a, crop; b, “stomach”; c, free ends of two urinary tubes; c′, common origin of other six tubes; d, cœcum; e, spinneret; ff, muscles for protruding its sheath; gg, maxillary glands.—After Meinert, from Sharp.

In the larvæ of weevils (Calandra sommeri) there is a crop (Fig. 305), but not in the larva of Calosoma; also, according to Beauregard, in the pollen-eating beetles Zonitis, Sitaris, and Malabris it is wanting, while in Meloe it is highly developed (Kolbe).

The crop forms a lateral dilatation of the end of the œsophagus in the larvæ of weevils and of saw-flies (Athalia centifoliæ, Fig. 306).

305The “sucking stomach” or food-reservoir.—This is a thin muscular pouch connected by a slender neck with the end of the œsophagus or the crop, when the latter is present. There is no such organ in Orthoptera, except in Gryllotalpa. It is wanting in the Odonata and in the Plectoptera (Ephemeridæ); in Platyptera (Perlidæ and Termitidæ), in Trichoptera, and in Mecoptera (Panorpidæ). In most adult Neuroptera (Myrmeleonidæ, Hemerobiidæ, and Sialidæ), but not in Rhaphidiidæ, the long œsophagus is dilated posteriorly into a kind of pouch or crop, and besides there is often a long “food-reservoir” arising on one of its sides, that of Myrmeleon (Fig. 307) and Hemerobius being on the right side.

Fig. 308.—Digestive canal of Sarcophaga carnaria: a, salivary gland; b, œsophagus; c, food reservoir; f-g, stomach; h, intestine; i, urinary tubes; k, rectum.—From Judeich and Nitsche.

A true food-reservoir is present in most Diptera (Fig. 308) as well as in the larvæ of the Muscidæ, but according to Dufour it is wanting in some Asilidæ and in Diptera pupipara, and according to Brauer in the Œstridæ. The food-reservoir in Diptera is always situated on the left side of the digestive canal; there is usually a long neck or canal, while the reservoir is either oval or more usually bilobed, and often each lobe is itself curiously lobed.

In Lepidoptera (Figs. 309, 310) the so-called “sucking stomach” is, as Graber has proved, simply a reservoir for the temporary reception of food; though generally found to contain nothing but air, Newport has observed that in flies it is filled with food after feeding. He has found this to be the case in the flesh fly, and in Eristalis he has found it “partially filled with yellow pollen from the flowers of the ragwort upon which the insect was captured,” the pollen grains also occurring in the canal leading to the bag, in the gullet, and in the stomach itself. Graber has further proved by feeding flies with a colored sweet fluid that this sac is only a food-receptacle. As he says: “It can be seen filling itself fuller and fuller with the colored fluid, the sac gradually distending until it occupies half the hind-body.”

The food-reservoir of the Hymenoptera is a lateral pouch at the end of the long, slender œsophagus, and has been seen in the bee to be filled with honey.

306

Fig. 309.—Digestive canal of Sphinx ligustri: h, œsophagus; i, rudiment of the gizzard; k, “stomach”; q, its pyloric end; t, food reservoir; p, urinary tubes; l, ilium; m, cœcum of colon; n, rectum; v, vent.—After Newport.

In the mole-cricket the hinder part of the crop is armed within with hook-like bristles directed backwards so as not to prevent the energetic pressure of the food backwards into the proventriculus, and to obviate the possibility of a regurgitation. (Eberli.)

The fore-stomach or proventriculus.—This is especially well developed in the Dermaptera, in the Orthopterous families Locustidæ, Gryllidæ, and Mantidæ, while in the Thysanura (Lepisma) there is a spherical gizzard provided with six teeth. It also occurs in many wood-boring insects, and in most carnivorous insects, notably the Carabidæ, Dyticidæ, Scolytidæ, in the Mecoptera (scorpion-flies), in the fleas, and in many kinds of ants, as well as Cynips, Leucospis, and Xyphidria. It is very muscular, lined within with chitin, which is usually provided with numerous teeth arising from the folds. These folds begin in the œsophagus or crop, and suddenly end where the mesenteron (“chylific stomach”) begins. It has been compared with the gizzard of birds, and is usually called by German authors the chewing or masticating stomach. (Kaumagen.)

The proventriculus is best developed in the Gryllidæ (Acrida viridissima), where the six folds at the end of the crop close together to form a valve between the crop and proventriculus. “They are each armed with five very minute hooked teeth; and, continued into the gizzard, develop many more in their course through that organ. These first teeth are arranged around the entrance to the gizzard, and seem designed to retain the insufficiently comminuted food and to pass it on to that organ.

307

Fig. 310.—Anatomy of Danais archippus after removal of right half of the body. Lettering of the head: a, antenna; ph, pharynx; pl, labial palpi; r, proboscis; g, brain; usg, subœsophageal ganglion. Lettering of the thorax: I. II. III. thoracic segments; b1, b2, b3, the coxal joints of the three pairs of legs; bm, muscles of the wings; ac cephalic aorta with its swelling; œ, œsophagus; bg, thoracic ganglia of the ventral cord; sd, salivary glands of one side, those of the other side cut off near their entrance into the common salivary duct. Lettering of the abdomen: 1–9. abdominal segments; h, heart; sm, so-called sucking-stomach (food-reservoir); cm, chyle-stomach; ag, abdominal ganglia: ed, hind intestine with colon (c) and rectum (r); rm, urinary vessels; ov, ovarial tubes, those of the right side cut off; ove, terminal filaments of the ovaries; bc, bursa copulatrix; obc, its outer aperture; od, oviduct; vag, vagina; wo, its outer aperture; ad, glandular appendages of the vagina partly cut away; vk, connective canal between the vagina and bursa copulatrix with swelling (receptaculum seminis); an, anus.—After Burgess, from Lang.

Fig. 311.—Transverse section of the proventriculus of Gryllus cinereus: muc, muscular walls; r, horny ridge between the large teeth (sp).—After Minot.

Fig. 312.—Transverse section of the proventriculus of the cockroach.—After Miall and Denny.

308

Fig. 313.—Digestive canal of the honey-bee: A, horizontal section of the body; lp, labial palpus; mx, maxilla; e, eye; pro. t, prothorax; mesa. t, mesothorax; meta. t, metathorax; dv, dorsal vessel; v, v, ventricles of the same; No. 1, No. 2, No. 3, salivary gland systems; œ œsophagus; g, g, ganglia of chief nerve-chain; n, nerves; hs, honey-sac; p, petaloid stopper or calyx of honey-sac or stomach-mouth; c. s, chyle stomach; bt, urinary tubes; si, small intestine (ilium); l, “lamellæ or gland-plates of colon,” rectal glands; li, rectum. B, cellular layer of stomach; gc, gastric cells, × 200. C, urinary tube; bc, cells; t, trachea. D, inner layer, with gastric teeth (gt).—After Cheshire.

“Next to these in succession on each of the longitudinal ridges are four flat, broad, somewhat quadrate teeth, each of which is very finely denticulated along its free margin. These extend about half-way through the gizzard. They appear to be alternately elevated and depressed during the action of the gizzard, and to serve to carry on the food to the twelve cutting teeth, with which each ridge is also armed, and which occupy the posterior part of the organ. These 309teeth are triangular, sharp-pointed, and directed posteriorly, and gradually decrease in size in succession from before backward. Each tooth is very strong, sharp-pointed, and of the color and consistence of tortoise shell, and is armed on each side by a smaller pointed tooth. These form the six longitudinal ridges of the gizzard, between each two of which there are two other rows of very minute teeth of a triangular form, somewhat resembling the larger one in structure, occupying the channels between the ridges. The muscular portion of the gizzard is equally interesting. It is not merely formed of transverse and longitudinal fibres, but sends from its inner surface into the cavity of each of the large teeth other minute but powerful muscles, a pair of which are inserted into each tooth. The number of teeth in the gizzard amounts to 270, which is the same number in these Gryllidæ as found formerly by Dr. Kidd in the mole-cricket. Of the different kinds of teeth there are as follows: 72 large treble teeth, 24 flat quadrate teeth, 30 small single-hooked teeth, and 12 rows of small triangular teeth, each row being formed of 12 teeth. This is the complicated gizzard of the higher Orthoptera.” (Newport.)

In the more generalized cockroach, there are six principal folds, the so-called teeth, which project so far inwards as to nearly meet (Fig. 312). The entire apparatus of muscles and teeth is, as Miall and Denny state, “an elaborate machine for squeezing and straining the food, and recalls the gastric mill and pyloric strainer of the crayfish. The powerful annular muscles approximate the teeth and folds, closing the passage, while small longitudinal muscles, which can be traced from the chitinous teeth to the cushions, appear to retract these last, and open a passage for the food.”

As in the fore-stomach or proventriculus of the lobster, the solid, rounded teeth do not appear to triturate the solid fragments found in the organ, but act rather as a pyloric strainer to keep such bodies out of the chylific stomach. We accept the view of Plateau that this section of the digestive canal in insects, which he compares to the psalterium of a ruminant, is a strainer rather than a masticatory stomach, and both Forel and Emery, as well as Cheshire, take this view.

The proventriculus of the honey-bee (Fig. 313, hs) is called by apiarians the “honey-sac” or “honey-stomach.” Cheshire states that if it be carefully removed from a freshly killed bee, its calyx-like “stomach-mouth” may be seen to gape open and shut with a rapid snapping movement. The entrance to the stomach is guarded by four valves, each of which is strongly chitinous within, and fringed along its edge with downward-pointing fine stiff bristles. By the contraction of the longitudinal muscles (lm), the valves open to allow the passage of food from the honey-sac to the “chyle-stomach.” It is closed at will by circular muscles (tm). Then the bee can carry food for a week’s necessities, either using it rapidly in the production of wax, or eking it out if the weather is unfavorable for the gathering of a new store.

310

Fig. 314.—“Honey-sac stopper,” “stomach-mouth,” or calyx-bell of honey-bee, × 50. A, front view of one of the lobes of the calyx-bell; l, lip-like point, covered by down-turned bristles (b); sm, side membrane. B, longitudinal section of the stomach-mouth, with continuations into entrance of chyle-stomach; l, l, lip-like ends of leaflets; s, setæ; lm, longitudinal muscles; tm, transverse muscles in cross-section; cl, cell-layer of honey-sac; LM, TM, longitudinal and transverse muscles of same; nc, nucleated cells of tubular extension of stomach-mouth into chyle-stomach; lm′, tm′, longitudinal and transverse muscles of chyle stomach; c, c, cells covered within by an intima. C, cross-section of stomach-mouth; m, cross-section of muscles seen at lm in B; tm, transverse muscles surrounding stomach-mouth. D, cross-section through small intestine; a and m, longitudinal and surrounding muscles.—After Cheshire.

Cheshire also shows that when bees suck up from composite and other flowers nectar together with much pollen, the outside wrinkled membrane (sm, A, Fig. 314) “is seen to continually run up in folds, and gather itself over the top of the stomach-mouth, bringing with it, by the aid of its setæ, the large pollen-grains the nectar contains.” The lips (l, l, B, Fig. 314), now opening, take in this pollen, which is driven forwards into the cavity made between the separating lips by an inflow of the fluid surrounding the granules. The lips in turn close, but the down-pointing bristles are thrown outwards from the face of the leaflet, in this way revealing their special function, as the pollen is prevented from receding while the nectar passes back into the honey-sac, strained through between the bristles aforesaid, the last parts escaping by the loop-like openings seen in the corners of C, Fig. 314. The whole process is immediately and very rapidly repeated, so that the pollen collects and the honey is cleared. “Three purposes, in addition to those previously enumerated, are thus subserved by this wondrous mechanism. First, the bee can either eat or drink from the mixed diet she carries, gulping down the pollen in pellets, or swallowing the nectar as her necessities demand. Second, when the collected pollen is driven 311forwards into the chyle-stomach, the tube extension, whose necessity now becomes apparent, prevents the pellets forming into plug-like masses just below p, Fig. 313, for, by the action of the tube, these pellets are delivered into the midst of the fluids of the stomach, to be at once broken up and subjected to the digestive process. And third, while the little gatherer is flying from flower to flower, her stomach-mouth is busy in separating pollen from nectar, so that the latter may be less liable to fermentation and better suited to winter consumption. She, in fact, carries with her, and at once puts into operation, the most ancient, and yet the most perfect and beautiful, of all ‘honey-strainers.’”

Forel’s experiments on the proventriculus of ants prove that through its valvular contrivance it closes the passage from the crop to the mid-intestine (“chylific stomach”), and allows the contents of the former to pass slowly and very gradually into the latter. Emery confirms this view, and concludes that the organ in the Camponotidæ and in the Dolichoderidæ provided with a calyx-bell, usually regarded as a triturating stomach (Kaumagen), but more correctly as a pumping stomach, consists of parts which perform two different functions. Under the operation of the muscles of the crop the entrance to the pumping stomach becomes closed, in order by such spasmodic contraction to prevent the flow of the contents of the crop into the proventriculus. By the pressure of the transverse muscles of the proventriculus its contents are emptied into the mid-intestine, while simultaneously a regurgitation into the crop is prevented. In the Dolichoderidæ and Plagiolepidinæ the closure in both cases is effected by the valves. In the true Camponotidæ there are two separate contrivances for closing; the calyx belonging to the crop-musculature, while the valves essentially belong to the proventricular pumping apparatus.

Opinions vary as to the use of this portion of the digestive canal. Graber compares it to the gizzard of birds, and likens the action of the rosette of teeth to the finer radiating teeth of the sea-urchin, and styles it a chopping machine, which works automatically, and allows no solid bits of food to pass in to injure the delicate walls of the stomach (mid-gut).

He also states that the food when taken from the proventriculus is very finely divided, while that found in the œsophagus contains large bits.

Kolbe says that this view has recently been completely abandoned, and that the teeth are used to pass the food backwards into the chylific stomach. “But Goldfuss had denied the triturating action of the proventriculus of the Orthoptera (Symbolæ ad Orthopterorum quorundam Œconomiam, 1843), stating that the contents of the same are already fluid in the gullet, so that the fore-stomach (Kaumagen) does not need to comminute the food” (Kolbe). In the Gryllidæ and Locustidæ, just before the posterior opening of the proventriculus into the stomach the chitinous lining swells into a ring and projects straight back as the inner wall of the cylindrical chylific stomach. The muscular layer forms two sac-like outgrowths or folds, which separate on the circular fold from the chitinous membrane. This apparatus only allows very finely comminuted food to pass into the stomach.

In the Acrydiidæ (Eremobia muricata) at the end of the proventriculus, where it passes into the stomach, is a small circular fold which hangs down like a curtain in the stomach.

The œsophageal valve.—Weismann[50] states that the origin of the proventriculus in the embryo of flies (Muscidæ) shows that it should 312be regarded as an intussusception of the œsophagus. While in the embryo the invaginated portion of the œsophagus is short, after the hatching of the larva it projects backwards into the mid-intestine. Kowalevsky also observed in a young muscid larva, 2.2 mm. in length, that the œsophagus, shaped like a tube, extends back into the expanded portion (proventriculus) and opens into the stomach (Fig. 315, A). In a larva 10 mm. long the funnel is shorter, the end being situated in the proventriculus (Fig. 315, B, pr). In the cavity between the outer (o) and inner wall (i) no food enters, and the use of this whole apparatus seems to be to prevent the larger bits of food from passing into the chylific stomach (Kowalevsky).

Fig. 315.—Œsophageal valve of young muscid larva: m, its opening: t, thickening of the cells; mes, mesoderm.—After Kowalevsky.

Beauregard has found a similar structure in the Meloidæ, and calls it the “cardiac valvule” (Fig. 318, Kl). It was observed by Mingazzini in the larvæ of phytophagic lamellicorn beetles, and Balbiani described it in a myriopod (Cryptops) under the name of the “œsophageal valvule.”

Gehuchten describes a homologous but more complicated structure in a tipulid larva (Ptychoptera contaminata), but differing in containing blood-cavities, as a tubular prolongation of the posterior end of the œsophagus which passes through the proventriculus and opens at various positions in the anterior part of the chylific stomach (Fig. 316).

The three layers composing this funnel are distant from each other and separated by blood-cavities, the whole forming “an immense blood-cavity extended between the epithelial proventricular lining and the muscular coat.”

According to Schneider the longitudinal muscular fibres of the fore and hind gut in insects pass into the stomach (mid-gut). The anterior part of the fore-gut has generally only circular fibres. When, however, the longitudinal fibres arise behind the middle, then they separate from the digestive canal and are inserted a little behind the beginning of the chylific stomach. Hence there is formed an invagination of the proventriculus, which projects into the cavity of the stomach.

Schneider describes this process, which he calls the “beak,” as an invagination of the fore-stomach which projects into the cavity of the stomach. The two layers of the invagination in growing together form a beak varying in shape, being either simple or lobed and armed with bristles or teeth. This beak is tolerably large in Lepisma, Dermaptera (Forficula), Orthoptera, and in the larvæ and adults of Diptera, but smaller in the Neuroptera and Coleoptera, while in other insects it is wanting.

313Proventricular valvule.—Gehuchten also describes in Ptychoptera what he calls “the proventricular valvule,” stating that it is “a circular fold of the intestinal wall” (Fig. 310, vpr). He claims that it has not before been found, the “proventricular beak” of Schneider being regarded by him as the œsophageal valvule.

Fig. 316.—Digestive canal of Ptychoptera contaminata: gs, salivary glands; ra, œsophagus; pr, proventriculus; gt, crown of eight small tubular glands; im, mid-intestine; ga, two accessory white glands; vm, urinary vessels; ig, small intestine; gi, large intestine; r, rectum; A, the proventriculus in which the hinder end of the œsophagus extends as far as the chyle-stomach. B, longitudinal section of the proventricular region; sph, muscular ring or œsophageal sphincter; ppr, wall of the proventriculus; e, circular constriction dividing the cavity of the proventriculus in two; vpr, circular fold of the wall of the mid-intestine forming the proventricular valve; , œsophageal valve.—After Gehuchten.

The peritrophic membrane.—This membrane appears first to have been noticed by Ramdohr in 1811 in Hemerobius perla. It has been found by Schneider, who calls it the “funnel.” On the hinder end of the fore-stomach, he says, the cuticula forms a fold enclosing the outlet of the fore-stomach, and extending back like a tube to the anus. This “funnel,” he adds, occurs in a great number of insects. It has been found in Thysanura, but is wanting in Hemiptera. In the Coleoptera it is absent in Carabidæ and Dyticidæ. It is generally present in Diptera and in the larvæ of Lepidoptera, but not in 314the adults. In Hymenoptera it has been found in ants and wasps, but is absent in Cynipidæ, Ichneumonidæ, and Tenthredinidæ. All those insects (including their larvæ) possessing this funnel eat solid, indigestible food, while those which do not possess it take fluid nourishment. It is elastic, and firmly encloses the contents of the digestive tract. Until Schneider’s discovery of its general occurrence, it had only been known to exist in the viviparous Cecidomyia larvæ (Miastor). Wagner, its discoverer, noticed in the stomach of this insect a second tube which contained food. Pagenstecher was inclined to regard the tube as a secretion of the salivary glands. Metschnikoff, however, more correctly stated that the tube consisted of chitin, but he regarded it as adapted for the removal of the secretions. (Schneider.) Plateau, however, as well as Balbiani, the latter calling it the “peritropic membrane,” considers this membrane as a secretion of the chylific stomach, and that it is formed at the surface of the epithelial cells. It surrounds the food along the entire digestive tract, forming an envelope around the fæcal masses. On the other hand, Gehuchten states that in the larva of Ptychoptera its mode of origin differs from that described by Plateau and by Schneider, and that it is a product of secretion of special cells in the proventriculus.

The mid-intestine.—This section of the digestive canal, often, though erroneously, called the “chylific stomach” or ventriculus, differs not only in its embryonic history, but also in its structure and physiology from the fore and hind intestine of arthropods, and also presents no analogy to the stomach of the vertebrate animals. In insects it is a simple tube, not usually lined with chitin, since it is not formed by the invagination of the ectoderm, as are the fore and hind intestine, the absence of the chitinous intima promoting the absorption of soluble food. Into the anterior end either open two or more large cœcal tubes (Fig. 299), or its whole outer surface is beset with very numerous fine glandular filaments like villi (Fig. 317 and Fig. 329).

The mid-intestine varies much in size and shape; it is very long in the lamellicorn beetles (Melolontha and Geotrupes), and while in Meloë it is very large, occupying the greatest part of the body-cavity, in the longicorn beetles and in Lepidoptera it is very small. The pyloric end consists of an internal circular fold projecting into the cavity. In the Psocidæ (Cæcilius) the pyloric end is prolonged into a slender tube nearly as long as the larger anterior portion.

The limits between the mid and hind intestine are in some insects difficult to define, the urinary tubes sometimes appearing to open 315into the end of the mid-intestine (“stomach”). The latter also is sometimes lined with an intima. The limits are also determined by a circular projection, directly behind which is an enlargement of the intestine in the shape of a trench (rigole), or circular cul-de-sac (the “pyloric valvule” of some authors, including Beauregard), while the walls of the small intestine contract so as to produce a considerable constriction of the cavity of the canal. This constriction exactly coincides with the beginning of the double layer of circular muscles in the wall of the small intestine. An internal layer, which is the continuation of the circular muscles of the chylific stomach, and an external layer much more developed probably belong to this part of the alimentary canal. Since the homologue of the circular fold occurs in the locust as well as in Diptera, it is probably common to insects in general.

Fig. 317.—Digestive canal of Carabus monilis: h, œsophagus; i, gizzard or proventriculus; k, “stomach,” with its cœca (r); p, urinary tubes; q, their point of insertion; m, n, colon, with cœcal glands; s, anal glands; a, b, c, a gastric cœcum; a, b, portion of lining of gizzard.—After Newport.

Fig. 318.—Digestive canal of Meloe: sch, œsophagus; Kl, œsophageal valve; mD, mid-intestine; eD, hind-intestine; Ei, eggs; g, sexual opening.—After Graber.

Gehuchten adds that the limit set by the circular projection does not exactly coincide with the opening into the intestine of the urinary tubes and the two annexed glands. He shows by a section (his Fig. 133) that the tubular glands open into the alimentary canal 316in front of the circular fold. It is the same with the Malpighian tubes. They are not, therefore, he claims, dependences of the terminal intestine, but of the mid-intestine. Beauregard has observed the same thing in the vesicating insects (Meloidæ). The Malpighian tubes, he says, open into the “chylific stomach” before the valvular crown. This arrangement does not seem to be general, because, according to Balbiani, the Malpighian vessels open into the beginning of the intestine in Cryptops. Compare also Minot’s account of the valve in locusts separating the stomach from the intestine, and in front of which the urinary or Malpighian tubes open.

Histology of the mid-intestine.—The walls of the stomach are composed of an internal epithelium, a layer of connective tissue, with two muscular layers, the inner of which is formed of unstriated circular muscular fibres, and the outer of striated longitudinal muscular fibres.

In the cockroach short processes are given off from the free ends of the epithelial cells, as in the intestine of many mammals and other animals. “Between the cells a reticulum is often to be seen, especially where the cells have burst; it extends between and among all the elements of the mucous lining, and probably serves, like the very similar structure met with in mammalian intestines, to absorb and conduct some of the products of digestion.” (Miall and Denny.)

Gehuchten shows that the epithelial lining of the mesenteron (chylific stomach) of the dipterous larva Ptychoptera is composed of two kinds of cells, i.e. secreting or glandular cells and absorbent cells, the former situated at each end of the stomach, and the absorbent cells occupying the middle region. The part played by these cells in digestion will be treated of beyond in the section on digestion. (See p. 327.)

The hind-intestine.—In many insects this is divided into the ileum, or short intestine, and the long intestine. The limit between the intestine and stomach is externally determined by the origin of the urinary tubes, which are outgrowths of the anterior end of the proctodæum. Like the fore-intestine the hind-intestine is lined with a thick muscular layer, and, as Gehuchten states, the passage from the epithelial lining of the stomach (mid-intestine) to the muscular lining of the intestine is abrupt.

Large intestine.—In Ptychoptera, as described by Gehuchten, there are no precise limits between the small and large intestine; the epithelium of the large intestine has a special character, and its constituents present a close resemblance to the absorbed cells of the chylific stomach, being like them large and polygonal. The muscular layer is not continuous, and is formed of longitudinal and circular fibres, the latter being the larger.

317The ileum—Though in most insects slender, and therefore called the small intestine, the ileum is in locusts (Fig. 298) and grasshoppers (Anabrus, Fig. 299) as thick as the stomach. In many carnivorous beetles (Dyticus, Fig. 320, il, and Necrophorus) it is very long, but rather slender and short in the Carabidæ and Cicindelidæ, as well as those insects whose food is liquid, such as Diptera. In the Lepidoptera it varies in length, being in Sphinx quite long and bent into seven folds (Fig. 309), while it is very short in the Psocidæ, Chrysomelidæ, and Tenthredinidæ.

In the locust the ileum is traversed by six longitudinal folds with intervening furrows; outside of each furrow is a longitudinal muscular band. Seen from the inner surface the epithelium has an unusual character, the cells in the middle of each of the flat folds being quite large, polygonal in outline, while towards the furrows the cells become very much smaller. The walls are double when seen in transverse section, the inner layer consisting of epithelial cells resting on connective tissue, the outer layer formed of circular muscles. The cuticula is thin, but probably chitinous; it resembles that on the gastro-ileal folds, except that there are no spinules, but unlike the cuticula of the stomach it extends equally over the folds and the furrows. (Minot.) In the cockroach the junction of the small intestine with the colon is abrupt, a well-developed annular fold assuming the nature of a circular valve. (Miall and Denny.)

The gastro-ileal folds.—In the locust the intestine is separated from the chylific stomach by what Minot calls “the gastro-ileal folds,” which form a peculiar valve. The urinary vessels open just underneath and in front of this valve. In Melanoplus, and probably in the entire family of Acrydiidæ, they are indicated as “dark spots, round in front and lying at the anterior end of the ileum so as to form a ring around the interior of the intestine.” They are 12 in number, and all alike. They are pigmented and round in front where they are broadest and stand up highest; they narrow down backwards, the pigment disappears, and they gradually fade out into the ileal folds. Directly beneath them, and just at the posterior end of the stomach, there is a strong band of circular striated muscular fibres. The epithelium of these folds is covered with minute conical spines, which are generally, but not always, wanting between the folds. (Minot.)

The colon.—This section of the intestine (Fig. 319) is sometimes regarded as a part of the rectum. In the locust the six longitudinal folds of the ileum are continued into the colon, but their surface, instead of being smooth as in the ileum, is thrown up into numerous 318irregular curved and zigzag secondary folds. The cells of the epithelium are of uniform size, and the layer is covered by a highly refringent cuticula without spines; and, like that in the ileum, it rests on a layer of connective tissue, beyond which follows (1) an internal coat of longitudinal, and (2) an external coat of circular striated muscular fibres. (Minot.)

Fig. 319.—Digestive canal of Lucanus cervus: G, anterior muscles of the pharynx; H, œsophagus; I, gizzard; K, chyle-stomach; L, ilium; M, colon (cœcal part of); N, colon; O, rectum; a, frontal ganglion of the vagus; b, vagus nerve; c, anterior lateral ganglion connected with the vagus.—After Newport.

In butterflies (Pontia brassicæ), in Sphinx ligustri, and probably in most Lepidoptera the colon is distinct from the rectum, and is anteriorly developed into a very large more or less pyriform or bladder-like cæcum (Figs. 309, 310), which in certain Coleoptera (Dyticus, Fig. 320, d; Silpha, Necrophorus, etc.) is of remarkable length and shape; it also occurs in Nepidæ (Fig. 327). In the cockroach a lateral cæcum “is occasionally, but not constantly, present towards its rectal end,” and a constriction divides the colon from the rectum. (Miall and Denny.)

The rectum.—The terminal section of the hind-gut varies in length and size, but is usually larger than the colon, and with thick, muscular walls. In Lepidoptera it is narrow and short.

The rectum is remarkable for containing structures called rectal glands (Fig. 298). Chun describes those of Locusta viridissima as six flat folds, formed by a high columnar epithelium and a distinct cuticula; there is a coat of circular bands corresponding to the furrows between the glands. Minot states that this description is applicable to the locusts (Acrydiidæ) he has investigated, the only difference being in the structural details of the single layers. He claims that the rectal folds “do not offer the least appearance of glandular structure,” neither is their function an absorbent one, as Chun supposed. From their structure and position Fernald regards the rectal glands of Passalus as acting like 319a valve, serving to retain the food in the absorptive portions of the digestive tract till all nutriment is extracted.

Fig. 320.Dyticus marginalis, ♂ opened from the back: a, crop; b, proventriculus; c, mid-intestine beset with fine cœcal glands; d, long cœcal appendage of the colon; apodemes; B1-B3, apodemes; vhm, coxal extensor muscle, moving the hind leg; ho, testis; dr, accessory gland; r, penis; e, reservoir of the secretion of the anal gland.—After Graber.

The epithelial folds of the larvæ of dragon-flies serve as organs of respiration, the water being admitted into this cavity, and when forcibly expelled serving to propel the creature forward. Paired and single anal glands (repugnatorial) enter the rectum of certain Coleoptera (Figs. 302, l; 317, s; 320, e).

The vent (anus).—The external opening of the rectum is situated in the end of the body, in the vestigial 10th or 11th abdominal segment, 320and is more or less eversible. It is protected above in caterpillars, and other insects with 10 free abdominal segments, by the suranal plate. It is bounded on the sides by the paranal lobes, while beneath is the infra-anal lobe.

The anus is wanting in certain insects, and where this is the case the hind-gut, owing to a retention of the embryonic condition, is usually separated from the mid-intestine. (See p. 300.)

Fig. 321.—Enteric canal of Psyllopsis fraxinicola: œ, œsophagus; md, mid-intestine; ed, hind-intestine; vm, urinary vessels; s, the coil formed by the hind-intestine and the most anterior part of the mid-intestine.—After Witlaczil, from Lang.

Some remarkable features of the digestive canal in hemipterous insects are noteworthy. In the Coccidæ, according to Mark, the anterior end of the long mid-intestine forms, with the hinder end of the œsophagus, a small loop, whose posterior end is firmly grown to the wall of the rectum, and forms a cup-like invagination of the latter. Then the rest of the tube-like stomach turns sidewise and forms a large loop, which turns back on itself and occupies a large part of the body-cavity. This loop receives on the anterior end, near the œsophagus, the two urinary vessels, and forms just below the opening into the rectum a short cæcum.

In other homopterous genera (Psyllidæ and some Cicadidæ) Witlaczil describes nearly the same peculiarity, the mid-gut and part of the intestine forming a loop growing together for a certain distance and winding round each other (Fig. 321).

Histology of the digestive canal.—In all the divisions of the digestive canal of insects the succession of the cellular layers composing it is the same: 1st, a cuticula; 2d, an epithelium; 3d, connective tissue; 4th, muscular tissue. In the locust, the first division of the canal (fore-gut), there are two muscular coats, an internal longitudinal and an external circular coat; the fibres are all striated. The lining epithelium is not much developed, but forms a thick, hard, and refringent cuticula, which is thrown up into spiny ridges. In the second division (mid-gut, “stomach”) the epithelium is composed of very high columnar cells, which make up the greater part of the thickness of the walls, while the cuticula is very delicate, slightly refringent, with no ridges, and is probably not chitinous; the fibres of the muscular coats are not striated, while this division is also distinguished by the presence of glandular follicles and folds. The stomach and the cæcal appendages have all these peculiarities in common, while no other part of the canal is thus characterized.

321The third division (intestine and rectum) is composed of an epithelium, the cells of which are intermediate in size between those of the fore and mid gut. The cells are often pigmented, and they are covered by a much thicker cuticula than that of the stomach, but which is not so thick and hard as that of the œsophagus and proventriculus. The very refringent cuticula is not thrown up into ridges, though in some parts it is covered with delicate conical spines, which are very short. “The epithelium and underlying connective tissue (tunica propria) are thrown up into six folds, which run longitudinally, being regular in the ileum and rectum (as the rectal glands), but very irregular in the colon. Outside the depression between each two neighboring folds there is a longitudinal muscular band, these making six bands. This peculiar disposition of the longitudinal muscles does not occur in any other part of the canal; it is, therefore, especially characteristic of the third division.” (Minot.)

LITERATURE ON THE ORGANS OF DIGESTION

Treviranus, G. R. Resultate einiger Untersuchungen über den inneren Bau der Insekten. (Verdauungsorgane von Cimex rufipes.) (Annal. d. Wetterau. Gesells., 1809, i, pp. 169–177, 1 Taf.)

Ramdohr, C. A. Abhandlungen über die Verdauungswerkzeuge der Insekten. 1811, vii, pp. 221, 30 Taf.

Dutrochet, R. J. H. Mémoire sur les métamorphoses du canal alimentaire dans les insectes. (Journal de Physique, 1818, lxxxvi, pp. 130–135, 189–204; Meckel’s Archiv, 1818, iv, pp. 285–293.)

Suckow, F. W. L. Verdauungsorgane der Insekten. (Heusinger’s Zeitschr. f. organ. Physik., 1828, iii, pp. 1–89.)

Doyère, L. Note sur le tube digestif des Cigales. (Ann. Sc. nat. Zool., 1839, 2 Sér., xi, pp. 81–85.)

Grube, A. E. Fehlt den Wespen- oder Hornissenlarven ein After oder nicht? 1 Taf. (Müller’s Archiv für Physiol., 1849, pp. 47–74.)

Sirodot. Recherches sur les sécrétions chez les insectes. (Ann. Sc. nat. Zool., 1858, 4 Sér., x, pp. 141–189, 251–328, 12 Pls.)

Milne-Edwards, H. Leçons sur la physiologie et l’anatomie comparée, v, 1859, pp. 498–536, 581–638.

Dufour, L. Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes Coléoptères. Appareil digestif. (Ann. Sc. nat., ii, 1824, pp. 462–482, 2 Pls.; iii, 1824, pp. 215–242, 5 Pls., pp. 476–491, 3 Pls.; iv, 1824, pp. 103–125, 4 Pls.; iv, 1825, pp. 265–283.)

—— Recherches anatomiques sur l’Hippobosque des chevaux. (Ann. Sc. nat., 1825, vi, pp. 299–322, 1 Pl.)

—— Description et figure de l’appareil digestif de l’Anobium striatum. (Ibid., xiv, 1828, pp. 219–222, 1 Pl.)

—— Recherches anatomiques sur les Labidoures. Appareil de la digestion. (Ibid., xiii, 1828, pp. 348–354, 2 Pls.)

322—— Recherches anatomiques et considerations entomologiques sur quelques insectes Coléoptères, compris dans les familles des Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles. Appareil digestif. (Ibid., Sér. 2, Zool., i, 1834, pp. 67–76, 2 Pls.)

—— Résumé des recherches anatomiques et physiologiques sur les Hémiptères. (Ibid., pp. 232–239.)

—— Mémoire sur les métamorphoses et l’anatomie de la Pyrochroa coccinea. Appareil digestif. (Ibid., Sér. 2, Zool., xiii, 1840, pp. 328–330, 334–337, 2 Pls.)

—— Histoire comparative des metamorphoses et de l’anatomie des Cetonia aurata et Dorcus parallelepipedus. Appareil digestif. (Ibid., Sér. 2, Zool., 1824, xviii, pp. 174–176, 2 Pls.)

—— Anatomie générale des Diptères. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1814, pp. 248, 249.)

—— Histoire des métamorphoses et de l’anatomie du Piophila petasionis. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1844, pp. 372–377, 1 Pl.)

—— Études anatomiques et physiologiques sur les insectes Diptères de la famille des Pupipares. Appareil digestif. (Ibid., Sér. 3, Zool., iii, 1845, pp. 67–73, 1 Pl.)

—— Recherches sur l’anatomie et l’histoire naturelle de l’Osmylus maculatus. Appareil digestif. (Ibid., Sér. 3, Zool., ix, 1848, pp. 346–349, 1 Pl.)

—— Études anatomiques et physiologiques, et observations sur les larves des Libellules. Appareil digestif. (Ibid., Sér. 3, Zool., xvii, 1852, pp. 101–108, 1 Pl.)

—— Recherches anatomiques sur les Hyménoptères de la famille des Urocerates. Appareil digestif. (Ibid., Sér. 4, Zool., i, 1854, pp. 212–216, 1 Pl.)

—— Fragments d’anatomie entomologique. Sur l’appareil digestif du Nemoptera lusitanica. (Ibid., Sér. 4, viii, 1857, pp. 6–9, 1 Pl.)

—— Recherches anatomique et considerations entomologiques sur les Hémiptères du genre Leptopus. Appareil digestif. (Ibid., Sér. 4, Zool., 1858, x, pp. 352–356, 1 Pl.)

—— Recherches anatomiques sur l’Ascalaphus meridionalis. Appareil digestif. (Ibid., Sér. 4, xiii, 1860, pp. 200–202, 1 Pl.)

Leydig, F. Zur Anatomie von Coccus hesperidum. (Zeitschr. f. wissens. Zool., v, 1853, pp. 1–12, 1 Taf.)

Lubbock, J. On the digestive and nervous System of Coccus hesperidum. (Proc. Roy. Soc., ix, 1886, pp. 480–486; also Ann. Mag. Nat. Hist., 1859, Ser. 3, iii, pp. 306–311.)

Scheiber, S. H. Vergleichende Anatomie und Physiologie der Œstriden-Larven. V. Das chylo- und uropœtische System. (Sitzber. d. k. Akad. d. Wissens. Wien. Math.-naturwiss. Cl., 1862, xlv, pp. 39–64, 1 Taf.)

Gerstaecker, A. Bronn’s Klassen und Ordnungen des Tierreichs. V. Gliederfüssler. (Ernährungsorgane, pp. 87–105.)

Graber, V. Zur naheren Kenntnis des Proventriculus und der Appendices ventriculares bei den Grillen und Laubheuschrecken. (Sitzber. d. k. Akad. d. Wissensch. Wien. Mathem.-naturwiss. Cl., lix, 1869, pp. 29–46, 3 Taf.)

—— Ueber die Ernährungsorgane der Insekten und nächstverwandten Gliederfüssler. (Mitteil. d. naturwiss. Vereins für Steiermark. Graz, 1871, ii, pp. 181, 182.)

—— Verdauungssystem des Prachtkäfers. (Ibid., Graz, 1875.)

—— Die Insekten., i, 1877. (Verdauungsapparat, pp. 308–328.)

Wilde, K. F. Untersuchungen über den Kaumagen der Orthopteren. (Archiv f. Naturgesch., xliii Jahrg., 1877, pp. 135–172, 3 Taf.)

323Simroth, H. Ueber den Darmkanal der Larven von Osmoderma eremita mit seinen Anhängen. (Giebel’s Zeitschr. f. d. ges. Naturwiss., 1878, li, pp. 493–518, 3 Taf.)

Müller, H. Ueber die angebliche Afterlösigkeit der Bienenlarven. (Zool. Anzeiger, 1881, pp. 530, 531.)

Schiemenz, Paulus. Ueber das Herkommen des Futtersaftes und die Speicheldrüsen der Bienen, nebst einem Anhänge über das Riechorgan. (Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)

Rovelli, G. Alcune ricerche sul tubo digerente degli Atteri, Ortotteri e Pseudo-Neurotteri. (Como, 1884, p. 15.)

Beauregard, H. Structure de l’appareil digestif des Insectes de la tribu des Vésicants. (Compt. rend. Acad. Paris, 1884, xcix, pp. 1083–1086.)

—— Recherches sur les Insectes vésicants., 1 Part, Anatomie. (Journ. Anat. Phys. Paris, 1885, xxi Année, pp. 483–524, 4 Pls.; 1886, xxii Année, pp. 85–108, 242–284, 5 Pls.)

—— Les Insectes vésicants, Paris, 1890, Chap. III, Appareil digestif, pp. 63–99; (Phénomènes digestifs, pp. 161–170; Pls. 6–9.)

Wertheimer, L. Sur la structure du tube digestif de l’Oryctes nasicornis. (Compt. rend. Soc. Biol. Paris, 1887, Sér. 8, iv, pp. 531, 532.)

Kowalevsky, A. Beitrage zur Kenntniss der nachembryonal Entwicklung der Musciden. (Zeitschr. f. wissens. Zool., xlv, 1887, pp. 542–594, 5 Taf.)

Schneider, A. Ueber den Darm der Arthropoden, besonders der Insekten. (Zool. Anzeiger, 1887, x Jahrg., pp. 139, 140.)

—— Ueber den Darmkanal der Arthropoden. (Zool. Beiträge von A. Schneider, ii, 1887, pp. 82–96, 3 Taf.)

Fritze, A. Ueber den Darmkanal der Ephemeriden. (Berichte der Naturforsch.-Gesellsch. zu Freiburg i. Br., 1888, iv, pp. 59–82, 2 Taf.)

Emery, C. Ueber den sogenannten Kaumagen einiger Ameisen. (Zeitschr. f. wissens. Zool., 1888, xlvi, pp. 378–412, 3 Taf.)

Meinert, F. Contribution à l’anatomie des Fourmilions. (Overs. Danske Vidensk. Selsk. Forhandl. Kjöbenhavn, 1889, pp. 43–66, 2 Pls.)

Mingazzini, P. Richerche sul canale digerente dei Lamellicorni fitofage (Larve e Insetti perfetti). (Mitteil. Zool. Station zu Neapel, ix, 1889–1891, pp. 1–112, 266–304, 7 Pls.)

Fernald, Henry T. Rectal glands in Coleoptera. (Amer. Naturalist, xxiv, pp. 100, 101, Jan., 1890.)

Visart, O. Digestive canal of Orthoptera. (Atti Soc. Toscana Scient. Natur., vii, 1891, pp. 277–285.)

Eberli, J. Untersuchungen an Verdauungstrakten von Gryllotalpa vulgaris. (Vierteljahresschr. d. Naturforsch. Gesells. Zurich, 1892, Sep., p. 46, Fig.)

Holmgren, Emil. Histologiska studier öfver några lepidopterlarvers digestionskanal och en del af deras Körtelartade bildningar. (Ent. Tidskr. Årg. xiii, pp. 129–170, 1892, 6 Pls.)

Ris, F. Untersuchung über die Gestalt des Kaumagens bei den Libellen und ihren Larven. (Zool. Jahrb. Abth. Syst., ix, 1896, pp. 596–624, 13 Figs.)

See also the works of Straus-Dürckheim, Newport, Mark, Witlaczil, Vayssière, Landois, Jordan, Oudemans, Berlese, List, Grassi, Verson, Miall and Denny, Leidy, Cheshire, Kowalevsky, Gehuchten, Locy, etc.

324

b. Digestion in insects

For the most complete and reliable investigation of the process of digestion, we are indebted to Plateau, whose results we give, besides the conclusions of later authors:

In mandibulate or biting insects, the food is conducted through the œsophagus by means of the muscular coating of this part of the digestive canal. Suctorial insects draw in their liquid food by the contractions followed by the dilatations of the mid-intestine (chylific stomach). Dragon-flies, Orthoptera, and Lepidoptera swallow some air with their food.

Where the salivary glands are present, the neutral alkaline fluid secreted by them has the same property as the salivary fluid of vertebrates of rapidly transforming starchy foods into soluble and assimilable glucose. In such forms as have no salivary glands, their place is almost always supplied by an epithelial lining of the œsophagus, or, as in the Hydrophilidæ, a fluid is secreted which has the same function as the true salivary fluid.

Nagel states that the saliva of the larva of Dyticus is powerfully digestive, and has a marked poisonous action, killing other insects, and even tadpoles of twice the size of the attacking larva, very rapidly. The larvæ not only suck the blood of their victims, but absorb the proteid substances. Drops of salivary juice seem to paralyze the victim, and to ferment the proteids. The secretion is neutral, the digestion tryptic. Similar extra-oral digestion seems to occur in larvæ of ant-lions, etc. (Biol. Centralbl., xvi, 1896, pp. 51–57, 103–112; Journ. Roy. Micr. Soc., 1896, p. 184.)

In carnivorous insects and in Orthoptera, the œsophagus dilates into a crop (ingluvies) ended by a narrow, valvular apparatus (or gizzard of authors). The food, more or less divided by the jaws, accumulates in the crop, which is very distensible; and, when the food is penetrated by the neutral or alkaline liquid, there undergoes an evident digestive action resulting, in carnivorous insects, in the transformation of albuminoid substances into soluble and assimilable matter analogous to peptones, and, in herbivorous insects, an abundant production of sugar from starch. This digestion in the crop, a food-reservoir, is very slow, and, until it is ended, the rest of the digestive canal remains empty.

“Any decided acidity found in the crop is due to the injection of acid food; but a very faint acidity may occur, which results from the presence in the crop of a fluid secreted by the cæcal diverticula of the mesenteron.” (Miall and Denny.)

325When digestion in the crop is accomplished, the matters are subjected to an energetic pressure of the walls through peristaltic contractions, and then, guided by the furrows and chitinous teeth, pass along or gradually filter through the valvular apparatus or proventriculus, whose function is that of a strainer.

At the beginning of the “chyle-stomach” (mesenteron) of Orthoptera are glandular cæca which secrete a feebly acid fluid. This fluid emulsifies fats, and converts albuminoids into peptones. It passes forwards into the crop, and there acts upon the food.

In the mesenteron (mid-intestine) the food is acted upon by an alkaline or neutral fluid, never acid, either secreted, as in Orthoptera, by local special glands, or by a multitude of minute glandular cæca, as in many Coleoptera, or by a simple epithelial layer. It has no analogy with the gastric juices of vertebrates; its function differs in insects of different groups; in carnivorous Coleoptera it actively emulsionizes greasy matters; in the Hydrophilidæ it continues the process of transformation of starch into glucose, begun in the œsophagus. In the Scarabæidæ, it also produces glucose, but this action is local, not occurring elsewhere; in caterpillars, it causes a production of glucose, and transforms the albuminoids into soluble and assimilable bodies analogous to peptones, and also emulsionizes greasy matters. Finally, in the herbivorous Orthoptera there does not seem to be any formation of sugar in the stomach itself, the production of glucose being confined to the crop (jabot).

When digestion in the crop is finished, the proventriculus relaxes, and the contents of the crop, now in a semi-fluid condition, guided by the furrows and teeth, passes into the mesenteron, which is without a chitinous lining, and is thus fitted for absorption.

The contents of the mid-intestine (chylific stomach) then slowly and gradually pass into the intestine, the first anterior portion of which, usually long and slender, is the seat of an active absorption. The epithelial lining observed in certain insects seems, however, to indicate that secondary digestion takes place in this section. The reaction of the contents is neutral or alkaline.

The second and larger division of the intestine only acts as a stercoral reservoir. (The voluminous cæcum occurring in Dyticidæ, Nepa, and Ranatra, whether full or empty, never contains gas, and it is not, as some have supposed, a swimming-bladder.) The liquid product secreted by the Malpighian tubes accumulates in this division, and, under certain circumstances, very large calculi are often formed. In his subsequent paper on the digestion of the cockroach, Plateau states that in the intestine are united the residue of 326the work of digestion and the secretion of the urinary or Malpighian tubes, this secretion being purely urinary.

These organs are exclusively depuratory and urinary, freeing the body from waste products of the organic elements. The liquid they secrete contains urea (?), uric acid and abundant urates, hippuric acid (?), chloride of sodium, phosphates, carbonate of lime, oxalate of lime in quantity, leucine, and coloring-matters.

The products of the rectal or anal glands vary much in different groups, but they take no part in digestion, nor are they depuratory in their nature.

Insects have nothing resembling chylific substances.[51] The products of digestion, dissolved salts, peptones, sugar in solution, emulsionized greasy matters, pass through the relatively delicate walls of the digestive canal by osmose, and mingle outside of the canal with the blood.

Whatever substances remain undigested are expelled with the excrements; such are the chitin of the integuments of insects, vegetable cellulose, and chlorophyll, which is detected by the microspectroscope all along the digestive canal of phytophagous insects.

In his experiments in feeding the larvæ of Musca with lacmus, Kowalevsky found that the œsophagus, food-reservoir, and proventriculus, with its cæcal appendages, always remained blue, and had an alkaline reaction; the mid-intestine, also, in its anterior portion, remained blue, but a portion of its posterior half became deep red, and also exhibited a strong reaction. The hind-intestine, however, always remained blue, and also had an alkaline reaction. (Biol. Centralbl., ix, 1889, p. 46.)

The mechanism of secretion.—Gehuchten describes the process of secretion in insects, the following extract being taken from his researches on the digestive apparatus of the larva of Ptychoptera. The products of secretion poured into the alimentary canal are more or less fluid; for this reason, it is impossible to say when an epithelial cell at rest contains these products. For the secreting nature of these cells is only apparent at the moment when they are ready 327for excretion; then the cellular membrane swells out, and a part of the protoplasmic body projects into the intestinal cavity.

Before going farther, the terms secretion and excretion should, he says, be defined. With Ranvier, he believes that the elaboration in the protoplasm of a definite fluid substance is, par excellence, the secretory act, while the removal of this substance is the act of excretion.

Fig. 322.—Different phases of the mechanism of secretion and of excretion.—After Gehuchten.

A glandular cell of the chylific stomach, when at rest, is always furnished with a striated “platform,” or flat surface, or face, on the side facing the cavity of the stomach, and the free edge of the platform, or plateau, is provided with filaments projecting into the digestive cavity (Fig. 322, f). These glandular cells, when active, differ much in appearance. In a great number, the platform (plateau) has disappeared, and is replaced by a simple, regular membrane. During the process of secretion, a finely granular mass, in direct continuity with the protoplasm, swells, and raises the membrane over the entire breadth of the cell, causing it to project into the intestinal cavity (Fig. 322, A, B). These vesicles, or drops of the secretion, whether free or still attached by a web to the cells, are clear and transparent in the living insect, but granular in the portions of the digestive canal fixed for cutting into sections. Gehuchten then asks: “How does a cell gorged with the products of secretion empty itself?” Both Ranvier and also Heidenhain believe that one and the same glandular cell may secrete and excrete several times without undergoing destruction, but their researches 328made on salivary glands have not answered the question. Gehuchten explains the process thus: when the epithelial cell begins to secrete, the clear fluid elaborated in the protoplasm of the cell increases the intracellular tension, until, finally, the fluid breaks through certain weak places in the swollen basal membrane of the platform, and then easily passes through the closely crowded filaments, and projects out into the intestinal cavity as a pear-shaped vesicle of a liquid rich in albumens at first attached to the free face of the cell, but finally becoming free, as at Fig. 322, A, B.

When the elaboration of the substance to be secreted is more active, the mechanism of the secretion is modified. The basal membrane of the platform may then be raised at several places at once; instead of a single vesicle projecting into the intestinal cavity, each cell may present a great number more or less voluminous. If all remain small and rapidly detach themselves from the glandular cell, the filaments of the platform are simply separated from each other at different points of the free face, as in Fig. 322, C. On the other hand, when the different vesicles of a single cell become larger, the filaments of the platform are compressed and crowded against each other in the spaces between the vesicles remaining free, and the undisturbed portions of the platform appear homogeneous (Fig. 322, D). After the excretion of the secretory products by this process of strangulation, the cell then assumes the aspect of a glandular cell at rest, and may begin again to form a new secretion.

To sum up: The process of excretion may occur in two ways:

1. Where the membrane ruptures and the substances secreted are sent directly out into the digestive cavity. 2. Where the vesicles become free by strangulation, floating in the glandular or intestinal cavity, and ending by rupturing and coming into contact with the neighboring vesicles or with the food.

Absorbent cells.—Besides the glandular or secreting cells in Ptychoptera, there is between the two regions of the chyle-stomach lined with these cells a region about a centimetre long composed of absorbent cells. The absorbent cells are very large, polygonal, and contain a large nucleus, in which is a striated convoluted chromatic cord.

The food on entering the chyle-stomach is brought into contact with the products secreted in the proventriculus, in the first part of the chyle-stomach, and in the tubular glands. These products of secretion act on the food, extracting from them useful substances which they render soluble. These substances, after having been absorbed by the absorbent cells in the middle region of the stomach, 329undergo special modifications, and are transformed into solid products, which are situated at the bottom of these cells. Afterwards the alimentary substances freed from a portion of their useful substances are again placed in contact with the products of secretion in the distal part of the chylific ventricle, and reach the terminal part of the intestine.

“The products of secretion,” adds Gehuchten, “diverted into the intestinal canal do not come into immediate contact with the alimentary substances; they are separated from it by a continuous, structureless, quite thick membrane (the peritrophic membrane), which directly envelops the cylinder of food matters, extending from the orifice of the œsophageal valvule to the end of the intestine. Between this membrane and the free face of the epithelial lining there exists a circular space, into which are thrown and accumulate the excreted substances. The latter then cannot directly mingle with the aliments; but when they are liquid they undoubtedly pass through this membrane by osmose, and thus come into contact with the nutritive substances. It is the same with the products of absorption. The absorption of soluble products of the intestinal cavity is not then so simple a phenomenon as it was at first thought to be, since these products are nowhere brought into immediate contact with the absorbent cells” (pp. 90, 91).

The most recent authority, Cuénot, states that absorption of the products of digestion takes place entirely in the mid-intestine, and in its cæca when these are present. The mid-intestine exercises a selective action on the constituents of the food comparable to the action of the vertebrate liver.

LITERATURE ON THE PHYSIOLOGY OF DIGESTION

Davy, J. Note on the excrements of certain insects, and on the urinary excrement of insects. (Edinburgh New Phil. Journ., 1846, xl, pp. 231–234, 335–340; 1848, xlv, pp. 17–29.)

—— Some observations on the excrements of insects, in a letter addressed to W. Spence. (Trans. Ent. Soc. London, Ser. 2, iii, 1854, pp. 18–32.)

Bouchardat, A. De la digestion chez le ver à soie. (Revue et Mag. de Zool., Sér. 2, 1851, iii, pp. 34–40.)

Lacaze-Duthiers, H., et A. Riche. Mémoire sur l’alimentation de quelques insects gallicoles et sur la production de la graisse. (Ann. Scienc. natur., 1854, Sér. 4, ii, pp. 81–105.)

Basch, S. Untersuchungen über das chylopoetische und uropoetische System von Blatta orientalis. (Sitzungsber. d. math.-naturwiss. Classe d. Akad. d. Wissensch. Wien., 1858, xxxiii, pp. 234–260, 5 Taf.)

Lambrecht, A. Der Verdauungsprozess der stickstoffreichen Nährmittel, welche unsere Bienen geniessen, in den dazu geschaffenen Organen derselben. (Bienenwirtschaftl. Centralbl., viii Jahrg., 1872, pp. 73–78, 83–89.)

330Plateau, F. Recherches sur les phénomènes de la digestion chez les insectes, (Mém. Acad. roy. de Belgique, Sér. 2, xli, 1 Part, 1873, pp. 124, 3 Pls.)

—— Note additionelle au mémoire sur les phénomènes de la digestion chez les insectes. (Bull. Acad. roy. de Belgique, Sér. 2, xliv, 1877, pp. 710–733.)

Tursini, G. Fr. Un primo passo nella ricerca dell’ assorbimento intestinale degli artropodi. (Rend. d. R. Accadem. di Sc. fis. e matemat. di Napoli, 1877, xvi, pp. 95–99, 1 Pl.)

Jousset de Bellesme, Physiologie comparée. Recherches expérimentales sur la digestion des insectes et en particulier de la blatte. Paris, 1876, vii, and 96 pp., 3 Pls.

—— Recherches sur les fonctions des glandes de l’appareil digestif des Insectes. (Compt. rend., lxxxii, Paris, 1876, pp. 97–99.)

—— Travaux originaux de physiologie comparée. (i, Insectes, Digestion, Métamorphoses.) Paris, 1878, 5 Pls.

Simroth, H. Einige Bemerkungen über die Verdauung der Kerfe. (Zeitschr. f. d. gesammten Naturwiss., xli, 1878, pp. 826–831.)

Krukenberg, C. Fr. W. Versuche zur vergleichenden Physiologie der Verdauung und vergleichende physiologische Beiträge zur Kenntnis der Verdauungsvorgange. (Untersuch, a. d. physiolog. Institut d. Universität Heidelberg, 1880, i, 4, pp 327, Figs.; ii, 1, p. 1, Figs.)

Metschnikoff, E. Untersuchungen über die intrazelluläre Verdauung bei wirbellosen Tieren. (Arb. d. zool. Instit. Wien., 1883, v, pp. 141–168, 2 Taf.)

Locy, William A. Anatomy and physiology of the family Nepidæ. (Amer. Naturalist, xviii, 1884, pp. 250–255, 353–367, 4 Pls.)

Vangel, E. Beiträge zur Anatomie, Histiologie, und Physiologie des Verdauungsapparates des Wasserkäfers, Hydrophilus piceus. (Termész. Füzet., x, 1886, pp. 111–126 (in Hungarian); pp. 190–208 (in German), 1 Pl.)

Schönfeld. Die physiologische Bedeutung des Magenmundes der Hönigbiene. (Archiv f. Anat. u. Physiol., Physiol. Abt., 1886, pp. 451–458.)

Faussek, V. Beiträge zur Histiologie des Darmkanals der Insekten. (Zeitschr. f. wiss. Zool., 1887, xl, pp. 694–712, 1 Taf.; Abstract in Zool. Anz., Jahrg. x, pp. 322, 323, 1 Taf.)

Frenzel, J. Ueber Bau und Thätigkeit des Verdauungskanals der Larve des Tenebrio molitor, mit Berücksichtigung anderer Arthropoden (Berlin. Ent. Zeitschr., 1882, pp. 267–316, 1 Taf.); Inaug.-Diss. Göttingen, 1882.

—— Einiges über den Mitteldarm der Insekten, sowie über Epithel-regeneration. (Archiv f. Mikrosk. Anat., 1885, xxvi, pp. 229–306, 3 Taf.)

—— Zum feineren Bau des Wimperapparates. (Ibid., 1886, xxviii, pp. 53–80, 1 Taf.)

—— Die Verdauung lebenden Gewebes und die Darmparasiten. (Archiv f. Anat., 1891.)

Gehuchten, A. van. Recherches histologiques sur l’appareil digestif de la Ptychoptera contaminata, I Part. Étude du revêtment épithélial et recherches sur la sécrétion. (La Cellule, 1890, vi, pp. 183–291, 6 Pls.)

Cuénot, L. Études physiologiques sur les Orthoptères. (Arch. Biol., xiv, 1895, pp. 293–341, 2 Pls.)

Needham, James G. The digestive epithelium of dragon-fly nymphs. (Zool. Bull., i, 1897, Chicago, pp. 104–113, 10 Figs.)

With the writings of Mingazzini (see p. 323), Kowalevsky, Ranvier, Haidenhain, Beauregard (p. 323), Sadones.

331

THE GLANDULAR AND EXCRETORY APPENDAGES OF THE DIGESTIVE CANAL

Into each primary division of the digestive canal open important glands. The salivary and silk-glands are offshoots of the œsophagus (stomodæum); the cœcal appendages open into the stomach (mesenteron), while the urinary tubes grow out in embryonic life from the primitive intestine (proctodæum), and there are other small glands which are connected with the end of the hind-intestine.

a. The salivary glands

We will begin our account of these glands with those of the Orthoptera, where they are well developed. In the cockroach a large salivary gland and accompanying reservoir lie on each side of the œsophagus and crop. The gland is a thin, leaf-like, lobulated mass, divided into two principal lobes. These open into a common trunk, which after receiving a branch from a small accessory lobe, and from the salivary reservoir, unites with its fellow to form the unpaired salivary duct which opens into the under side of the lingua. Each salivary reservoir is a large oval sac with transparent walls. (Miall and Denny, also Figs. 299, sr, and 327.) The ducts and reservoirs have a chitinous lining, and the ducts are, like the tracheæ, surrounded by a so-called spiral thread, or by separate, incomplete, hooplike bands, which serve to keep the duct permanently distended. In the locust (Fig. 298) the lobules are more scattered, forming small separate groups of acinose glands. In the embryo of Forficula Heymons has observed a pair of salivary glands opening on the inner angle of the mandibles, a second pair opening in the second maxillæ, while a third pair of glands, whose function is doubtful, is situated in the hinder part of the head, opening to the right and left on the chitinous plate (postgula) behind the submentum. In Perla, there are two pairs segmentally arranged (Fig. 343).

332

Fig. 323.—Left side of the head of the silkworm: a, adductor muscle of the mandible, from which the muscular fibres have been removed; b, upper fibres of the same; c, lower fibres cut away to show the adductor muscle (e); d, fibres inserted on the accessory adductor lamella; f, œsophagus, much swollen; g, salivary gland; h, dorsal vessel; i, l, tracheæ of the mandibular muscles; k, trachea; n, optic nerve.—After Blanc.

Fig. 324.—Lower side of the head of the silkworm exposed, the spinning apparatus, the œsophageal ganglion, and the adductor of the left mandible removed: M, mandible; P, abductor of the mandible; R, adductor; N, salivary gland attached at O to the edge of the adductor muscle; o, o, transverse portion of the “hyoid”; 3, masticator nerve and its recurrent branch (7); L, tongue cut horizontally.—After Blanc.

Here we might refer to a pair of glands regarded by Blanc as the true salivary glands. They do not appear to be the homologues of the salivary glands of other insects, though probably functioning as such. The functional salivary glands of lepidopterous larvæ have been overlooked by most entomotomists, and the spinning glands have been, it seems to us, correctly supposed to be modified salivary glands. Lucas also regards those of case-worms (Trichoptera) as morphologically salivary glands. Those of the silkworm were figured by Réaumur (Tom. i, Pl. v, Fig. 1), but not described; while those of Cossus, which are voluminous, were regarded by Lyonet as “vaisseaux dissolvans.” Dr. Auzoux (1849), in his celebrated model of the silkworm, represented them accurately, while Cornalia briefly described them as opening into the mouth. The first satisfactory description is that of Blanc (1891), who states that in the silkworm “the two salivary glands” are small, flexuous, yellow tubes, which occupy a variable position on the sides of the œsophagus (Fig. 323). The glandular portion passes into the head, ending at the level of the adductor plate of the mandibles (Fig. 324, o), and entering the buccal cavity at the base of the mandible, as seen in Fig. 323. It is plain, when we recognize the direct homology of the silk-glands of the caterpillars with the salivary glands of other insects, and of the spinneret with the hypopharynx, that 333these so-called “salivary glands” in lepidopterous larvæ are different structures. They are probably modified coxal glands, belonging to the mandibular segment.

Fig. 325.—One of the two salivary glands of Cæcilius burmeisteri: d, excretory duct; cn, the lumen or canal; cg, gland-cells; ct, salivary fluid.—After Kolbe.

The polygonal epithelial cells of these glands contain branched nuclei, recalling those of the spinning-glands. In those caterpillars which feed on leaves, the salivary glands are slightly developed, but in such as bore into and eat wood, as the Cossidæ, the glands are, as figured by Lyonet, very large, forming two sausage-shaped bodies passing back to the beginning of the mid-intestine, each ending in a long convoluted filament. The salivary glands of the imago are very long and convoluted (Fig. 310, sd).

In the Panorpidæ these glands differ in the sexes, the males having three pairs of very long tortuous tubes, while, in the females, they are reduced to two indistinct vesicles. (Siebold.)

In the Diptera in general there are two pairs, one situated in the beak, the other in the thorax. In the larvæ there is a single pair (Fig. 341). Kraepelin describes a third pair in the Muscidæ at the point of transition from the fulcrum to the œsophagus, but Knüppel has apparently found only what may be fat cells at this point, so that the supposed presence of a third pair in Diptera needs confirmation. In the Psocidæ there are two salivary glands, of simple tubular shape (Fig. 325).

In the Nepidæ the salivary glands are four in number, and of conglomerate structure, two being long and extending back into the beginning of the abdomen, while the other two are about one-fourth as long. (Figs. 327, 328.) In Cicada, besides a pair of simple tortuous tubes, there is in the head another pair of glands, each composed of two tufts of short lobes, situated one behind the other. (Dufour.) In many Hemiptera (Pyrrhocoris, Capsus, etc.) there is but a single pair, each gland consisting of four lobes; in the Coccidæ each gland is divided into two lobes (Fig. 326); in the Aphidæ, according to Witlaczil, they consist of two lobes grown together. In the Psyllidæ they are said to be absent.

In Phylloxera vastatrix the saliva is forced through a salivary 334passage out of the duct and into the mouth by a pumping apparatus furnished with special muscles. (Krassilstschik.)

In the Odonata acinose glands are present in the imago, but not in the nymph until in its last stage, Poletaiew accounting for their absence in the earlier stages by the fact that the larva swallows more or less water while taking its food.

In the Coleoptera, as we have observed in Anopthalmus, there are three pairs of salivary glands (Fig. 74). In the Blapsidæ these glands consist of many ramifying tubes united on each side of the œsophagus into a single duct; in others they are but slightly developed, while in still others they are wanting.

The salivary glands are most highly differentiated in the Hymenoptera, and especially in the bees (Bombus and Apis), where Schiemenz found not less than five systems of glands (Fig. 329; also 87), of which four systems are paired. One pair of these glands lies in the tongue, three in the head, and one in the thorax.

Fig. 326.—Acinous salivary glands of Orthezia cataphracta. In some acini the nuclei and boundaries of the cells are shown.—After List, from Field’s Hertwig.

System I is situated in the head, and consists of unicellular glands; the duct from each cell leads into a common, strongly chitinized duct, opening into the gullet.

System II, composed of acinose glands, lies also in the head; its duct is united with that of System III, situated in the thorax. (Fig. 329, 2, 3.)

System IV is situated at the base of the upper surface of the mandibles, and forms a delicate sac lined within with glandular cells; its duct opens at the insertion of the mandibles.

System V lies in the beak, and is a single gland consisting of unicellular glands; it opens into the common opening of Systems II and III. This system is wanting in the honey-bee, but occurs in Bombus and other genera.

335

Fig. 327.—Appendages of digestive canal of Belostoma.—After Locy.

Fig. 328.—Salivary and other glands of Ranatra.—After Locy.

In all the five systems there constantly occur three cellular layers: the intima, epithelial, and propria. As regards their origin Schiemenz states that Systems I and IV are new structures, that System III arises in part, and Systems II and V wholly, from the silk-glands of the larva. As the glands differ much in the sexes, and in different species and genera, Schiemenz believes that their function is very manifold.

In addition to those previously discovered by Schiementz, Bordas has detected two additional pairs of salivary glands in the worker and male honey-bee, i.e. the internal mandibular and sublingual glands, so that in Apis there are in all six pairs, and apparently one unpaired.

The delicate chitinous external layer of the gland is perforated by many very fine pores through which the salivary fluid secreted 336by the epithelial cells passes into the salivary duct. The glands are externally bathed by the blood.

In many insects, including lepidopterous larvæ, the single median opening of the salivary duct is converted into a spraying apparatus.

In the adult Lepidoptera, according to Kirbach:—

Fig. 329.—Salivary glands of the honey-bee: systems No. 1–3, × 15: sv, salivary valve (of systems 2 and 3) at base of tongue; lp, labial palpus; mx, maxilla; so, salivary opening of system 1 in hypopharyngeal plate; no, openings in plate for termination of taste-nerve; œ, œsophagus; sd, salivary duct; b, junction of ducts of system No. 2; c, junction of ducts of system No. 3; sc, sc, salivary sacs; fl, front lobe; bl, back lobe; a, chitinous duct, with spiral thread. B, single acinus of system No. 1, × 70: n, nucleus; st, salivary tract; d, large duct. C, single pouch, or acinus, from system No. 2: a, propria or outer membrane; sc, secreting cells. D, termination of system No. 3:1,2,3,4, lines marking end of section; d, duct in section; sc, secreting cells in section; n, nucleus.—After Cheshire.

“Its lower half forms a thick chitinous gutter, with a concave cover above, in which the similarly shaped upper half lies encased, so that between the two only a small semicircular opening remains. Powerful muscles extend from the cover to the lower side and to the two ridges of the bottom plate; through their contraction the upper channel is elevated, and presses out of the hinder part of the ducts into the space thus formed a great quantity of the saliva, which by allowing the contraction of the cover-muscle through the crevice-like opening, which is situated in the lower edge of the mouth-opening, becomes 337squeezed out in order either to mix with the fluid where the 2d maxillæ fuse, passing up into the canal in the proboscis, or to penetrate into and thus dilute the semi-fluid or solid substances taken, into the proboscis.”

The morphology and general relations of the salivary glands have been sketched out by Hatschek, Patten, and by Lucas, from observations on those of the case-worms or larval Trichoptera.

Fig. 330.—Eight pairs of glands of Andrena: I, thoracic; II, postcerebral; III, supracerebral; IV, lateropharyngeal; V, mandibular; VI, internomandibular; VII, sublingual; VIII, lingual; Md, mandible; L, tongue; o, eye; œ, œsophagus; J, honey-sac.—After Bordas.

Patten states that the spinning-glands in Neophylax are formed by a pair of ectodermal invaginations on the ventral side of the embryo, between the base of the 2d maxillæ and the nervous cord. They increase rapidly in length, and “they also unite to form a common duct, which opens at the end of the upper lip.”

The salivary glands in the same insect are “formed by invagination of the ectoderm on the inner sides of the mandibles, in the same manner as are the spinning glands.”

Lucas has shown that in trichopterous larvæ (Anabolia) there are three pairs of salivary glands in the head, which are serially arranged. The first pair belong to the mandibular, the second pair to the 1st maxillary, and the third pair, or spinning glands, to the 2d maxillary segment. The first or mandibular glands open into the mouth at the base of the mandibles directly behind the dorsal condyle. The second pair open between the 1st and 2d maxillæ; at the base of the latter, near the ventral condyle of the mandibles. The third pair open into the hypopharynx, which is modified to form the spinneret. Lucas agrees with Korschelt in regarding them as modified coxal glands, Schiemenz having previously regarded the headglands of the imago of the bee as belonging to the segments bearing the three pairs of buccal appendages, so that each segment originally contained a pair of glands. It is thus proven that the silk-glands are modified salivary glands adapted to the needs of spinning larvæ, and indeed in the imago the sericteries revert to their primitive shape and use as salivary glands.

The serial arrangement of the salivary glands in the Hymenoptera, where the number varies from five to ten pairs, is clearly proved by Bordas. He has detected five more pairs than were previously known, and names the whole series as follows:—1, the thoracic salivary glands, which are larger than the others, and nine other pairs, which are all contained in the head as follows: 2, postcerebral; 3, supracerebral; 4, lateropharyngeal; 5, mandibular; 6, internomandibular, situated on the inner side of the base of mandible; 7, sublingual; 8, lingual (these and 1 to 7 common to all Hymenoptera); 9, paraglossal (in Vespidæ); 10, maxillary (very distinct in most wasps). These glands do not all occur in the same species, being more or less atrophied.

Bordas further shows the segmental arrangement of the cephalic glands by stating that the supracerebral glands correspond to the antennal segment, the sublingual glands to the labial, the mandibular glands (external and internal) 338to the mandibular segment, the maxillary glands to the 1st maxillary segment, the lingual glands to the 2d maxillary segment, while the thoracic and postcerebral salivary glands, he thinks, correspond to the ocular segment, a view with which we are indisposed to agree, although conceding that each of the six segments of the head has in it at least one pair of salivary glands.

Functions of the different salivary glands in Hymenoptera.—The secretion of the thoracic glands is feebly alkaline. The postcerebral salivary glands, considered by Ramdohr to be organs of smell, secrete, like the preceding, a distinctively alkaline fluid, which mingles with the products of the thoracic glands. The supracerebral glands, also equally well developed in all Hymenoptera, though much atrophied in the females and especially the males of Apis mellifica, also in the Vespinæ and Polistinæ, secrete an abundant, feebly acid liquid, which is actively concerned in digestion.

As to the mandibular glands, which Wolf supposed to be olfactory organs, their acid secretion, though smelling strongly, acts energetically on the food as soon as introduced into the mouth.

The sublingual glands, atrophied in most Apidæ, but relatively voluminous in Sphegidæ, Vespinæ, Polistinæ, Crabronidæ, etc., empty their secretion into a small prebuccal excavation, where accumulate vegetable and earthy matters collected by the tongue, and the saliva secreted by these glands, acts upon them before they pass into the pharynx. The lingual glands secrete a thick, sticky liquid, which causes foreign bodies to adhere to the tongue, and also agglutinates alimentary substances. The uses of the other glands, maxillary and paraglossal, are from their minuteness undetermined. (Bordas.)

LITERATURE ON THE SALIVARY GLANDS

Leydig, F. Zur Anatomie der Insekten. (Archiv Anat. und Phys. 1859.)

—— Untersuchungen zur Anatomie und Histiologie der Tiere. Bonn, 1883, pp. 174, 8 Taf.

—— Intra- und interzellulare Gänge. (Biolog. Centralblatt, x, 1890, pp. 392–396.)

Dohrn, A. Zur Anatomie der Hemipteren. (Stettin. Entom. Zeit., 1866, salivary glands, pp. 328–332.)

Kupffer, C. Die Speicheldrüsen von Periplaneta orientalis und ihr Nervenapparat. (Beiträge zur Anatomie und Physiol., 1875.)

Schiemenz, P. Ueber das Herkommen des Futtersaftes und die Speicheldrüsen der Biene. (Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)

Korschelt, E. Ueber die eigentümlichen Bildungen in den Zellkernen der Speicheldrüsen von Chironomus plumosus. (Zool. Anzeiger, 1884, pp. 189–194, 221–225, 241–246.)

Hofer, B. Untersuchungen über den Bau der Speicheldrüsen und des dazu gehörenden Nervenapparates von Blatta. (Nova Acta d. Kais. Leopold.-Carol. Deutsch. Akad. d. Naturforscher, li, 1887, pp. 345–395, 3 Taf.)

Knüppel, A. Ueber Speicheldrüsen von Insekten. (Archiv für Naturg., 1887, Jahrg. 52, pp. 269–303, 2 Taf.)

Blanc, Louis. La tête du Bombyx mori à l’état larvaire, anatomie et physiologie. (Extrait des Travaux du Laboratoire d’Études de la Soie, 1889–1890; Lyon, 1891, p. 180, many figs.)

Bordas, L. Anatomie des glandes salivaires des Hyménoptères de la famille des Ichneumonidæ. (Zool. Anzeiger, 1894, pp. 131–133.)

—— Glandes salivaires des Apides, Apis mellifica. ♂ and ♀. (Comptes rendus Acad. Sc., Paris, cxix, pp. 363, 483, 693–695, 1894; also two articles in Bull. Soc. Philomath. Paris, 1894, pp. 5, 12, 66.)

339—— Appareil glandulaire des Hyménoptères. (Ann. Sc. Nat. Zool., xix, Paris, 1894, pp. 1–362, 11 Pls.) (See also p. 366.)

Berlese, Antonio. Le cocciniglie Italiane viventi sugli agrumi. Firenze, 1896, 12 Pls. and 200 Figs.

With the writings of Mark, Minot, Locy, List, Krassilstschik, Nagel (1896).

b. The silk or spinning glands, and the spinning apparatus

The larvæ of certain insects, chiefly those of the Lepidoptera, possess a pair of silk or spinning glands (sericteries) which unite to form a single duct opening in the upper lip at the end of the lingua, which is modified to form the spinneret. (See pp. 71, 75.) All caterpillars possess them, and they are best developed in the silkworms, which spin the most complete cocoon. Silk-glands also occur in the larvæ of the Tenthredinidæ, in the case-worms or larval Trichoptera, also in certain chrysomelid beetles (Donacia, Hæmonia), and in a weevil (Hypera). In a common caddis-worm (Limnophilus) the glands are of a beautiful pale violet-blue tint, and two and a half times as long as the larva itself; viz. the body is 20 mm. and the glands 55 mm. in length.

In caterpillars the glands are of tubular shape, shining white, and much like the ordinary simple tubular salivary glands of the imago. When only slightly longer than the body they are twice folded, the folds parallel and situated partly beneath and partly on the side of the digestive canal; not usually, when folded in their natural position, extending much behind the end of the stomach; but in the silkworms they are so long and folded as to envelop the hinder part of the canal. In geometrid caterpillars the glands when stretched out only reach slightly beyond the end of the body; in Datana they are half again as long as the body. Helm thus gives their relative length in certain Eurasian caterpillars, and we add that of Telea polyphemus:—

Vanessa io length of body 32 mm.; of the silk glands 26 mm.
Smerinthus tiliæ length of body 63 mm.; of the silk glands 205 mm.
Bombyx mori length of body 56 mm.; of the silk glands 262 mm.
Antheræa yamamaya length of body 100 mm.; of the silk glands 625 mm.
Telea polyphemus length of body 60 mm.; of the silk glands 450 mm.

Thus in Telea the silk-glands are about 18.50 inches in length, being about seven times as long as the body.

For the most complete accounts of the spinning glands of Lepidoptera and their mechanism we are indebted to Helm and to Blanc, and for that of the Trichoptera to Gilson.

340The unpaired portion, or spinning apparatus (filière of Lyonet), is divided into two portions; the hinder half being the “thread-press,” the anterior division the “directing tubes.” The silk material, stored up in the thickest portion of the glands, passes into the thread-press (Fig. 334, A), which is provided with muscles which force the two double ribbon-like threads through the directing tube, as wire is made by molten iron being driven through an iron plate perforated with fine holes. The entire spinning apparatus, or filator, as we may call it, is situated in the tubular spinneret. The opening of the spinneret is directed anteriorly, and the anterior end of the directing tube passes directly into this opening so that the directing tube may be regarded as an invagination of the lingua.

The silk thread which issues from the mouth of the spinneret is, as Leeuwenhoek discovered, a double ribbon-like band, as may be seen in examining the silk of any cocoon.

The process of spinning.—Since the appearance of Helm’s account, Gilson, and also Blanc, have added to our knowledge of the way in which the silk is spun and of the mechanism of the process. Gilson has arrived, in regard to the function of the press or filator, at the following conclusions: 1, the press regulates the thread, it compresses it, gives it its flattened shape; 2, it regulates the layer of gum[52] (grès) which surrounds the thread; 3, it may render the thread immovable by compressing it as if held by pincers.

The process of spinning in the silkworm, says Blanc, comprises all the phenomena by which the mass of silk contained in the reservoir is transformed into the silk fluid of which the cocoon is spun. The excretory canals each contain a cylindrical thread of silk having a mean diameter of 0.2 mm. and surrounded by a layer of gum (grès) which in the fresh living organ exactly fills the annular space situated between the fibroin cylinder and the wall. Arrived within the common duct, the two threads receive the secretion of Filippi’s gland, where the silken fluid is formed, but has not yet assumed its definite external characters. The two threads press through the common canal and arrive at the infundibulum (Fig. 334, c) of the press, at 341the bottom of which is situated the orifice of the spinning canal, almost completely divided into two by the sharp edge of the rachis (Figs. 334, a, 335, l). The threads each pass into one of the two grooves, and the layer of gum (grès) fills the rest of the canal of the press or filator.

Fig. 331.—Longitudinal section of the spinneret: a, horizontal portion of the tongue; b, vertical portion; c, f, circle of the tongue; d, tongue-pad; e, orifice of the spinneret; g, body of the lyre; h, prebasilar membrane forming a fold; i, internal canal of the spinneret; k, filator.—After Blanc.

Fig. 332.—The lower lip (labium) of Bombyx mori, isolated, seen from the left side: A, lyre; B, spinneret; C, labial palpus; D, vertical part of the labium; E, horizontal part of the same; H, L, silk-canal; K, right gland of Filippi; L, canal of the left gland; N, labial nerve; a, oblique fibre of the elevator of the labium; b, right fibre of the same; c, depressor of the labium; d, superior spinning muscles.

Fig. 333.—The labium in a horizontal position, seen from the side: f, the filator or press situated under the external part of the spinneret (d), between the branches (b), of the lyre (a); e, labial palp; c, tongue.

Fig. 334.—Longitudinal section of the spinneret and press (filator): A, filator or press; B, spinneret; CD, body of the lyre; F, lower part of the labium; E, common canal; eh, its epithelium; G, superior muscle of the press; a, rachis; b, its posterior enlargement; c, infundibulum; d, cuticle; o, orifice of the spinning canal; op, central canal of the lyre and of the spinneret; fi, hypodermis of the lyre; f, f, hypodermic pad of the lyre.

The silken substance is then pressed by the more or less powerful contractions of the muscles of the filator, so 342that the passage of the threads is facilitated. If the muscles totally contract, the spinning canal is opened wide, the threads pass easily upwards and assume the form of a triangular prism (Fig. 336).

Fig. 335.—Spinning apparatus, seen from above: A, opening of the spinneret; B, central canal of the spinneret (C); D, common canal; E, canal of Filippi; F, excretory canal of a silk-gland; i, orifice of the canal of Filippi’s gland; l, rachis; k, ring of the infundibulum; b, c, d, e, f, cavity of the different canals; h, spur which separates the two excretory canals.—This and Figs. 331–334 after Blanc.

If this contraction diminishes, the chitinous wall of the spinneret comes together, owing to its elasticity; the ceiling of the canal approaches the floor; the cavity tends to take the form of a semicircular slit, and the threads are compressed, flattened. As each mass or thread of silk is much more voluminous than the canal, except when the latter is extremely dilated, it follows that the two threads are always compressed, or squeezed together, and that each of them is compelled to mould itself in the groove it occupies and to take its shape. Hence the variations in the appearance of the two masses or divided portions of silk, which as stated present all grades between the form of an isosceles-triangular prism and that of a nearly flat ribbon; but this last case is quite rare. The use of the spinneret, then, is to compress the thread and to change its form more or less considerably, at the same time as it diminishes its diameter.

Fig. 336.—Diagram of the press and its muscles: a, lower; b, lateral; c, upper muscles of the press.—After Blanc.

Moreover, this constant compression of the thread as it passes through the press keeps it in a certain state of tension so as to allow the caterpillar while spinning to firmly hold its thread.

Finally, when the worm suspends the contraction of its spinning muscles, the press flattens, vigorously compresses the thread, and arrests its motion, in such a way that if there was a strain on the silken fluid (bave), it would break rather than oblige the caterpillar to let go any more of it.

The press does not act directly on the silken thread, but through the gummy layer (grès) which transmits over the whole surface of the silken fluid (brin) the pressure exerted on it. After having overcome this difficult passage, the silk thread has acquired its definite form; it rapidly passes out of the spinneret.

343How the thread is drawn out.—Having seen, says Blanc, how the two masses of silk (brins), in passing through the spinning apparatus (or press), join each other, constituting the frothy silken fluid, thus becoming modified in form, it remains to examine the way in which the thread is drawn out of the spinneret. If we examine a caterpillar while spinning, it will be seen that in moving its head it draws on the frothy mass of silk fixed to the web of the cocoon. This traction certainly aids very much the exit of the thread, but it is not the only cause.

The silk, Blanc affirms, is pushed out by a force a tergo, developed by different agents, such as the pressure of the distended cuticle or the silky mass contained in the reservoir, as seen in the section of a worm which has spun its cocoon. But if we consider a caterpillar before it has begun to spin, it is difficult to explain the mechanism of spinning. As Blanc has often observed, in making sections of the heads of silkworms, two cases arise. Sometimes the worm has already spun a little, and a certain length of the frothy silk (bave) issues from the orifice of the spinneret, where it forms a small twisted bundle. At other times the worm has not spun since its last moult or the frothy mass of silk has broken within the head, and we find the end in the common tube. In the first of these two cases, the worm, dilating its press, is able by a general contraction to discharge a little of the gritty material (grès) which lines the ball of silk hanging at the end of the spinneret. It can also reject a certain quantity of the secretion of Filippi’s glands and thus soften the gritty substance. The little plug of silk can then adhere to the body with which it comes in contact.

In the same case it is necessary that the two bits or portions of silk traverse the press, and this normally has a calibre less than their diameter. The worm should then distend the spinning tube as much as is practicable, so as to make the openings as large as possible. It has been stated that the press is, in this condition, at least as large as the mass of frothy silk. This Blanc believes (although Gilson thinks otherwise) is pushed by a force a tergo, and reaches the funnel of the spinning canal; its two bits of silk (brins) unite there, penetrate into the canal itself, and, owing to successive impulses produced by the general contractions of the worm, press through and pass out of the spinneret.

While the silkworm is engaged in spinning its cocoon, the spinneret and press execute very varied movements, determined by the elevator, depressor, retractor, and protractor muscles of the labium, as well as those of the press. These movements, originally very 344numerous, may combine among themselves, so that the spinneret is susceptible of assuming during the process of spinning still more diverse positions.

Fig. 337.—Portion of the silk-gland of Bombyx mori: p, tunica propria; i, tunica intima; s, secretion-cell with branched nuclei; a, separate secretion-cell from the anterior part of the silk-gland of Amphidasis betularia; b, the same of Vanessa urticæ; c, the same in Smerinthus tiliæ.—After Helm.

Fig. 338.A, section of gland of lepidopter: B, section of silk-gland of a saw-fly larva; n, nucleus; i.d, canals; d.s, cavity.—After Gilson.

Histologically the silk-glands are composed of three layers,—the outer, or tunica propria (Fig 337); the inner, the tunica intima; the middle layer being composed of extraordinarily large epithelial cells which can be seen with the naked eye, and are also remarkable for the branched shape of the nuclei (a, b, c, 337), the branches being 345more or less lobed, and the larger the cells the more numerous are the branches of the nucleus. Gilson[53] finds that those of Trichoptera, Lepidoptera, Diptera, and Hymenoptera ordinarily consist of a small number of cells; and it is quite common, he says, to find only two cells in a transverse section (Fig. 338, A). In the Tenthredinidæ, however, “the organ still consists of a tube, the wall of which is composed of flat cells, but in addition to that, two series of spheroidal cells are attached to the sides. Each of these cells contains a system of tiny canals running through their cytoplasm (B, i. d). These cells are the secreting elements; they continually cast the silk substance into the tube.” A peculiarity of the tunica intima is its distinct transverse striation.

Fig. 339.—Branching nucleus of spinning gland of Pieris larva.—After Korschelt, from Wilson.

Fig. 340.—Filippi’s glands (G) isolated and seen from above: e, e, its lobules; d, its excretory canal; E, silk-duct; C, common canal; c, upper spinning muscle; b, lower muscle; a, lateral muscle; T, spinneret.—After Blanc.

The lining of the glands and of their common duct is moulted when the caterpillar casts its skin, and this, as well as the mode of development, shows that the glands are invaginations of the ectoderm. Gilson finds that the silk-glands and silk-apparatus of Trichoptera are very similar to those of caterpillars, and that the silk is formed in the same way.

Appendages of the silk-gland (Filippi’s glands).—In most larvæ there is either a single or a pair of secondary glands which open into the spinning glands near their anterior end. They are outgrowths of the gland provided with peculiarly modified excretory 346cells or evaginations of the entire glandular epithelium. Those of Bombyx mori (Fig. 340) are very well developed, and, according to Blanc, form two whitish, lobulated masses in the labium on each side of the common duct of the spinning gland. Externally they appear to be acinose; but their structure, as described by Blanc and by Gilson, is very peculiar. Helm thinks, with Cornalia, that the function of these glands is to secrete the adhesive fluid which unites the silk threads, and also to make the silk more adhesive in the process of spinning, but Blanc states that this is done before the thread passes into the common excretory tubes, and he is inclined to think that the secretion serves to lubricate the spinneret, and thus to facilitate the passage of the thread. On the other hand, in certain caterpillars these glands are situated quite far from the spinning apparatus.

The silk-glands in the pupa state undergo a process of degeneration, and finally completely disappear. They are specific larval organs evolved in adaptation to the necessity of the insect’s being protected during its pupal life by a cocoon. (Helm.)

Morphologically the silk-glands are by Lang regarded as modified coxal glands, and homologues of the setiparous parapodial glands of chætopod worms, the coxal glands of Peripatus, and the spinning glands of spiders.

In Scolopendrella, spinning glands are situated in the two last segments of the body, opening out at the end of the cercopods (Fig. 15, s.gl), and the larvæ of the true Neuroptera (Chrysopa, Myrmeleon, etc.) which spin cocoons, have spinning glands opening into the rectum. The silk forming the cocoon of the ant-lion, as Siebold and the older observers have stated, is secreted by the walls of the rectal or anal sac. Siebold (Anatomy of the Invertebrates, p. 445) states that in the larva of Myrmeleon, the silk-apparatus is very remarkable, “for the rectum itself is changed into a large sac and secretes this substance which escapes through an articulated spinneret projecting from the opening of the anus”[54] (Fig. 307, e). The larvæ of the Mycetophilidæ have spinning glands at the hinder end of the body, as also the imago of the female of the tineid moth Euplocamus. (Kennel.) The larvæ of ichneumons, wasps, bees, of Cecidomyia, and other Diptera, spin silken cocoons, but their glands have not yet been examined.

It should also be observed that during the process of pupation the larvæ of butterflies, of certain flies (Syrphus), and beetles (Coccinellidæ and some Chrysomelidæ) attach themselves by silk spun from the anus, so that the pupa is suspended by its tail; such glands are probably homogenetic with the coxal glands.

The silk in its fluid or soft state is mucilaginous, and according to Mulder, in the silkworm consists of the following substances, varying somewhat in their relative proportions by weight:

Silk-fibre material 53.67
Glue (Leim) 20.66
Protoplasm 24.43
Wax 1.39
Coloring matter 0.05
Fat and resin 0.10
347

LITERATURE ON THE SPINNING GLANDS

Helm, E. Anatomische und histiologische Darstellung der Spinndrüsen der Schmetterlingsraupen. (Zeitschr. f. wissens. Zool., xxvi, 1876, pp. 434–469, 2 Taf.)

Lidth de Jeude, Th. W. van. Zur Anatomie und Physiologie der Spinndrüsen der Seidenraupe. (Zool. Anzeiger, 1878, pp. 100–102.)

Engelmann, W. Zur Anatomie und Physiologie der Spinndrüsen der Seidenraupe. (Onderz. Phys. Lab. Utrecht, iii, 1880, pp. 115–119.)

Joseph, G. Vorläufige Mitteilung über Innervation und Entwickelung der Spinnorgane bei Insekten. (Zool. Anzeiger, 1880, pp. 326–328.)

Poletajew, N. Ueber die Spinndrüsen der Blattwespen. (Zool. Anzeiger, 1885, pp. 22–23.)

Meinert, Fr. Contribution à l’anatomie des fourmilions. (Overs. Danske Vidensk. Selsk. Forh. Kjöbenhavn, 1889, pp. 43–66, 2 Pls.)

Blanc, Louis. Étude sur la sécrétion de la soie et la structure du brin et de la bave dans le Bombyx mori. Lyon, 1889, pp. 48, 4 Pls.

—— La tête du Bombyx mori à l’état larvaire. Anatomie et physiologie. (Extrait du volume des Travaux du Laboratoire d’Études de la Soie. Années 1889–1890, Lyon, 1891, pp. 180, 95 figs.)

Gilson, G. Recherches sur les cellules sécrétantes. La soie et les appareils séricigènes: I. Lépidoptères. (La Cellule, 1890, vi, pp. 115–182, 3 Pls. I, Lépidoptères (suite); II, Trichoptères. Ibid., x, pp. 71–93, 1893, 1 Pl.)

Garman, H. Silk-spinning dipterous larvæ (Science, xx, 1893, p. 215).

Also the writings of Meckel, Pictet, Duméril, Klapálek, Wistinghausen, Loew, Hagen, Fritz Müller, Kolbe, McLachlan, de Selys-Longchamps.

c. The cæcal appendages.

These diverticula of the mid-intestine (“stomach”) are appended to the anterior end, and in the living, transparent larva of Sciara, which has two large, long, slender cœca (Fig. 341), the partly digested food may be seen oscillating back and forth from the anterior end of the stomach into and out of the base of each cæcum. In the Locustidæ (Anabrus, Fig. 299) and Gryllidæ (Fig. 344, e) there are two large, short cæca, and in the locusts (Caloptenus) there are six cæca, while cockroaches have eight. In the Coleoptera (Carabidæ and Dyticidæ) these large cæca appear to be replaced by very numerous slender, minute villi or tubules, which arise from the anterior part of the stomach (Figs. 317, r, also 342).

These cæca differ in structure from the stomach, as shown by Graber, as well as by Plateau and by Minot. The latter states that a single transverse section of one of the diverticula of the locust demonstrates at once that its structure is entirely different from that of the stomach.

348

Fig. 341.—Larva of Sciara: s.gl, salivary gland; ur.t, urinary tubes; i intestine; st, stomach; cae cæcal appendages; t, testis.

Its inner surface is thrown up into longitudinal folds, generally twelve in number. These folds shine through the outer walls, and are accordingly indicated in the drawings of Dufour, Graber, and others. The entire cæcum has an external muscular envelope, outside of which are a few isolated longitudinal muscular bands. The folds within are formed mainly by the high cylindrical epithelium which lines the whole interior of the cavity. Tracheæ ramify throughout all the layers outside the epithelium. There are appearances of glandular follicles in the bottom of the spaces between the folds. (Minot.)

Burmeister supposed that these cæca were analogous to the pancreas, and this view has been confirmed by Hoppe Seyler, Krukenberg, Plateau, and others, who claim that the digestive properties of the fluid secreted in them agrees with the pancreatic fluid of vertebrates.

Fig. 342—Cross-section of mid-intestine of Acilius sulcatus, showing the arrangement of the cæca, two tracheæ passing into each cæcum.—After Plateau.

d. The excretory system (urinary or Malpighian tubes)

The excretory matters or waste products of the blood tissue of worms are carried out of the body by segmentally arranged tubes called nephridia. As a rule they arise in the blood sinuses of the body and open externally through minute openings in the skin. As there is a pair to each segment (in certain oligochete worms two or three pairs to a segment), they are often called segmental organs. In the annulate worms each segment of the body, even the cephalic or oral segment, originally contains a pair of these excretory organs. These 349vessels may have survived in myriopods and perhaps do exist in insects as urinary tubes, and also occur in many of the Arachnida, and thus are characteristic of each important class of land arthropods, but are either wanting or are very rudimentary or much modified in the marine classes, notably the Crustacea and Merostomata (Limulus), where they are represented by the shell-glands of Copepoda, green glands of the lobster, and the brick-red glands of Limulus.

Fig. 343.—Digestive canal of Perla maxima: l, upper lip; mh, buccal cavity; ap, common end of salivary ducts (ag); o, œsophagus; s, s, salivary glands, arranged segmentally; b, cæca of chyle-stomach; lg, their ligaments of attachment; mp, urinary tubes; r, rectum; af, anal orifice.—After Imhof, from Sharp.

In the earliest tracheate arthropod, Peripatus, these tubes are well developed and are highly characteristic, each segment behind the head bearing a pair (Fig. 4, so4-so9). It has been suggested by some, but not yet proved, that the urinary tubes of insects are morphologically the same as the segmental organs of worms and of Peripatus; but there are no facts directly supporting this view, and, as Sograff states, it is a pure hypothesis and can only be confirmed or disproved by very detailed researches on the development of the urinary tubes of myriopods and of insects. Others regard them as probably homologous with the tracheæ, since they have a similar origin. As, however, they arise in the embryo as outgrowths of the proctodæum they may have arisen in myriopods and insects independently, and not be vermian heirlooms.

While in worms and in Peripatus a pair of these segmental organs occur in each segment, in insects this serial arrangement is not apparent; those with a purely excretory function are not segmentally arranged, with outlets opening externally, but arise as outgrowths of the hind-intestine or proctodæum of the embryo, not being segmentally arranged. The place of their origin is usually the dividing line between the mid and hind intestine (Fig. 343, mp); this applies to Scolopendrella (Fig. 15, urt) as well as to insects.

The urinary tubes are usually long, slender, blind, tubular glands varying in number from two to over a hundred, which generally arise at the constriction between the mid and hind intestine, and which lie loosely in the cavity of the body, often extending towards the head, and then ending near the rectum (Figs. 301, 310, vm). 350They were first discovered by the Italian anatomist Malpighi, after whom they were called the Malpighian tubes. While at first generally regarded as “biliary” tubes, they are now universally considered to be exclusively excretory organs, corresponding to the kidneys of the higher animals.

Fig. 344.—Digestive canal and appendages of the mole-cricket; a, head: b, salivary glands and receptacle; c, lateral pouch; d, stomatogastric nerves; e, anterior lobes of stomach; f, peculiar organ; g, neck of stomach; h, plicate part of same; i, rectum; k, anal gland; m, urinary tubes.—After Dufour, from Sharp.

Usually arising from the anterior end of the hind-intestine where it passes into the mid-intestine, in certain forms they shift their position, in some Hemiptera (Lygæus, Cimex) opening into the rectum, while in the Psyllidæ they arise from the slender hinder part of the mid-intestine, being widely separated at their origin. (Fig. 321.)

The length varies in different groups; where they are few in number (two to four, six to eight), they are very long, but where very numerous they are often short, forming dense tufts, each tuft connecting with the intestine by a common duct (ureter), or, as in the mole-cricket, the numerous tubes empty into a single duct (Fig. 344); in the locusts (Acrydiidæ), however, they are arranged in 10 groups, each group consisting of about 15 tubes, making about 150 in all; and are much convoluted and wound irregularly around the digestive canal, and when stretched out being about as long as the entire body.

The urinary tubes occur in twos, or in multiples of two, though a remarkable exception is presented in the dipterous genera Culex and Psychodes, in which there are five tubes; the young and fully grown larvæ, as well as the pupa and imago of Culex, having this number (Fig. 433, mg.)

351In many insects (Pentatoma, Cimex, Velia, Gerris, Haltica, Donacia, and often in caterpillars), the vessels open into a sort of urinary bladder connecting with the intestine on one side.

Fig. 345.A, section of urinary tube of Periplaneta; B, part of tube of Perla; p, peritoneal membrane; c, cavity or lumen; n, nucleus of a secreting cell.—After Schindler.

In the larvæ of some insects the blind ends of the tubes are often externally bound to the rectum, in the silkworms being attached by fine threads to the intestine, while in some flies (Tipula and Ctenophora), two vessels may unite to form a loop. In all larval Cecidomyiæ, the two tubes are united to form a loop which curves backward, opening near the vent, the proctodæum being very short. (Giard.)

Fig. 346.—Portion of a urinary tube of Calliphora vomitoria: tr, trachea; l, lumen; k, nucleus.—After Gegenbaur.

While usually the urinary vessels form simple tubes, in many species of Lepidoptera and Diptera they are branched, thus resembling those of spiders and scorpions. Moreover, in many Lepidoptera and Diptera (Fig. 308), the tubes are not simple, but are lobulated, and in some Hemiptera (Pentatoma, Notonecta, and Tettigonia) are twisted or lace-like. In rare cases there are two kinds of urinary tubes; in Melolontha vulgaris, two of them are partly lobulated and yellow, while the other two are simple and white. Their color in beetles varies, some being whitish or yellowish; in Geotrupes, Dyticidæ, Hydrophilidæ, etc.