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 — A. S. Packard

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

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

Norwood Press
J. S. Cushing & Co.—Berwick & Smith
Norwood Mass. U.S.A.

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.

This 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.

Professor 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.

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

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 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 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 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

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 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

PART 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 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

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 under 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.

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 composed 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.

We 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 more 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 abdominal), 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.

[Illustration:

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. ]

The 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.

=Relations 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.

[Illustration:

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; _so_{4}_, _so_{5}_, _so_{6}_, _so_{9}_, 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. ]

The 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.

[Illustration:

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 inclined 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 known, 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.