PRINCIPLES OF INSECT IMORPHOLOGY McGraw-Hill Publications in the Zoological Sciences E. J. Boell, Consulting Editor Baitsell Human Biology Breland Manual of Comparative Anatomy Chapman Animal Ecology Haupt Fundamentals op Biology Hyman The Invertebrates; Protozoa through Ctenophora (Vol. I) The Invertebrates; Platyhelminthes and Rhynchocoela (Vol. II) The Invertebrates ; Acanthocephala, Aschelminthes, and Entoprocta (V ol. Ill) The Invertebrates; Echinodermata (Vol. IV) Leach Functional Anatomy of the Mammal M ayr, Linsley, and V singer Methods and Principles of Systematic Zoology Metcalf and Flint Fundamentals of Insect Life Mitchell A Textbook of General Physiology Mitchell and Taylor Laboratory Manual of General Physiology Pearse Animal Ecology Quiring Functional Anatomy of the Vertebrates Rogers Textbook of Comparative Physiology Senning Laboratory Studies in Comparative Anatomy Shull Evolution Shidl Heredity Shull, LaRue, and Ruthven Principles of Animal Biology Shull, LaRue, and Ruthven Laboratory Directions in Principles op Animal Biology Snodgrass Principles of Insect Morphology Storer General Zoology Storer Laboratory Manual for General Zoology Storer and Usinger Elements of Zoology Weichert Anatomy of the Chordates Weichert Elements of Chordate Anatomy Weichert Representative Chordates Welch Limnology Wieman An Introduction to Vertebrate Embryology Wolcott Animal Biology Wolcott and Powell Laboratory Guide in Animal Biology There are also the related series of McGraw-Hill Publications in the Botanical Sciences, of which Edmund W . Sinnott is Consulting Editor, and in the Agricultural Sciences, of which R. A. Brink is Consulting Editor. ^PRINCIPLES OF INSECT MOEPHOLOGY R. E. SNODGRASS United Slates Dcparlmcnl of Agriculture Bureau of Entomology and Plant Quarantine McGRAW-HILL BOOK COMPANY, Inc. NEW YORK AND LONDON 1935 COPTHIGHT, 1935, BY THE McGhaw-Hilij Book Company, Inc. PRINTED IN THE HOTTED STATES OP AMERICA All rights reserved. This book, o, 'parts thereof, may not be reproduced in any form vnthout permission of the vvblishers. xiii 59510 PREFACE 'J'he principal value of facts is that they give us something to think about. A scientific textbook, therefore, should contain a fair amount of reliable information, though it may be a matter of choice with the author whether he leaves it to the reader to formulate his own ideas as to the meaning of the facts, or whether he attempts to guide the reader’s thoughts along what seem to liim to be the proper channels. The writer of the present text, being convinced that generalizations ai’e more important than mere knowledge of facts, and being also somewhat partial to his own way of thinking about insects, has not been able to refrain entirely from presenting the facts of insect anatomy in a way to suggest relations between them that possibly exist only in his own mind. Each of the several chapters of this book, in other words, is an attempt to give a coherent morphological view of the fundamental nature and the apparent evolution of a particular group of organs or associated struc- tures. It is more than likely, practically certain, that many of the generalizations here offered will soon be modified or superseded by other generalizations, but they will have served their purpose if they induce critical students to make a wider and more thorough study of the prob- lems of insect morphology. Two notable books have appeared recently in entomology: one, “Lehrbucb der Entomologie,” by Dr. Hermann Weber of Danzig, in its first edition; the other, “A General Textbook of Entomology,” by Dr. A. D. Imms of Cambridge, England, in its third edition. In preparing the present text the writer has made a special effort to concur with the authors of these books in the matter of anatomical terms, in order that students may as far as possible be spared confusion in turning from one treatise to another. Unfortunately, however, there is still much unavoid- able discrepancy in the use and application of anatomical names in entomology. The troi^e, ,ii^ Jgrge measure, can be blamed on the insects themselves, since they will not entirely conform with any plan of nomenclature or with any scheme we can devise for naming their parts consistently. To make clear the meaning of terms as used in this text, therefore, a glossary of definitions is appended to each chapter, wherein, also, will be found the German equivalents of many of our English and Latinized technical names. In making acknowledgments, the writer must first of all declare his indebtedness to the Bureau of Entomology, United States Department vi PREFACE of Agriculture, for the experience and information acquired in the course of his many years of official service. Most of the illustrations accom- panying the text that are not accredited to particular sources are the property of the Bureau of Entomology and Plant Quarantine, and many of them have been published in the Miscellaneous Collections and the Annual Reports of the Smithsonian Institution of Washington, D.C. For the use of these figures the writer hereby expresses his thanks both to the Bureau and to the Smithsonion Institution. With regard to illustrations borrowed from other works, the writer is particularly indebted to Professor Hermann Weber, of Danzig, for permission to use figures from his “Biologie der Hemipteren” and “Lehrbuch der Entomologie.” The rest of the illustrations, each accredited to its proper source in the scientific iournals, have been freely drawn from the common heritage of entomology contributed by the many workers in many lands who have devoted themselves to the study of insects. To my wife, Ruth H. Snodgrass, credit is due for the typing of the manu- script and for much of the work of indexing and proofreading. R. E. Snodgrass. Washington, D. C. May, 1935. CONTENTS i’RBlACE V CHAPTER I Introduction 1 CHAPTER II General Organization and Development 14 Glossary of Embryological Terms 44 CHAPTER III The Body Wall and Its Derivatives 48 1. The Body Wall 48 2. External Processes of the Body Wall 55 3. Sense Organs 59 4. Ectodermal Glands, Corpora Allata, Oenocytes 60 5. Muscle Attachments on the Body Wall 62 6. Moulting 64 Glossary of Terms AppUed to the Body Wall 68 CHAPTER IV Body Regions, Sclerites, and Segmentation 70 Glossary of General Terms Applied to the Body Segments and the Skeletal Plates 81 CHAPTER V The Segmental Appendages of Arthropods 83 Glossary of Terms Applied to the Appendages 98 CHAPTER VI The Head 100 1. General Morphology of the Arthropod Head 100 2. Structure of the Definitive Insect Head 104 3. Special Modifications in the Structure of the Head 118 Glossary of Terms Applied to the Head 127 CHAPTER VII The Head Appendages 130 1. Preantennal Appendages 130 2. The Antennae 131 3. The Postantennal Appendages 133 4. The Mandibles 133 5. The Superlinguae 139 6. The Maxillae 141 7. The Labium 145 8. Glands of the Head Appendages 153 Glossary of Terms Applied to the Head Appendages 155 vii CONTENTS viii CHAPTER VIII Pj,qb The Thohax 157 1. Evolution of the Thorax 157 2. The Neck 159 3. General Structure of the Thorax 160 4. The Prothorax 172 5. The Pterothorax 173 6. The Thoracic Muscles 186 Glossary of Terms Applied to the Thorax 190 CHAPTER IX The Thoracic Legs 193 1. The Structure of the Legs 193 2. Muscles and Mechanism of the Legs 200 Glossary of Terms Applied to the Parts of an Insect’s Leg 209 CHAPTER X The Wings 211 1. Origin and Evolution of the Wings 211 2. Development of the Wings 214 3. Structure of the Wings 215 4. The Wing Muscles 228 5. The Wing Movements 233 6. Insect Flight 240 Glossary of Terms Applied to the Wings 243 CHAPTER XI The Abdomen 246 1. General Structure of the Abdominal Segments 247 2. The Abdominal Musculature 257 3. The Abdominal Appendages 267 CHAPTER XII The Organs of Ingestion 280 1. The Preoral Cavity 281 2. The Cephahc Stomadaeum 284 3. The Feeding Mechanism of Neuroptera and Coleoptera 286 4. The Feeding Mechanism of Hymenoptera 295 5. The Feeding Mechanism of Lepidoptera 302 6. The Feeding Mechanism of Diptera 311 7. The Mouth Parts of Siphonaptera 325 8. The Feeding Mechanism of Thysanoptera 326 9. The Feeding Mechanism of Hemiptera 328 10. The Feeding Mechanism of Anoplura 344 CHAPTER XIII The Alimentary Canal 347 1. Development of the Alimentary Canal 347 2. General Structure of the Alimentary Canal 348 3. The Stomodaeum 349 4. The Mesenteron 359 5. The Proctodaeum . 374 CONTENTS ix Page 6. The Filter Chamber 383 Glossary of Terms Applied to the Alimentary Canal 387 CHAPTER XIV The Organs of Distribution, Conservation, and Elijhnation 389 1. The Blood 389 2. The Organs of Circulation 397 3. The Fat-body 407 4. The Ocnocytcs 410 5. The Corpora allata 411 6. The Organs of Elimination 413 Glossary of Terms Used in This Chapter 420 CHAPTER XV The Respiratory System 422 1. The Integument as a Respiratory Organ 423 2. Blood Gills 424 3. The Tracheal System 425 4. General jMechanism of Tracheal Respiration 452 Glossary of Terms Applied to the Respiratory System 461 CHAPTER XVI The Nervous System 464 1. General Structure, Organization, and Function of the Nervous System. . 465 2. The Central Nen'ous System 472 3. The Stomodacal Ners’ous System 501 4. The Peripheral Nervous System 503 Glossary of Terms Applied to the Ncia'ous System 507 CHAPTER XVII The Sense Organs 510 1. General Structure and Classification of Insect Sense Organs 512 2. The Hair Organs 515 3. The Campaniform Organs 521 4. The Plate Organs 523 5. The Scolopophorous Organs 525 6. The Eyes 528 Glossary of Terms Applied to the Sense Organs 548 CHAPTER XVIII The Internal Organs of Reproduction 550 1. The Female Organs 552 2. The Male Organs 567 3. General Morphology of the Reproductive Organs 573 Glossary of Terms Applied to the Internal Reproductive Organs 578 CHAPTER XIX The Organs of Copulation and Oviposition 581 1. The Male Genitalia 582 2. The Female Genitalia 607 Glossary of Terms Applied to the External Genitalia 620 References 625 Index 647 PRINCIPLES OF INSECT MORPHOLOGY CHAPTER I INTRODUCTION Morphology, in the biological sense, is the science of form in living organisms. Anatomy is the determination of structural facts. Morphol- ogy seeks to find the reason for structure, and to understand the relation of different structural forms to one another. Morphology, therefore, must be intimate with function, since it must see forms as plastic physical adaptations to the work to be performed. A few physiological functions are basic to all organisms ; they are essential to the continuance of matter in a lining state. The v^arious structural types of organisms are special ways of accomplishing these functions, that is, for doing the same things in different w'ays or under different circumstances. Some represent improvements in the machinery along established lines ; others represent changes or new ideas developed along new and divergent lines. The morphologist, therefore, though primarily a comparative anatomist, in order properly to develop his subject, must give attention to the worldng of the physical mechanisms with which he deals in his anatomical studies, he must look for the significance of structural modifications and innova- tions, and he must understand the basic physiological functions that underlie organic form. In the study of insect morphology we cannot confine ourselves to the limits of entomology. The fundamental organization of insects was established long before insects became a specialized group wdthin the phylum of the Arthropoda, and the basic structure of the arthropods is much older than the arthropods themselves. As organisms evolve, important structures are often so modified that their true nature becomes obscured; but the same structure is not likely to be modified to the same degree in all related groups, or in all members of the same group. Struc- tural modification has been carried to a high degree in all the arthropods, affecting some organs in one group, others in another group ; and particu- larly is this true of the insects. Hence, in the discussion of the mor- phology of insect organs given in the following chapters, many references 1 2 PRINCIPLES OF INSECT MORPHOLOGY will be made to corresponding parts in other arthropods, while, for an understanding of the more fundamental structures of the arthropods, it will be found necessary to go back to the segmented worms and to those wormlike creatures known as onychophorans. Before taking up the particular subject in hand, therefore, we must know something of the distinctive structural features of the Annelida the Onychophora, and the major groups of the Arthropoda. THE ANNELIDA The typical annelid worms are elongate cylindrical animals divided transversely into a series of segments. The mouth is situated ventrally between the first segment and a preoral lobe (prostomium) ; the anus is terminal in the last segment (periproct). The segments between the prostomium and the periproct are true metameres, or somites, produced by segmentation of the primitive body region anterior to the periproct. Some of the annelids are provided with lateral segmental appendages (parapodia), which are hollow evaginations of the body wall, movable by muscles inserted on or within their bases. The body cavity of the Annelida is the coelome. It is often divided transversely by intersegmental septa into segmental coelomic cavities. The alimentary canal is a tube extending through the body from the mouth, situated ventrally between the prostomium and the first somite, to the anus, which is terminal on the periproct. A blood vascular sys- tem is well developed in some forms by enclosure of tracts of the haemo- coele in mesodermal walls. Usually there is a median dorsal vessel and a median ventral vessel connected by lateral trunks, from which are given off branches to the various organs of the body. The excretory system consists of paired segmental tubes (nephridia) opening, on the one hand, into the coelome, and, on the other, to the exterior. Respiration takes place either directly through the body wall or by means of gills, which are evaginations of the integument. The annelid nervous system includes a median prostomial ganglion, the brain (archicerebrum), lying dorsal to the alimentary canal, and a ventral nerve cord consisting of double segmental ganglia united by paired connectives. The brain and the first ventral ganglia are united by connectives embracing the oesophagus. The brain innervates whatever sense organs, such as palpi, tentacles, and eyes, may be located on the prostomium; it is often differentiated into a forebrain (protocerebrum) and a hindbrain (deutocerebrum). The forebrain may contain well- developed association centers in the form of stalked bodies, or corpora pedunculata. The germ cells of the mature annelid occur in groups imbedded in the mesodermal lining of the coelome, the simple organs thus formed being INTRODUCTION 3 (he gonads (ovaries and testes). The ripening ova and spermatozoa are discharged from the gonads either into the general coelomic cavity, from Avliich they escape through tlie nephridia or through pores of the body wall, or into siiccial coelomic receptacles connected bj’’ ducts with the. exterior. The young annelid larva has a characteristic form and is known as a trochophore. THE ONYCHOPHORA The onyehophorans, including Pcnpalus and related genera, are wormlike animals resembling the annelids in many respects. Though .‘segmentation is not evident in the C3'lindrical bodj’’ or in the somatic musculature, the presence of a scries of paired lateroventral ambulatoi'y appendages gives the animal a segmented appearance. The “legs” resemble the annelid parapodia in that each is .a hollow evagination of the bodj’ wall movable bj’’ four sets of muscles reflected into the appendage from the somatic wall. The mouth is situated anteriorly on the ventral surface at the base of a prostomial lobe. The prostomium bears a pair of tentacles and a pair of simiile ej’es. An extraoral mouth c.avity contains the true oral opening and a pair of stronglj’- muscul.atcd mouth hooks. The bodj' cavitj* of the Onj'chophora is continuous through the length of the animal. The circulatoiy .sj'stem consists of a dorsal vessel only, which has paired openings into the bodj' cavity between each pair of legs. The excretorj’ organs arc ncjihridia similar to those of Ajinclida, opening cxtcrnallj’ on the bases of the legs. Delicate internal air tubes (tracheae), arising in grouixs from pits scattered irrcgularlj' over the integument, probablj' subserve respiration. The nervous sj'stem consists of a dorsal brain located in the head, and of two long lateral nerve cords in which ganglia are but little difTcrentiated. The brain inncrv.ates the tentacles, the cj'cs, and the mouth hooks. The reproductive organs in each sex are a pair of long tubular sacs, the ducts of which unite in a median e.xit tube that oijcns vcntrallj' near the posterior end of the body. Most species of Onychophora arc viviparous, the embryo being developed within the oviducts of the female. The young animal takes on directly the form of the adult. The Onj'chophora h.ave often been regarded as primitive arthropods, but there is little in their organization that conforms with arthropod structure. Their relationships are undoubtedly with the Annelida, but the fact that the young .at no stage have any resemblance to a trochophore larva would seem to indicate that the Onychophora are not derived from typical annelids. The Onychophora, in fact, have .an ancient lineage of their own; fossil forms arc known from the Middle Cambrian that closely resemble modern species, except for the smaller number of legs. 4 PRINCIPLES OF INSECT MORPHOLOGY THE ARTHROPODA The arthropods have an annulate body and segmental appendages. Their distinctive features are the jointing of the appendages and a group- ing of the body segments, each appendage being composed of a number of lim b segments (podites) individually movable by muscles, while the body segments are segregated to form more or less distinct trunk sections (tagmata). The integument is usually hardened by the deposition of sclerotizing substances in definite areas of the cuticula, forming exo- skeletal plates (sclerites) to which most of the muscles are attached. The intervening membranous areas allow of movement between the plates. This character has given the arthropods an unlimited field for the development and evolution of exoskeletal mechanisms both in the trunk and in the appendages. The composition and specialization of the trunk sections, or tagmata, are characteristic of each of the several major groups of arthropods. The most constant and distinctive tagma is the head. In its simplest form the definitive head represents the embryonic protocephalon, consisting of a large preoral region and usually the first postoral somite. Generally, however, it includes a gnathal region (gnathocephalon) formed of at least three succeeding somites. The body region following the head may preserve a uniform segmentation and simple structure, or it may be variously differentiated into a thorax and an abdomen. In some forms the cephalic region and a varying number of succeeding somites are combined in a “cephalo thorax,” or prosoma, distinct from the abdomen. The primitive terminal segment (telson) is probably not a true somite, but an endpiece of the body bearing the anus, corresponding to the peri- proct of the Annelida. The appendages of the trunk include a pair of procephalic antennae (antermules), and a double series of segmented, postoral, ventrolateral limbs, potentially a pair on each segment but the last. The postoral appendages become variously modified in adaptation to functional special- izations. A typical arthropod limb consists of a basis (coxopodite) movable anteroposteriorly on the body, and of a six-segmented shaft (telopodite) movable in a vertical plane on the basis. Endite and exite lobes of the limb segments are frequently developed into specialized appendicular processes. The definitive alimentary canal includes long anterior and posterior sections (stomodaeum and proctodaeum) derived from the ectoderm. Typical segmental nephridia are absent. The blood vascular system is variously developed, but in plan it conforms with that of the Annelida. Respiration takes place either through the general body integument or by means of evaginations (gills) or invaginations (tracheae) of the body wall. INTRODUCTION 5 The nervous system of the arthropods has the same general structure as that of the Annelida. The primitive brain consists of a preoral body of nerve tissue lying above the stomodaeum, which is differentiated into protocerebral (ocular) and deutocerebral (antennular) regions, except in forms lacking antennae. The definitive brain in most groups, however, is a syncerebrum, since it includes also the first pair of ganglia of the ventral nerve cord, which secondarily become tritocerebral brain lobes. The protocerebrum contains often highly developed corpora pedunculata and the ocular centers; the deutocerebrum innervates the antennules (first antennae) ; and the tritocerebrum the first pair of postoral append- ages (chelicerae or second antennae). The visual organs of the arthropods include dorsal (median) and lateral eyes innervated from the protocerebrum. The dorsal eyes are always simple ocelli, located usually on the upper or anterior surface of the head, but in Xiphosura, and possibly in the trdobites and eurypterids, there is a pair of rudimentary ventral eyes on the deflected under surface of the head before the mouth. The number of dorsal eyes varies from one to eight, but often none is present; primitively there were perhaps two pairs; a single median dorsal ocellus probably represents the ocelli of one pair united. The lateral eyes are typically compound, beiug formed of groups of simple optic units composing a single organ, but often they are represented by groups of distinct ocelli. The reproductive organs are mesodermal sacs enclosing the germ cells. The paired mesodermal exit ducts open either separately to the exterior or into a common median outlet tube of ectodermal origin. The position of the genital aperture is variable. The Arthropoda include three major groups, namely, the Trilobita, the Chelicerata, and the Mandibulata. The Trilobita The trilobites are extinct creatures that flourished throughout the Paleozoic era but were most abundant during the Cambrian and Ordo- vician periods. They are the most generalized of known arthropods. The body is usually oval and flattened and carries ventrally a double series of jointed limbs. The trunk is divided into a head and two body regions known as the thorax and the pygidium, but the name "trilobite” is derived from the apparent triple division of the trunk lengthwise into an elevated median area (the axis, or rhachis) and two depressed lateral areas (pleurae). The head, which appears to include the prostomium and four somites, is covered by a dorsal carapace; the thorax consists of a variable number of free segments; the pygidium con- tains several segments, which, except in certain earlier forms, are united in a caudal shield. Each body segment, except the last, bears ventrally 6 PRINCIPLES OF INSECT MORPHOLOGY a pair of jointed appendages. On the upper surface of the head in most species are a pair of compound lateral eyes, and in some forms a median tubercle which appears to be a simple dorsal eye; on the under surface is a pair of small spots which some writers believe to be ventral eyes. The distinctively generalized feature of the trilobites, as compared with the other arthropods, is the lack of specialization and structural differentiation in the segmental appendages. The first pair of appendages, which probably are procephalic antennules, are filamentous and multiar- ticulate. The rest are without doubt postoral limbs. They are practi- cally all alike except that some of the more anterior ones may have a greater number of segments than the others. The trilobite limb pre- serves the typical form and fundamental^ structure of all arthropod appendages. The basis supports a large exite (epipodite) bearing a series of thin, closely set plates or filaments, which probably functioned as gills. The telopodite is usually six segmented. The trilobites appear to be related, on the one hand, to the Xiphosura, and, on the other, to the phyllopod crustaceans, since they have features characteristic of both these groups. They are not literally the ancestors of the other arthropods, however, since along with the trilobites there lived the highly specialized eurypterids and a large and varied crustacean fauna, but the trilobites are probably more closely related to the ancestral arthropods than are any other known forms. The Chelicerata In both the chelicerate and the mandibulate arthropods the seg- mental appendages are diversified in form and function, and some of them are suppressed. The most generally distinctive features that separate the Chelicerata from the Mandibulata are the suppression of the antennules, and the modification of the first postoral appendages to form a pair of chelicerae, which typically are pincerlike feeding organs. The body segments are grouped into two trunk regions, a prosoma and an abdomen. The first includes the protocephalon and the first six postoral somites, which are always more or less united. The abdomen varies in length and may be distinctly segmented, though in the higher forms it is usually short and its segmentation indistinct or suppressed. The prosoma bears six pairs of limbs, including the chelicerae, all of which, except the chelicerae, are generally leglike in form. The telopodites of some of the appendages often contain seven segments instead of six by the interpolation of a "patella” between the femur (meropodite) and the tibia (carpopodite). Abdominal appendages are usually absent, but in the more generalized forms they are retained in modified shape and may have gill-bearing epipodites as in the Trilobita. INTRODUCTION 7 The brain of the Chelicerata is a syncerebrum composed of the primi- tive cerebrum and the ganglia of the cheliceral segment, but, owing to the loss of the procephalic antennules, the deutocerebral centers are suppressed, and the brain consists of the protocerebrum, which innervates the eyes, and of the tritocerebrum, which innervates the chelicerae. The ocular organs include median dorsal eyes, lateral eyes, and in some cases ventral eyes. The lateral eyes are compound in more primitive forms; in others they are represented by groups of simple eyes. The Chelicerata include the Eurypierida, the Xiphosura, the Pycno- gonida, and the Arachnzda, Eurypterida. ^The eurypterids are extinct Paleozoic arthropods that lived from the Cambrian to the Carboniferous period but attained their greatest development in the Silurian and Devonian. They were aquatic, mostly fresh-water, or mud-inhabiting creatures. While the majority were relatively small, less than a foot in length, some became the largest of all known arthropods, reaching a length of (5 or 7 feet. The segments of the prosoma are united; the abdomen consists of 12 free segments, the last bearing a telson, which is usually a long tapering spine but is plate-like in some forms. The chelicerae are long or short. The next four pairs of limbs are generally small, and the sixth pair long and flat, evidently swimming organs ; but in some species the legs are all long and slender. The first five segments of the abdomen have plate-Jike appendages overlapping each other and concealing gills. On the upper surface of the cephalic region are a pair of compound lateral eyes and two small simple dorsal eyes; on the under surface, according to Stormer (1934), there appears to be in some forms a pair of ventral eyes. Xiphosura. The modern members of this group, commonly known as horseshoe crabs or king crabs, have so many points of resemblance with the ancient eurypterids that the two groups are often classed together as the Merostomata, and certain exi^met forms make the connection even closer. The body of the horseshoe crab is distinctly divided between the prosoma and the abdomen; in each, part the segments are united and covered by a large dorsal carapace, the second ending in a long spine-like telson. The six thoracic appendages are all chelate in the female. The abdomen contains only six segments, which bear plate-like appendages veiy similar to those of the anterior part of the abdomen of tlie euryp- terids, each except the first having a large gill-bearing epipodite. On the head are a pair of compound lateral eyes, a pair of simple median dorsal eyes, and a pair of rudimentary median ventral eyes. Pycnogonida, or Pantapoda. The pycnogonids are aberrant arthro- pods commonly kiiown as sea spiders. From the nature of their append- ages they appear to belong to the Chelicerata. 8 PRINCIPLES OF INSECT MORPHOLOGY Arachnida. Here are included the scorpions, the solpugids, the phalangids, the spiders, the ticks, and the mites. In most forms the trunk is divided in the typical chelicerate manner into a prosoma and an abdomen, but in the solpugids the prosoma is constricted between the fourth and fifth pairs of limbs. The abdomen may be long and distinctly segmented, in which case the distal part is narrowed and has the form of a jointed “tail,” but in the spiders and mites the abdomen is short and rotund, with indistinct or suppressed segmentation. The prosomal appendages include the chelicerae, a pair of pedipalps, and four pairs of legs. Abdominal appendages are either absent or represented by modi- fied rudimentary structures. The cephalic region of the prosoma generally bears a group of simple dorsal eyes, and, in some forms, on each side one to five simple lateral eyes ; the lateral eyes are never compound. Paleontologically the oldest known arachnids are contemporaneous with the trilobites and eurypterids. The Mandibulata The mandibulate arthropods differ from the Chelicerata in two characteristic features, namely, (1) the retention of the procephalic antennules, and (2) the modification of the bases of the second postoral appendages to form a pair of biting, jawlike feeding organs, the mandibles. The appendages of the first postoral segment, corresponding to the chelicerae of the Chelicerata, are present in most Crustacea as a pair of large biramous antennae (second antennae), but in the other mandibulate groups they are suppressed or are represented only by embryonic rudi ments. The telopodites of the mandibles may be retained in Crustacea in the form of “palpi,” but otherwise they are lost. The first two post- mandibular appendages are generally modified as accessory feeding organs (the first and second maxillae), though in some forms they are reduced and more or less rudimentary. The following appendages may all have a uniform structure, or they may be variously modified on differ- ent regions of the body; but a patellar segment is never present in any of them. Tagmosis is variable in the Mandibulata. The head may consist of the proto cephalon only (including the second antennal somite), but generally it contains a gnathocephalic section composed of the mandibular and the two maxillary somites, to which may be added the appendages, at least, of the next trunk segment. In the malacostracan Crustacea the protocephalon, the gnathal segments, and a varying number of succeeding segments are more or less combined in a “cephalothorax.” The Myriapoda and Hexapoda have a distinct head including four post- oral somites; the body of the first group shows no tagmosis, but in the hexapods it is differentiated into a thorax and an abdomen. INTRODUCTION 9 The brain of the Mandibulata is well differentiated into a proto- cerebrum and a deutocerebrum, innervating respectively the eyes and the antennules (first antennae) ; generally it includes also the first postoral ganglia of the ventral nerve cord, innervating the second antennae when these appendages are present; these ganglia become the tritocerebral lobes of the definitive brain, though they retain their primitive ventral commissure. The mandibular and both maxillary pairs of ganglia are united in a composite suboesophageal ganglion contained in the head when the latter includes the gnathal segments. A stomodaeal nervous system is usually present, having its principal connections with the tritocerebral ganglia. The major groups of the Mandibulata are the Crustacea, the Myriapoda, and the Hexapoda. Crustacea. ^The crustaceans include the phyllopods, the barnacles, the shrimps, crayfish and lobsters, the crabs, and related forms. The Crustacea are distinguished from all the other arthropods by a biramous structure of the limbs, each appendage typically having an outer branch (exopodite), arising from the basal segment (basipodite) of the telopodite, and an inner branch (endopodite), which is the shaft of the telopodite distal to the basipodite. The coxopodite, in many forms, supports a gill- bearing epipodite, which apparently corresponds to the similar basal appendicular organ of the limbs of Trilobita and the abdominal append- ages of Xiphosura. Tagmosis is variable in the Crustacea. The proto cephalon, including the tritocerebral somite, may constitute a small primitive head distinct from the rest of the trunk, or it may be united with several somites following to form a composite head; or, again, a variable number of segments in the thoracic region are more or less united with the head in a cephalothorax distinct from an abdomen. The first antennal appendages (antennules) are usually filamentous and multiarticulate; they are never biramous. The tritocerebral appendages (second antennae) are typically biramous and thus show that they belong to the series of postoral limbs, though the body segment bearing them forms a part of the protocephalon. The mandibles are always well developed; the two pairs of maxillae are small, sometimes rudimentary. The next three pairs of appendages are termed maxillipeds : the following five are differentiated in higher forms primarily as ambula- tory organs (periopods). The abdominal appendages (pleopods) are usually different from the thoracic appendages and are often specially modified. Most of the appendages may be modified for purposes of swimming. The brain in some of the lower Crustacea is a primitive cerebrum differentiated into a protocerebrum and deutocerebrum containing respec- 10 PRINCIPLES OF INSECT MORPHOLOGY lively the ocular and first antennal centers. In most forms, however, the definitive brain is a syncerebrum including the second antennal ganglia as tritocerebral lobes. The Crustacea have compound lateral eyes similar in structure to those of insects. Myriapoda. The common myriapods are easily distinguished from the other terrestrial arthropods by their slender forms and many legs, but a more generally distinctive character is the division of the trunk into only two parts, a head and a body. The head bears a pair of preoral antennae (antennules) and probably includes not more than four postoral somites. The appendages of the first somite are absent, those of the second are the mandibles, the other two pairs are variable in structure. The body is typically long and uniformly segmented, though some of the segments may be reduced, and in two groups the segments are united in pairs. Most of the primitive segments bear each a pair of legs. The myriapods are terrestrial animals; the majority of them have tracheal invaginations of the body wall for respiration. Eyes, when present in modern forms, consist of groups of simple lateral eyes, which approach the compound type in Scutigera. Certain Permian diplopods are said to have had large compound eyes. The Myriapoda include two principal groups. In the members of one group (progoneate forms) the reproductive organs open near the anterior end of the body; in those of the other group (opisthogoneate forms) the genital opening is at the posterior end of the body. The progoneate myriapods include the Diplopoda, the Pauropoda, and the Symphyla; the opisthogoneate group consists of the Chilopoda (centipedes). In the diplopods and pauropods most of the body somites appear to be united in successive pairs, at least dorsally; the legs of the diplopods occur in pairs on each double segment. The pauropods have branched antennae. Hexapoda. The hexapods are well characterized by the feature of their organization from which they get their name, which is the invariable specialization of three pairs of appendages as legs. The legs are always the appendages of the first three postgnathal somites ; the latter constitute a definite locomotor center, the thorax, distinct from the abdomen, which seldom bears organs of locomotion. The head has apparently the same composition as in the Myriapoda, since it always includes three pairs of gnathal somites. The abdomen never has more than 12 segments (11 true somites), of which the last is the periproct. The appendages of the head include a pair of pro cephalic antennae (antennules), embryonic rudiments of second antennae, a pair of man- dibles, and two pairs of maxillae. The second maxillae are united in a median composite organ known as the labium. The thoracic appendages are the three pairs of legs. Abdominal appendages may be present on INTRODUCTION 11 any of the 11 true somites of the abdomen, but they are always greatly modified or rudimentary and when present assume a variety of forms; generally most of them are absent in the adult stage. The appendages are never biramous in a manner comparable with the appendages of Crustacea, but epipodites may be present on the coxopodites. The brain of the hexapods has distinct protocerebral, deutocerebral, and trito cerebral centers, the last formed of the ganglia of the second antennal somite. The lateral eyes, when present, are typically com- pound, but in some adults and in many larval forms they are replaced by groups of simple eyes. Three dorsal ocelli are commonly present, the unpaired ocellus being median, anterior (or ventral) to the others, and probably double in its origin. The genital ducts open either separately or through a median tube of ectodermal origin. The paired duets of the female may open between the eleventh and twelfth segments of the abdomen (Protura), or between the seventh and eighth (Ephemerida). The median oviduct opens on the seventh, eighth, or ninth abdominal segment, the median ejaculatory duct on the ninth segment, except in Collembola where the genital open- ing in each sex is between the fifth and sixth definitive segments of the abdomen. The Hexapoda include several more or less distinct groups, which are the Protura, the Collembola, the Diplura, the Thysanura, and the Ptery- gota, but systematists are not agreed as to the relationships of these groups. Since the Pterygota are the winged insects, the other forms are generally termed collectively the Apterygota. Or, again, the Protura are set apart from the others, which are regarded as the true Insecta. The Diplura are usually classed with the Thysanura, the two groups then being distinguished as Thysanura entotrophica and Thysanura ectotrophica. Protura . ^The proturans are minute creatures resembling insects except that they lack antennae. They are more fundamentally distin- guished from the other hexapods by the fact that the body does not con- tain the definitive number of segments at hatching, there being added during growth two segments between the periproct and the preceding somite. This postembryonic development of segments is unknown in the other hexapods but is usual in the myriapods. The reproductive ducts in both sexes of Protura open behind the penultimate segment of the adult, as in Chilopoda. Notwithstanding these features, however, the Protura appear to be more closely allied to the insects than to the myriapods. The body is differentiated into a principal locomotor center, the thorax, composed of three segments bearing three pairs of legs, and into an abdomen of 12 complete segmental annuli, which has small or rudimentary tubular appendages on its first three segments only. The thoracic legs end each in a simple clawlike segment (dactylopodite) 12 PRINCIPLES OF INSECT MORPHOLOGY another myriapod feature, but one occurring also in some apterygote insects and in many pterygote larvae. Collembola— The collembolans are small creatures that constitute a sharply defined group of hexapods with many distinctive features. The abdomen never has more than six segments and is usually provided ■with three highly specialized appendicular organs. The first of these appendages is a large thick tube, the collophore, that projects from the ventral side of the first abdominal segment. The third is a leaping organ, the furcula, arising from the fifth segment, consisting of a large base and two long terminal prongs. When bent forward in repose, the furcula is held in place by the small forked second appendage, or tenaculum, arising from the third abdominal segment. The genital opening in both sexes occurs between the fifth and sixth segments of the abdomen. The repro- ductive organs closely resemble those of Protura. Diplura {Thysanura entotrophica ). The members of this group are characterized by having the mandibles and maxillae retracted into a pouch above the labium. In this respect and in certain other respects they are more specialized than the true Thysanura, but the latter are more closely related to the Pterygota. The common genera are Cam- podea and Japyx. Thysanura {Thysanura ectotrophica ) . The thysanurans have the mandibles and maxillae exposed in the usual manner, and they retain more complete remnants of abdominal appendages than do the Diplura. The coxopodites of the genital appendages in most species are provided with gonapophyses that form an ovipositor in the female, which appears to be the prototype of the egg-laying organ of pterygote insects. Of the two principal families, Machilidae and Lepismatidae, the second shows closer aflinities with the Pterygota in the structure and mechanism of the mandibles. Pterygota . The Pterygota include all the winged insects and their ■wingless relatives ; the latter presumably have lost their wings secondarily. The wings are lateral extensions of the dorsum of the second and third thoracic segments and were fully developed as organs of flight in the oldest known fossil insects. During the Carboniferous period, or probably earlier, a group of winged insects evolved a mechanism in the wing base for flexing the wings horizontally over the back when not in use. The descendants of this group (Neopterygota) include the majority of modern winged insects, while the more primitive nonwing-flexing insects are represented today by only two orders (Odonata and Ephemerida), both of which have descended from Carboniferous times but are not closely related to each other. Ontogenetically the wings are developed during postembryonic growth, and a postembryonic metamorphosis, in varying degrees, is INTRODUCTION 13 common in all the Pterygota. Among the wing-flexing insects, the wings in one group are developed during the larval stages within closed pouches of the body wall, and the larvae of members of this group have become so specialized that their transformation to the adult involves the inter- polation of a preimaginal instar, the pupa, between the larva and the fully formed adult. Insects of this group are known as the Endoptery- gota, because of the internal development of the wings, or as the Holo- metabola, in reference to their high degree of metamorphosis. Most of the other wing-flexing insects, as well as those that do not flex the wings, develop the wings externally in the usual manner of growth of appendic- ular organs, and they have in general a simpler metamorphosis that allows the nymph or larva to change directly into the form of the adult. These insects are distinguished, therefore, as the Exopterygota, or Hemi- metabola, but it should be observed that they do not constitute a mono- phyletic assemblage of forms, since they include a large group of wing-flexing insects and also the nonwing-flexing Odonata and Ephemerida. CHAPTER II GENERAL ORGANIZATION AND DEVELOPMENT To understand the structural organization of any animal, it is neces- sary to know that animal's history, for no living creature has arrived at its present organization by a direct line of development from its beginning. Structure generally is an adaptation to function; but many of the organs of complex animals have served a series of quite different functions during the course of their evolution and, as a consequence, have had their struc- ture many times remodeled by way of adaptation to their changing function or to new functions. The history of an animal cannot be known from the imperfectly preserved records of its past. Embryonic development may give us a suggestion of phylogenetic evolution, since in a general way the embryo repeats the history of its race; but the embryo, for purposes of its own, usually digresses much from the ancestral story, and it commonly abbrevi- ates its version of the earlier chapters or often omits these parts altogether. Fortunately, however, the less highly evolved the final form of an organism, the more of its early history is it likely to retain in its ontogeny. Hence, we may approximate a reconstruction of the phylogeny of a species by filling out the obscure passages in its embryonic story, or by suppl3dng those deleted, with material judiciously selected from the facts of embryonic development in related forms successively lower in the scale of organization. To the evidence derived from embryology, however, we must always add that to be deduced from a study of comparative anatomy, for in the structure of serially related adult animals we often get an insight into the course of evolution more reliable than that to be obtained from any other source. In the present chapter, therefore, we shall endeavor to build up a concept of the fundamental organization of an insect on evidence derived from embryology and from a study of the adult structure of other arthro- pods, of Peripaius, and of the annelid worms. The Germ Cells and the Soma. The generative cell from which any individual of the metazoic animals is produced begins development by division. After successive repetitions of division there are soon formed two distinct groups of cells in the resulting cell mass ; those of one group continue to be germ cells, those of the other group become the somatic cells. It is the organization of this second and always larger group of 14 GENERAL ORGANIZATION AND DEVELOPMENT 15 cells forming the soma, or body of the future animal, that we study in ordinary anatomy. The purpose of the soma primarily is to give protection to the germ cells during the period in which these cells develop to maturity. It then becomes obligatory upon the soma, in most forms of animals, to bring the germ cells of opposite sexes together in such a manner that a sufficient number of them may unite to insure perpetuation of the species. In the higher forms of animals, the soma has also taken upon itself the responsibility of securing protection to the fertilized eggs and to the early developmental stages of the new somata formed by them. Some ani- mals acquit themselves of this duty merely by depositing their eggs in places where the developing embryos will have a reasonable guaranty against destruction; others retain the fertilized eggs and give the embryos space for development within their own bodies. Finally, assuming stUl greater responsibility, the soma of the more highly endowed types of animals charges itself with the duty of protecting and nourishing the young during the adolescent period of postembryonic life. In addition to fulfilling its many parental functions, and in order to accomplish these functions, the soma must also maintain itself as a living unity. The methods that the soma has adopted for carrying on its various functions are the reasons for its structure; and the continual reorganizing of its structure in adaptation to more efficient ways of accomplishing its functions has resulted in its evolution from a simple to a complex organ- ization. The different methods that the somata of different animals have adopted and perfected to meet their obligations are expressed in the many forms of life existing today and in those that have existed during the past. Structure, vegetative functions, sensitivity to environmental conditions and changes, the power of automatic reaction to impinging forces, instincts, consciousness, intelligence, and the faculty of making voluntary adjustments to external conditions all these things are properties devel- oped and perfected in the soma for the benefit of the germ cells or for the somata that are to accompany the succeeding generations of germ cells. The soma of the more complex animals cannot reproduce itself ; when its purposes are accomplished, its physical elements disintegrate, its vitality is reduced, and sooner or later its enemies or adverse circumstances bring about its death. The germ cells appear to be the reproductive agents of the soma; but “reproduction,” so called, is more truly a repeated production of somata and germ cells. Each germ cell multiplies by division, and the resulting cells undergo a development into spermatozoa or ova, accord- ing to their sex, or some of them may become merely nutritive cells for the others. The persisting germ cells remain as separate entities, except in the union of male and female cells to form a composite cell 16 PRINCIPLES OF INSECT MORPHOLOGY from whicli the development of both germ cells and soma usually proceeds. The ovum alone, however, may produce a complete soma; in insects parthenogenesis is of frequent occurrence. The germ cells carry the determinants of heredity, called genes, whatever they may be, and apparently in most cases receive no direct influence from the accompany- ing soma. Typical Early Stages of Development, The egg, being a cell, has aU the constituent parts of an ordinary cell; but, since it is destined to a much greater activity than any of the body cells, it is provisioned with a supply of nutritive material, known as yolk, or deutoplasm, stored in the meshes of its cytoplasm. The known facts of comparative embryology lead us to believe that the early developmental stages in all animals were once essentially the same, though actually they may now be very different Fig. 1. Typical early stages in general embryonic development. A, the blastula (diagrammatic). B, C, D, three stages in the differentiation and invagination of the endoderm (End), and the formation of the mesoderm (Msd) of a chiton. (From Kowalevsky, 1883.) in different animals. Some of the differences are clearly correlated with the quantity of yolk contained in the egg; others are to be attributed to other causes, which may be termed embryonic expediency, for the embryo, as well as the postembryonic instars of the animal, departs from the ancestral line of development wherever an advantage is offered in so doing. The development of the insect presents one of the greatest puzzles that the embryologist encounters, and it is certain that the insect embryo does not structurally reproduce free-living stages of its ancestry. In order to understand insect structure as presented by the embryo, there- fore, we must discover the fundamental things in its development that underlie those that have been built up to suit the convenience of the embryo. In the typical, generalized form of animal development, which proceeds from eggs containing a minimum quantity of yolk, the first division, or cleavage, of the egg cuts the egg into approximately equal halves, and the sunilar succeeding divisions soon produce a globular GENERAL ORGANIZATION AND DEVELOPMENT 17 mass of cells, or blastomeres, called a morula. Then a cavity, the blasto- coele, appears in the center of the mass, and the cells become arranged in a single layer at the surface. This stage is the blastula (Fig. 1 A); the superficial layer of cells is the blastoderm {Bid). The term blast, so fre- quently recurring in embryological names, comes from the Greek word ^'Kaarbs, meaning a bud or sprout. The cells on one side of the blastula become distinguished from the others by an increase in size (Fig. 1 B). The larger cells then sink into the blastocoele cavity, usually as a hollow invagination of the blastula wall, and the primitive creature takes on the form of a double-walled sac (C). This stage in general embryonic development is the (^asirwZa. The blastoderm is now differentiated into an outer layer, or ectoderm {Bed), and an inner layer, or endoderm {End), between which is the remnant of the blastocoele {Blc). The new cavity is the gastrocoele {Gc), or archen- teron, and its external opening is the blastopore {Bpr). The endoderm is not necessarily produced in all animals by invagina- tion. Gastrulation is often accomplished by an internal proliferation of cells from the blastoderm forming an inner cell mass that later becomes excavated by a cavity which is the gastrocoele. Invagination and proliferation, therefore, are but two variants of the developmental process of gastrulation. Some of the lowest metazoic animals, such as the Coelenterata, never progress beyond the two-layered stage; but in all higher forms the two-layered gastrula becomes three layered by the differentiation of a middle layer, or mesoblast, from the primary inner layer or from points where the outer and inner layers meet. The mesoblast may be produced from scattered cells proliferated from the outer layer, and in such cases it is called a mesenchyme; but generally it takes the form of a definite cell layer, known as the mesoderm. The definitive middle layer may include cells produced in both ways. In the three-layered stage, the embryo thus has become differentiated into an outer ectoderm (Fig. 1 D, Bed), an innermost endoderm {End) lining the gastrocoele {Gc), and an intermediate mesoderm {Msd). The gastrocoele becomes the stomach of the future animal, the epithelial walls of which are formed of the endoderm. The mesoderm, in its typical mode of development, is given off laterally in each side of the body where the endoderm joins the ectoderm just within the lips of the blasto- pore (Figs. 1 D, 3 A, Msd). Animals possessing a mesoderm are typically elongate in form and are bipolar, since one end is habitually forward in progression; they acquire a dorsoventral differentiation of structure and consequently have a bilateral symmetry in their organization. The blastopore, originally posterior, as in the free-swimming larvae of certain Coelenterata, becomes ventral either by a migration in position or by a 18 PRINCIPLES OF INSECT MORPHOLOGY forward elongation on the under surface of the embryo. These stages of development, which probably took place in the ancestors of the arthro- pods, are well illustrated in the larvae of annelid worms and in the embryo of Peripatus (Fig. 2). Few animals, however, actually follow the simple form of early development outlined above; many appear to depart widely from it. Yet it is believed that in all cases the actual development is but a modifica- Fio. 2. jEarly stages in the embryonic development of Peripatus capensis. (From Balfour, 1883.) tion of the simple type. The arthropods particularly are aberrant in their ontogeny. The student of entomology, therefore, must keep clearly in mind the basic course of development in order to understand the depar- tures from it that will be encountered in the growth of the insect embryo. The Insect Egg. ^The egg of an insect is usually contained in a shell, called the chorion (Fig. 4 B, Cho). The chorion has the appearance and texture of the body-wall cuticula, but it is said to be nonchitinous. It is Fig. 3. Transverse sections of a young embryo of Peripatus capensis, shoiving the lateral mesoderm bands and their coelomic cavities. (From Balfour, 1883.) formed in the ovary as a product of the walls of the egg follicle, and it bears externally the imprint of the follicular cells. At the anterior end of the egg in most cases the chorion is perforated by a minute opening or group of pores known as the micropyle {Mi), which permits the entrance of the spermatozoa. The delicate cell wall of the ovum becomes the vitelline membrane {Vit) of the mature egg. When the egg contains much yolk, or deutoplasm (Fig. 4 B), most of its cytoplasmic material is distributed in a layer of clear cortical cytoplasm {cpl) about the periphery GENERAL ORGANIZATION AND DEVELOPMENT 19 of the egg, and in a small mass of nuclear cytoplasm (npl) around the nucleus, the remainder being reduced to a network of strands or sheets, in the meshes of which is held the yolk material (F). The typical shape of an insect egg rich in deutoplasm is elongate oval, which is the usual form of the egg within the ovarian tubule (Fig. 287) ; but when extruded the egg may take on various shapes, ranging from that of an elliptical globe to that of a flattened disc. The chorion may be a smooth shell having the form of the egg, or it may assume curious and bizarre shapes. Pro- tected eggs are usually of the simple elliptical, oval, or elongate type (Fig. 4 B), with one side commonly a little less convex than the other or sometimes slightly incurved. A few insects enclose their eggs in cases, or Fig. 4. Two types of insect eggs, sectional views. A, egg of a collembolan. Isotoma cinerea, with small quantity of yolk. {From PMliptschenko, 1912.) B, diagram of usual type of insect egg containing much yolk. oothecae, formed by a secretion discharged from the oviduct or from special colleterial glands. Cleavage and the Formation of the Blastoderm. Most insect eggs contain such a large quantity of yolk that their size is quite out of proportion to the amount of their protoplasmic matter. The ordinary insect egg, therefore, to avoid what would be a cumbrous procedure if its first cleavage division were to cut through its entire mass, limits its activities to the nucleus and the nuclear cytoplasm. This method of cleavage in the egg is called meroblastic, in distinction from total cleavage, which is holoilastic. Not all insect eggs, however, are meroblastic; the eggs of Collembola, which have a comparatively small quantity of yolk (Fig. 4 A), divide by entire cleavage, and the holoblastic form of division continues until a typical morula is formed (Fig. 5). The early development of Anurida has been described by Claypole (1898), that of Isotoma by Philiptschenko (1912). The blastomeres in the morula of Anurida, Claypole says, are prac- tically all of equal size (Fig. 5 G, H). In the follovdng stage, however, there ensues a period of disintegration in the blastomeres (Fig. 6 A, Blm), accompanied by a migration of the nuclei in small masses of cytoplasm (a) toward the periphery of the egg. The yolk is thus left behind in the 20 PRINCIPLES OF INSECT MORPHOLOGY central part of the egg as an inert mass in which the original cell boundaries are lost. The migrating nucleated fragments of cytoplasm, which are now virtually small cells, divide as they proceed to the exterior and arrange themselves in two layers (B) . The outer, continuous A Fig. 6 . Example of holoblastic cleavage in an insect egg, Anurida maritima.^ {From Claypole, 1898.) A, group of eggs. B-G, successive stages of cleavage resulting in a typical morula (H). layer is the ectoderm {Ecd), the inner, incomplete layer is the mesoderm (Msd), Some of the cells, however, remain behind in the yolk (B, End). These cells, according to Claypole, later form the stomach epithelium in Anurida and are therefore endodermal. Fig. 6. Developmental stages of Anurida maritima following holoblastic cleavage of the egg. (From Claypole, 1898.) A, disintegration of the blastomeres. B, differentia- tion of the germ layers. In Isofoma, as described by Philiptschenko, the holoblastic cleavage of the egg produces a compact morula in which the cleavage cavity disap- pears. Then each of the blastomeres divides into a smaller outer cell containing the nucleus, and into a large, nonnucleate inner sphere which retains the yolk. The small nucleated cells migrate outward between the yolk spheres and come to rest at the surface of the egg, where they GENERAL ORGANIZATION ANB DEVELOPMENT 21 form at first a single continuous layer, which is the blastoderm. The blastoderm cells later differentiate into an outer layer of ectoderm and an inner layer, which, in Isotoma, Philiptschenko claims is the common rudiment of the future mesoderm and endoderm. In meroblastic eggs the nucleus and its containing mass of cytoplasm, situated in the interior of the egg, behave in the manner of an ordinary cell. By repeated division, together with division of the daughter cells (Fig. 7 A), they produce an increasing number of cleavage cells (CCls), which migrate to the periphery of the egg (B) and there become disposed in a single layer just beneath the vitelline membrane (B, C). The cortical cytoplasm then unites with the cleavage bodies to form definitive cells, which constitute the blastoderm (C, D, Bid). In this stage, the insect embryo derived from a meroblastic egg is equivalent to the blastula Fig. 7. Diagrams of meroblastic cleavage and the process of blastoderm formation typical of insect eggs having a large quantity of yolk. in the generalized form of development (Fig. 1 A), but the cells of the blastoderm are all contained within the original egg wall, or vitelline membrane. Within the blastoderm is the mass of yolk material held in the meshes of the inner cytoplasm of the egg, and usually a few yolk cells (Fig. 7 C, YCls). The latter are nucleated cytoplasmic masses derived from the egg nucleus that did not take part in the formation of the blastoderm. In addition to these primary yolk cells, other cells are often found in the yolk after the completion of the blastula, which are said to be proliferated from the blastoderm. These cells, some investigators claim, are endo- derm cells that accomplish a partial digestion of the yolk before the embryonic stomach is formed, and for this reason they have been named vitellophags. It appears, as we shall presently see, that the walls of the stomach in some insects are formed in part or entirely from migratory endoderm cells proliferated into the yolk at an early stage of development. The Germ Cells. The germ cells of many insects become recognizable at the time the blastoderm is being formed. In the collembolan Isotoma, 22 PRINCIPLES OF INSECT MORPHOLOGY Philiptschenko (1912) says, the germ cells first appear as a small, compact group of cells in the yolk near the posterior end of the blastula (Fig. 8 A, GCls). In insects with meroblastic cleavage the germ cells are usually observed first at the posterior end of the blastoderm (Fig. 7 C, GCls), where they lie in a differentiated protoplasmic area called the germ tract (Keimbahn) and in some cases protrude somewhat from the surface of the Fig. 8. Early differentiation of the germ cells (OCls) from the somatic cells. A, blastula of Isotoma cinerea. (From Philiptschenko, 1912.) B, blastula of Drosophila melanogaster with germ cells segregated at posterior pole. (From Huettner, 1923.) blastoderm (Fig. 8 B, GCls). The protoplasm of the germ tract is often marked by the presence of dark-staining granules, and certain writers have suggested that it is some peculiar quality of the germ-band proto- plasm (Huettner, 1923), or even of the granules themselves (Hegner, 1914), that differentiates as germ cells whatever cleavage cells happen to wander into the germ tract. Huettner shows that even at this early stage Fig. 9. Diagrams of the formation of the germ band (OB) on the ventral side of the blasto- derm. A, cross section. B, longitudinal section. there is a difference in the nuclei between the germ cells and the blastoderm cells. The germ cells undergo a short period of activity during which they increase slightly in numbers, and then they enter a long period of rest while the somatic cells are developing. The germ cells move into the interior of the body and are finally lodged in the dorsal part of the meso- derm that forms the ovaries or testes (Fig. 22, GCls). The Germ Band. When the insect blastoderm has completely sur- rounded the egg, the creature is in the blastula stage (Figs. 1 A, 7 C). Its cleavage cavity, or blastocoele, is filled with the yolk. The first GENERAL ORGANIZATION AND DEVELOPMENT 23 differentiation that leads to the formation of the specific embryo consists of a thickening of the cells in that wall of the blastula which is to be the ventral side of the future insect (Fig. 9). This region of enlarged cells (GB) appears on the surface as an opaque oval or elongate area, which is known as the germ band, germinal disc, or primitive streak (Keimscheibe, Keimstreif). The blastoderm is now differentiated into an embryonic area {GB) and an extraembryonic field (DBl), the latter being the dorsal part of the blastoderm, composed of small, flat, often attenuate cells, termed the dorsal blastoderm, or serosa. Formation of the Inner Germ Layers. This subject is a most difficult one to understand in insect embryology, if by “understanding” we mean an interpretation of the Imown facts of the formation of the germ layers in insects according to the terms of the general gastrulation theory. The crux of the difficulty lies in the fact that the apparent gastrulation of the insect embryo gives rise directly to a cell layer that forms the tissues which are of mesoblastic origin in other animals, while the epithelium of the mesenteron, which should be the endodermal wall of the gastrula- tion cavity, appears in most cases to be derived independently from the blastoderm. In the typical and general mode of development, as we have seen, the embryo becomes two layered by the invagination of one side of the blastoderm (Fig. 1 C) ; but it was noted also that the invagination method of gastrulation may be modified into a process of internal proliferation of cells from a closed blastopore area of the blastoderm. The typical embryo next becomes three layered by the generation of a mesoderm from invaginations or proliferations of cells from the blastoderm just within the lips of the blastopore where the ectodermal and endodermal layers meet (Figs. 1 D, 3 A, Msd). It is evident, therefore, that the normal relations of the endoderm and the mesoderm might be obscured if the endoderm were formed in part or entirely by the proliferation of scattered cells, for it would then appear that gastrulation produces only the mesoderm. A discontinuous formation of the endoderm commonly takes place in the arthropods, and this condition has led to much mis- understanding of the true relations of the germ layers; but if we keep in mind the important fact that it is the function of the endoderm to absorb and digest the yolk, we need not be disconcerted by the various ways it adopts for fulfilling its destiny. In some of the Crustacea an archenteron is formed by typical gas- trulation in which an invaginated endoderm becomes directly the stomach of the mature animal. In such cases the endoderm absorbs the yolk mass and passes it into the archenteric cavity, from which it is drawn for use by the growing tissues. In certain other crustaceans the walls of the archenteron disintegrate, and the liberated endoderm cells migrate into 24 PRINCIPLES OF INSECT MORPHOLOGY the yolk, whence they later emerge and arrange themselves again in an epithelial layer, but this time surrounding the yolk to form the walls of the definitive stomach. In other cases, again, though the major part of the endoderm is fragmented, two flat masses of its cells remain intact beneath the yolk, and from these rudiments the mesenteron epithelium is later regenerated. In this case the endoderm cells lost to the yolk remain there as vitellophags. These various modifications of mesenteron formation by the endoderm are generally correlated with the amount of yolk in the egg. When we turn now to the insects, we find, so far as known, that the primitive method of forming the mesenteron by the typical invagination Fig. 10. Diagrams suggesting the evolution of germ-layer formation in pterygote insects. A, primitive gastrula with open blastopore and lateral bands of mesoderm. B, blastopore closing. C, blastopore closed, the archenteron in state of disintegration. D, most of endoderm cells dispersed in yolk as vitellophags {Vph), leaving only a ventral remnant {End) intact. E, differentiation of the ventral endoderm remnant into anterior, intermediate, and posterior mesenteron rudiments {AMR, IMR, PMR). process of gastrulation does not exist. We must presume, then, that, having become impracticable owing to the amount of yolk in the insect egg, it has been completely eliminated from insect ontogeny. Various stages may be traced, however, in the fragmentation of the major part of the endoderm, and the differentiation of its ventral remnant into several groups of cells that regenerate the embryonic stomach, or mesen- teron. These groups of cells are the so-called mesenteron rudiments. In the insect embryo the blastopore is never an open aperture, but the blastopore area, as indicated by the region from which the endoderm and mesoderm are formed, becomes elongated forward on the ventral side of the embryo, appearing sometimes as a groove (Fig. 23 A, Bpr) comparable with the long, open, slitlike blastopore of Peripatus (Fig. 2 B). We may GENERAL ORGANIZATION AND DEVELOPMENT 25 assume, therefore, that gastrulation and the formation of the mesoderm originally took place along this line (Fig. 10 A). With the closure of the blastopore, then, the mesoderm comes to be the first layer immediately within the blastopore area (B, C, Msd). In this case it is clear that the endoderm {End) would appear to be generated from the mesoderm, and that by complete disintegration it might take the form of cells scattered through the yolk (C) or, on the other hand, a ventral remnant of it might be preserved beneath the yolk (D). In the light of this concept we can understand many of the processes of mesoderm and endoderm formation described by different VTiters on insect embryology, the discrepancies between which have seemed irreconcilable. The Mesoderm. In the Collembola, according to Claypole (1898) and Philiptschenko (1912), the first-formed inner layer is the mesoderm. It is produced by a general tangential division of the blastoderm cells, the division taking place either at the time the cleavage cells reach the surface of the egg (Anurida, Claypole) or afterwards {Isotoma, Philip- tschenko). This method of mesoderm formation may be conceived as an unlocalized form of proliferation from the blastoderm. In the Thysanura the mesoderm is formed by a localized proliferation of cells from the midventral line of the blastoderm, the area of prolifera- tion being sometimes marked externally by a pit or a groove. In Cam- podea, according to Uzel (1897), and in Leptsma, as described by Heymons (1897), the mesoderm proliferation arises at a central point of the blasto- derm, the cells spreading out into a disc as they are given off. Heymons does not believe that the limited extent of the proliferation area here represents a polar gastrulation; he attributes it rather to the small size of the egg and to the oval shape of the germ band. In the Pterygota the surface of the usually elongate germ band becomes differentiated into a median area, or middle plate (Fig. 11 A, MP), and into two lateral areas known as the lateral plates {LP). From the middle plate is formed the mesoderm, usually in one of three ways; in some cases by an infolding or invagination of the middle plate (B, C), in others by an overgrovdh of the median plate by the lateral plates (D, E), and in stUl others by a proliferation of cells from the inner surface of the middle plate (F). In any case the formation of the mesoderm evidently represents a modified gastrulation, from which the endoderm appears to have been eliminated; but, as we shall presently see, the middle plate more probably represents a common mesoderm-endoderm rudiment, since a part of the endoderm is usually generated from the mesoderm or in close association with it. The Endoderm. 'V\Tiere the processes of gastrulation take place by proliferation from the area of a closed blastopore, it is clear, as we have already noted (Fig. 10 C), that the endoderm, if produced in connection 26 PRINCIPLES OF INSECT MORPHOLOGY with the mesoderm, will appear to arise from the latter. In the collem- bolan Isotoma, Philiptschenko (1912) says, the endoderm is generated from that part of the inner germ layer along the midventral line of the embryo that does not take part in the formation of the mesoderm somites (see Fig. 19 A). The endoderm tissue consists at first of three groups of cells an anterior group, which is the anterior mesenteron rudiment, a posterior group, which is the posterior mesenteron rudiment, and a median strand of loosely associated cells forming an intermediate mesenteron rudiment lying beneath the yolk and connecting the two terminal rudi- ments. The definitive embryonic stomach, or mesenteron, is produced by the multiplication of cells in each of the rudiments and the growth of Fig. 11. Diagrams showing three methods of mesoderm formation in pterygote insects. A, cross section of egg with germ band differentiated into middle plate {MP) and lateral plates (iP). B, C, the middle plate invaginated to form the mesoderm. D, E, the middle plate overgrown by the lateral plates. F, mesoderm proliferated internally from the middle plate. the cells around the yolk until they form an epithelial sac enclosing the latter. Here, therefore, it is clear that the primitive inner layer is a mesoderm-endoderm layer, since the endoderm is differentiated from its entire length. A similar condition is said by Strindberg (1913) to exist in termites {Eutermes) and in ants, where the endoderm arises by a proliferation of cells from the entire length of the mesoderm (or meso- derm-endoderm rudiment) ; these cells lie beneath the yolk and eventually surround it to form the walls of the mesenteron. There is reason to believe that the enclosure of the yolk by endoderm cells derived from the mesoderm-endoderm rudiment does not strictly represent the process of gastrulation in insects, but that it is a secondary regeneration of the mesenteron taking place subsequent to an earlier fragmentation of the walls of the true gastrulation cavity. This con- clusion is to be deduced from the observation of several investigators. GENERAL ORGANIZATION AND DEVELOPMENT 27 According to Claypole (1898), the endoderm cells of the collembolan Anurida originate during cleavage of the egg, but they remain in the yolk, while the blastoderm cells that give rise to the ectoderm and mesoderm are migrating to the surface. Finally, however, the endoderm cells engulf the yolk and then arrange themselves to form the definitiA^e mesenteron, which contains practically no yolk except that in the bodies of its cells. A similar method of mesenteron formation is described by Heymons (1897) for Lepisma. In this insect, Heymons says, the mesenteron epithelium is generated from cells found at an early stage of develop- ment within the yolk. These cells later migrate to the periphery of the yolk mass where they become aggregated into scattered islands that grow in extent by multiplication of their cells until eventually they sur- round the yolk and constitute the wall of the definitive mesenteron. In the Odonata the mesenteron is said by Tschuproff (1904) to be a composite organ made up of cells derived from three distinct sources, its anterior and posterior parts being formed from cells proliferated from the inner ends of the stomodaeum and proctodaeum (Fig. 13, Siam, Proc), while its middle part is formed of cells that migrate outward from the yolk. The cells derived from the yolk Tschuproff regards as the true endoderm; the others she claims are ectoderm cells. This conclusion concerning the nature of the anterior and posterior parts of the mesen- teron is not necessary, for it is well known that the anterior and posterior mesenteron rudiments derived from the middle plate of the embryo may be carried inward by the stomodaeal and proctodaeal invaginations; but the observations of Tschuproff, if true, do show that the Odonata present a gradient condition in the endodermal activities between one in which the mesenteron is re-formed from a disintegrated archenteron and one in which it is regenerated from anterior and posterior groups of endo- dermal cells. It is interesting to note that Eastham (1927) finds in Pieris rapae a median proliferation of cells from the middle plate of the embryo (Fig. 12, MPCls); these cells take no part in the mesoderm formation but pass into the yolk and there disintegrate. The proliferation of these cells, Eastham says, begins at the anterior end of the middle-plate region and “continues along the middle line of the embryo from before backward,” until there is formed “a median track of proliferation passing almost from end to end of the embryo.” T^Tiile Eastham apparently does not definitely commit himself to the view that these ceUs are endodermal, he mentions ha^dng observed in one series of sections a continuity at an early stage of development between them and the posterior mesenteron rudiment. It seems clear, however, that the line of proliferating cells must represent the true gastrulation of the insect in a rudimentary and 28 PRINCIPLES OF INSECT MORPHOLOGY disintegrated state. The cells, Eastham says, “in all probability help in the liquefaction of the yolk, rendering the latter capable of being assimilated by the germ band.” These " middle-plate cells that wander into the yolk are evidently endodermal vitellophags. In the majority of pterygote insects, mesenteron rudiments have been found only at the two ends of the mesoderm (Fig. 13 B, AMR, PMR ) ; but, as we have observed, it is claimed by Strindberg that in the termites and in ants the anterior and posterior rudiments are connected by an intermediate rudiment (A, IMR), thus duplicating the condition described by Philiptschenko in Isotoma. It is evident, therefore, that the usual condition, in which the mesenteron is formed from anterior and posterior rudiments alone, has resulted from a suppression of the inter- Fig. 12. Proliferation of cells (MPCls) from tlie middle plate of the germ band of Pieria rapae. (From Eastham, 1927.) mediate rudiment, or from the migration of its cells into the yolk, as in Pieris, and that the two persisting end rudiments represent the final remnants of a disintegrated archenteron (Fig. 10 C). In the Lepidoptera, according to Schwangart (1904) and Eastham (1927), the anterior and posterior mesenteron rudiments are formed in continuity with the meso- derm ; but in other insects the two rudiments often appear to arise either directly from the blastoderm at the two ends of the mesoderm (Fig. 10 E) or from the walls of the ectodermal stomodaeal and proctodaeal invaginations (Fig. 13). Resume of Gastrulation . ^The formation of a stomach by the ancestral process of open gastrulation (Fig. 10 A) becomes impracticable in embryos containing a large quantity of yolk. In insect phytogeny the blastopore, after elongating forward on the ventral side of the embryo, has become permanently closed (B, C) and gastrulation takes place by proliferation instead of by open invagination, the cells thus formed migrating sepa- rately into the yolk. This process represents a fragmentation of most of the wails of the archenteron (C), an embryonic device by which the GENERAL ORGANIZATION AND DEVELOPMENT 29 endoderm cells may surround the yolk. In some of the Apterygota the definitive stomach (mesenteron) is said to be regenerated by the scattered endoderm cells; in Odonata it is perhaps regenerated in part by these cells. In some of the Apterygota and in most of the Pterygota, however, the proliferated endoderm cells remain as vitellophags in the jmlk (D, Vpli), and the definitive stomach is regenerated from a ventral remnant of the endoderm (C, D, End) that remains in contact with the meso- derm below the yolk. This ventral endoderm remnant may consist of a continuous band of cells coextensive with the meso- derm (E, AMR, IMR, PMR), or it may be reduced to anterior and posterior groups of cells, which are the usual anterior and posterior mesenteron rudiments (AMR, PMR). Throughout the entire course of the life cycle of most insects the endodermal epithelium of the mesenteron is subject to disintegration and regeneration. The Alimentary Canal. The stomach, or mesenteron, of the insect embryo is clearly not the primitive archenteron, though when completed it is a sac com- posed of endoderm cells. As we have just seen, it may be formed by the reassembling of the primi- tive endoderm cells scattered in the yolk, or in part from such cells and in part from ventral endoderm remnants. In most insects, however, it is regenerated entirely from a median ventral remnant of the archenteron, including anterior, inter- mediate, and posterior rudiments (Fig. 13 A, AMR, IMR, PMR) or more commonly from anterior and posterior rudiments only (B, C, D, AMR, PMR). The cells of these mesenteron rudiments multipl}’- and extend over the surface of the yolk (C) until those proliferated from opposite ends meet and finally constitute a complete epithelial sac containing the ^mlk (D) . The sac is later enswathed by sheets of muscle tissue and even- tually becomes the definitive stomach, or vcniricidus, of the adult insect. Fig. 13. Diagrams shon'ing the formation of the definitive alimentary canal in pterygote insects. A, the germ layers in longitudinal section, including the three regenerative endodermal rudiments of the mesenteron (AMR, IMR, PMR). B, C, D, envelopment of the yolk by the growth of the mesenteron rudiments to form the stomach (Menl), and the ingrowth of the ectodermal stomodaeum (Slom) and proctodaeum (Proc). 30 PRINCIPLES OF INSECT MORPHOLOGY The complete alimentary canal (Figs. 13 D, 14) comprises, in addition to the stomach, anterior and posterior sections derived as secondary invaginations from the ectoderm. These parts, known respectively as the stomodaeum (Stom) and proctodaeum (Proc), later open into the mesenteron and thus give the alimentary canal its final form of a complete tube extending through the body (Fig. 20) . The embryonic development Fig. 14. Median longitudinal vertical section of a mature male embryo of the honey bee surrounded by the serosa and chorion. {From Pclrunkevilch, 1901.) of the alimentary canal, however, may entirely ignore the phylogenetic order of events, for it is often found that the stomodaeal and proctodaeal invaginations are formed prior to the generation of the mesenteron. The rudiments of the latter may then be carried inward by the ectodermal ingrowths, in which case the mesenteron appears to be generated from the opposing ends of the ectodermal parts of the alimentary canal. Fig. 15. Diagrams suggesting the evolution of the synaptic sensory-motor nervous system. A, the primary sensory nerve cells (SCI') located in the ectoderm and connected directly -with muscles (Md). B, the primary nerve cells internal (now motor cells) and stimulated indirectly by nerves from a second set of sensory cells (SCI") in the ectoderm. Origin of the Nervous System. All nerve tissue originates in the ectoderm. Nerve cells are ectodermal cells in which the properties of irritability and conductivity are highly developed. If their inner ends are connected with the motor tissues, the primary nerve cells (Fig. 15 A, SCI') become the agents for transmitting external stimuli to the motor elements (Mel), and thus they may directly incite the latter to action. In most animals, however, the nerve cells connected with the effector mechanisms sink beneath the surface of the ectoderm where they become shut off GENERAL ORGANIZATION AND DEVELOPMENT 31 from direct contact with the outer world (B, SCls'). The sunken nerve cells must then be themselves stimulated through the agency of other nerve cells {Scl”) that retain surface relations. In this way the general nervous system of complex animals comes to consist functionally of a motor system lying entirely within the body and of a sensory system main- taining, on the one hand, a connection with the exterior and, on the other, establishing connections with the motor system (B). In the annelids and arthropods the motor nerve cells are contained mostly in two more or less closely associated tracts of nerve tissue forming the ventral nerve cord (Fig. 16 C, VNC) Ijdng along the midventral line of the body. It is probable that this double median nerve cord has been Fig. 16. Diagrams showing the probable evolution of the annelid-arthropod type of central nervous system from a preoral nerve center (Arc) and two latoroventral nerve cords {NC). evolved from two lateroventral bands of ectodermal nerve cells forming two primitive lateral cords (A, B) which have approached each other medially. In the annelids the two cords are united anteriorly with a small nerve mass, or primitive brain, the so-called archicerebrum (Arc), lying in the anterior end of the body primarily before the mouth. In the ontogeny of insects most of the nerve tissues originate at an early stage of development from two longitudinal thickenings of the ectoderm near the midline of the germ band, known as the neural ridges (Fig. 17 A, NIR), between which is a median neural groove (NIG). From the inner surfaces of the ridges are proliferated longitudinal rows of large cells, the neuroblasts (Nbl), which are the primarj’- nerve cells of the future nerve cords. A median row of neuroblasts also is formed above the neural groove. The neuroblasts multiply by division and produce three strands 32 PRINCIPLES OF INSECT MORPHOLOGY of ganglionic cells (B, GngCls). Thus the definitive ventral nerve cord of insects appears to take its origin from two primitive lateral cords of nerve cells (LC) and a median cord {MC). The nerve cells formed from the neuroblasts send out processes which become the nerve fibers. Some of the fibers remain in the nerve cords where they establish communica- tion between the nerve cells; others extend outward to the other body organs and constitute the motor nerves. The sensory nerves of insects proceed inward from sensory cells of the ectoderm to the ventral nerve cord and here form intimate associa- tions with the motor cells. Little is definitely known, however, con- cerning the actual development of the sensory nerves in arthropods. When segmentation takes place in the body, the nerve cords are differentiated into segmental regions, or ganglia (Fig. 16 B, Gng), containing the nerve cells and into intersegmental connectives {Con) composed of nerve fibers. The two ganglia of each segment become connected crosswise by fibrous commissures {Com) and finally are so closely approximated and bound to each other that they constitute a single segmental ganglion (C), though the intersegmental con- Fig. 17. Embryonic origin of the ventral ,. j 1. 1 rnt. nervous system from the ectoderm. {Dia- nectives remain double. Ine grams based on figures from Wheeler, 1891.) successive paired ganglia in the Ecd, ectoderm; GngCls, ganglion cells; LC, , i i j. lateral nerve cords; MC, median nerve cord; lUture nead region 01 the inseCt Nbl, neuroblasts; NIG, neural groove; NIR, always COaleSCe Ul tWO grOUpS .' those neural ridge. anterior part of the head unite with the primitive archicerebrum to form the brain (Fig. 14, Br), which lies above the stomodaeum {Stom)] those of the posterior part compose the suboesophageal ganglion {SoeGng), which lies below the stomodaeum. The Embryonic Coverings. ^The embryos of most insects become separated in one way or another from contact with the egg shell during a part or all of the period of their development. We may distinguish four methods of separation, namely, (1) by invagination of the embryo, (2) by involution of the embryo, (3) by the formation of cellular protective membranes, (4) by the production of cuticular coverings. The last constitutes a distinct process, probably allied to moulting; the others MC GngCls g Nbl GENERAL ORGANIZATION AND DEVELOPMENT 33 intergrade, and the second and third are possibly derivations from the first. Invagination of the Embryo. In the Apterygota the germ band doubles upon itself ventrally at an early stage of development and sinks into the egg or the yolk in the form of an inverted U, in which position it may remain until the embryo is almost fully formed (Fig. 18 A, B). This method of development is characteristic of many other arthropods, and its retention in the apterygote insects apparently has a phylogenetic significance. In the Collembola (Fig. 18 A) the embryo fills the entire egg after the ventral flexure takes place. In Lepisma, as described by Heymons (1897), the embryo begins its development on the surface of the egg in the usual manner, but soon it curves ven- trally and sinks into the yolk (B). As the submergence increases, a part of the surrounding extraem- bryonic blastoderm is turned in also, and the mouth of the cavity contracts to a small pore, but it does not entirely close. Later, the Lepisma embryo partially emerges and completes its devel- opment with its head and thoracic parts again on the surface (C). Involution of the Embryo. In several other orders of insects, including the Odonata, some of the Orthoptera, and the Homoptera, the embryo, which begins its development on the surface of the egg (Fig. 18 D), turns into the yolk tail end first, by a process of revolving on its transverse axis, and stretches out within the egg in a reversed and inverted position (E). The revolution of the embryo carries a part of the extraembryonic blasto- derm into the yolk, and when the reversal of the embryo is complete the opening into the embryonic cavity is closed. There is thus differentiated from the extraembryonic blastoderm an inner part lining the embrjmnic cavity, which is the amnion {Am), and an outer part, or serosa (Ser), which surrounds the egg. Before the odonate or hemipteran embrjm Ser- F ' I Fig. is. T hree methods by which the insect embryo may become separated from the egg shell. A-C, invaoination of the embryo (A, Isoloma cincrca, from Philiptschcnko 1912; B, C, Lepisma, from Heymons, 1897). D-F, involution of the embrj’O and its return to the sur- face of the egg, diagrammatic. G-I, formation of cellular covering membranes, diagrammatic. 34 PRINCIPLES OF INSECT MORPHOLOGY completes its development, it reverts again to the surface by a counter- revolution that restores it to its original position (F). Cellular Embryonic Membranes.— The more usual type of embryonic covering is that produced by outgrowths of the blastoderm forming a cellular sheath over the embryo. In typical cases of this kind folds from the extraembryonic part of the blastoderm grow out around the ends or along the sides of the germ band (Fig. 18 G, b), the edges of which come together (H) and unite beneath the embryo (I) . The corresponding layers of the opposite folds then become continuous, and the embryo is shut in beneath two cellular membranes, the outer of which is the serosa {Ser ) , the inner the amnion {A m ) . Usually the amnion and serosa remain in contact over the ventral surface of the embryo; but in some cases, particularly in the Lepidoptera, the embryo and the amnion sink into the yolk, an,d a part of the yolk then fills the space betAveen the amnion and the serosa, producing a submerged condition of the embryo. In the honey bee, according to Nelson (1915), the embryonic covering consists of a single layer of cells, formed by two outgrowths from the serosa along the edges of the germ band, which eventually meet and unite beneath the embryo. The cellular embryonic membranes usually disappear before the embryo reaches maturity. In most cases they separate in a longitudinal cleft beneath the embryo, and the resulting folds are carried upward as the dorsal blastoderm contracts above the expanding lateral walls of the growing embryo. The cells of the dorsal blastoderm and the amniotic folds eventually sink into the yolk on the dorsal side of the egg, where they are finally absorbed. In the honey bee, the single embryonic membrane is said to persist until the time of hatching, when it is broken up by the movements of the young larva in the egg. Embryonic coverings formed of blastodermic folds are characteristic of insects; a similar structure is said to be found in other arthropods only in the scorpions. Cuticular Embryonic Membranes . Embryonic coverings of a non- cellular structure are of common occurrence in all groups of the Arthro- poda. These membranes, apparently, are of the nature of cuticular exuviae, for it is stated by Campbell (1929) that the embryonic invest- ment of the cockroach is a chitinous tissue. Their separation from the embryo, therefore, may be regarded as an embryonic moult. The embryonic membranes are shed at the time the young arthropod leaves the egg or shortly thereafter. Cuticular embryonic coverings occur in all insects with incomplete metamorphosis and have been observed in some holometabolous forms. In Anurida, as recorded by Claypole (1898), three distinct membranes are given off from the entire surface of the embryo before the appendages GENERAL ORGANIZATION AND DEVELOPMENT 35 are formed and before the embryo curves into the egg. In most insects, however, only one cuticular embiyonic membrane is known to exist. In some cases, as in the aphids, the membrane has the form of a simple sac enveloping the embryo, but in other insects the sac may be provided with pouches that individually ensheath the appendages. Evidentl}’-, then, the period of embryonic growth at which the moult is given off varies in different insects. A similar embrimnic covering occurs in Arachnida, Crustacea, ChUopoda, and Diplopoda, in some cases envelop- ing the embryo as a cuticular sac, in others having close-fitting extensions over the appendages. Segmentation of the Body. The first important step in the evolution of the annelids and arthropods was that by which the bod}’’ lost its unity of structure and became broken up into a series of parts, or somites. The somites are in general called body segments, but in the embryo they are more specifically termed metameres. In ontogenetic development, metamerism is usually described as originating in the mesoderm, the segmental regions bemg first marked by a closer massing of the mesodermal cells, which thus appear as a series of opaque areas in the germ band alternating with more transparent lines. It is pointed out by Eastham (1927, 1930), however, that in the embrj’^o of Pieris external segmentation, shown by indentations of the ectoderm, precedes the formation of the mesodermal somites. In the larvae of the annelid worms, which develop two lateral bands of mesoderm tissue, the segmental masses of mesodermal cells soon become hollowed by the formation of clefts between their dorsal and ventral cells, there being thus produced a series of paired mesodermal pouches that eventually e.xtends through the entire length of the body. These pouches are the coelomic sacs. As development proceeds, the coelomic sacs enlarge upward in the sides of the body and either remain thus as definitive segmental body cavities or unite in a common body cavity coextensive with the length of the animal. In the Arthropoda the mesoderm becomes distinctly segmented, but coelomic cavities are not so definitely formed as in the Annelida, and in many cases they are entirely absent. Where they appear they generally take the form of small clefts in the lateral parts of the mesoderm (Fig. 19 A, Coel, Cocl), but the series on each side soon becomes converted into a coelomic tube by the disappearance of the partitions, and the irmer walls of the tubes break down (B), so that the segmental cavities are at best but evanescent. Segmentation in the arthropods, as in the annelids, is not a process of “budding.” The somites are formed alwa 5 "s anterior to a small terminal piece, the telson, or pcriproct (Fig. 20, Prpt), which contains the anus and gives rise to the proctodaeal invagination. Even in arthropods in which 36 PRINCIPLES OF INSECT MORPHOLOGY segmentation is not complete at hatching, the segments added during postembryonic development are formed in the region immediately anterior to the periproct. Embryonic segmentation usually appears first in the gnathal region and proceeds posteriorly, though there is often much irregularity in the sequence of the newly forming somites. The seg- Fiq. 19. Formation of the coelomia sacs and the ventral nerve cord in the embryo of Forficula. (From Heymons, 1895.) A, cross-section of young embryo showing ooelomic clefts (Coel) in the mesoderm. B, later stage in which the ventral nerve cord is differenti- ated from the ectoderm, and the ooelomic sacs open into the epineural sinus (EpnS) of the haemocoele. mented area of the trunk, however, finally extends from the mouth to the periproct, but, since the mouth lies a short distance behind the anterior pole of the embryo, there is always an unsegmented preoral region of the animal, which is the prostomium (Fig 20, Prst). The prostomium is Fig. 20. Diagram of the fundamental structure of a segmented animal of the annelid- artoopod type. An, anus; Arc, archicerebrum; Ment, mesenteron; Mth, mouth; Prpt, periproct; Proc, proctodaeum; Prst, prostomium; Stom, stomodaeum; VNC, ventral nerve cord. represented in the arthropod embryo probably by the cephalic lobes (Fig. 23, Prc ) ; in the annelid trochophore larvae it is the large preoral swelling of the trunk. The segments between the prostomium and the periproct are the true somites, of which in the insects there are at least 18 when segmentation is complete. The insect embryo attains its defini- GENERAL ORGANIZATION AND DEVELOPMENT 37 tive segmentation before leaving the egg, but in the Protura the last two somites are formed just anterior to the periproct during postembryonic life. Since segmentation affects primarily the mesoderm and the ectoderm, but not the endoderm, the alimentary canal is never segmented ; but the metamerism of the body influences most of the organs subsequently formed, such as the nervous system, the circulatorj’^ organs, the body appendages, the tracheal invaginations, and the reproductive organs. The Segmental Appendages. ^The acquisition of segmentation was undoubtedly in itself an important event in the evolution of the seg- mented animals toward a higher development because of the increased facilities of movement that came with it ; but the advantages of metamer- ism were not fully available until the segments became equipped vith movable appendages. The segmental appendages of arthropods are developed in the embrim as hollow, paired outgrowths of the body wall, appearing soon after segmentation, on the lateroventral parts of the germ band (Fig. 23 B). Each appendage contains an extension of the mesoderm and becomes differentiated into a series of parts, the limb segments, or podites, which are eventually movable on each other through the development of muscle fibers in the mesoderm of the appendage. Each body segment between the prostomium and the periproct potentially may develop a pair of appendages; but various appendages suppressed in the adult may not appear even in the embryo. The possession of jointed segmental append- ages separates the arthropods from the annelids and undoubtedly has given them the possibility of attaining the high degree of development that sets them so far above their wormlike relatives. Completion of the Body Wall. ^The embrjm, we have seen, begins its development as a disc or band of thickened cells on the ventral side of the egg (Fig. 9, GB). The germinal area may be but a small part of the blastoderm, or it may almost encircle it, but there is always left a dorsal region {DBl) consisting of very thin attenuate cells that take no direct part in the formation of the definitive insect. As the marginal parts of the germ band increase in extent, they spread dorsallj’’ over the egg, and the extraembryonic field of the blastoderm contracts until it is con- densed into a cell mass that sinks into the yolk. The contracting dorsal blastoderm carries with it the ruptured amniotic folds, if the embrj*o has an amniotic covering, and all these tissues drawn into the j’olk are there absorbed to be utilized as nutriment bj' the growing embrj'o. With the disappearance of the dorsal blastoderm, the back of the embrjm is completed bj' the dorsal closure of the extended lateral plates, and the bodj' of the embryo thus becomes an OA'al or elongate sac with continuous walls formed from the ectodermal layer of the primitive germ band. 38 PRINCIPLES OF INSECT MORPHOLOGY Rep DMcl '■ Cdbl CdS Cdbl ^Epd EMsd / ' _ , ... .. ’.O'. .■■■%] IMsd The Definitive Body Cavity —In many adult annelids the body cavity consists of a series of segmental compartments which are the enlarged cavities of the coelomic sacs of the embryo. The arthropods have a continuous body cavity formed in part only from the rudimentary coelomic clefts in the embryonic mesoderm (Fig. 19 A, Cod). As the arthropod embryo develops, the median strand of the mesoderm breaks apart and some of its cells are said to be converted into free blood cells (Fig. 19 B, BCh). The space in •Ment which the blood cells lie is known as the epineural sinus {EpnS) ; it is a part of the haemocoele, or blood cavity of the insect, which is the remnant of the blastocoele after the invasion of the latter by the meso- ,r!,^ .rL derm. The coelomic clefts become BCls VNC VMol . V -J r .V j Fig. 21. Diagrammatic cross section COntUlUOUS HI 63,011 SldG 01 trllG DOCly of a nearly mature insect embryo. Apd, disintegration of the tranSVerse segmental appendage; BC, definitive body , , i ii cai-ity; scis, blood cells; Cdbl, cardio- partitions, and theiT inner walls blasts; CdS, cardiac sinus; Z)il/cZ, rudiment qj. Jggg break doWU, The of dorsal muscles; EMsd, external meso- ... derm layer (somatopleure) ; Epd, epider- epmeural SmUS thus becomes COn- misj/ilfsd, inner mesoderm layer (splanch- the COelomic SpaceS. nopleure); Merit, mesenteron; Rep, ft-, j rudiment of reproductive organ; Tra, The cavity SO formed enlarges Upward trachea; YMcl, rudiment of ventral .^^,jth the growth of the meSoderm muscles; Five, ventral nerve cord; y, yolk. ° around the yolk or the alimentary canal, penetrates into the appendages, and forms eventually the continuous body cavity of the segmented body of the arthropod (Fig. 21, BC). The mesoderm in each side of the body is split by the coelomic clefts into an outer lamina, or somatic layer (Fig. 21, EMsd), and into an inner lamina, or splanchnic layer (IMsd). The first becomes applied to the body waU, the second to the wall of the alimentary canal. The Mesodermal Organs. ^From the mesoderm are formed the muscles, the heart, the blood cells, the fat body, connective tissue, and the parts of the reproductive organs that are not of ectodermal origin. Since the development of the principal mesodermal organs will be given in the chapters on the circulatory system, the fat body, and the repro- ductive system, we need give but brief attention to the subject here. The Muscles. The muscle fibers of arthropods are all of the striated type of structure and are multinucleate, the nuclei in some cases being superficial and in others buried within the body of the fiber. Investi- gators differ as to whether each fiber represents a single multinucleate cell or is the product of many united cells. The muscles of the body wall and GENERAL ORGANIZATION AND DEVELOPMENT 39 the appendages and probablj'' the dilator or suspensory muscles of the alimentary canal are derived from the somatic layer of tlic mesodei-m; those that surround the digestive tube, including the ectodermal stomo- daeum and proctodaeum, are said to be all derived from the splanchnic layer. The Dorsal Blood Vessel. The dorsal blood vessel, or principal cir- culatory organ of the insect, is a muscular tube Ijdng along the dorsal midline of the body. Its lumen is derived from the dorsal part of the haemocoele which formed a cardiac sinus (Fig. 21, CdS) enclosed between the upper undivided ends of the lateral mesoderm bands. As the latter, or cardidblasis (Cdbl), approach each other above the alimentary canal, their inner faces become hollowed as two opposing furrows; and when DV Fia. 22. Cross section through the dorsal part of the abdomen of a male ombr.vo of tlio honey bee. (From Pclrunkccilch, 1903.) finally the two layers meet, the margins of their grooves unite to form a tube, which is the dorsal blood vessel (Fig. 22, DV). The walls of the vessel are converted into transverse muscle fibers, which give the tube a strong contractile power. The blood gains access to the heart cavity through lateral openings, oslia, in its walls; it is driven forward and expelled from the anterior end beneath the brain. The Fat Body. The jirincipal fat-containing tissue of insects consti- tutes a definite structure known as the /a/ body, which consists of a loose aggregate of mesodermal cells lying in the body cavit 3 ^ The first cells of the fat body are formed in the embryo, but the organ increases greatly in size bj’’ cell multiplication and cell growth during j^ostcmbrj'onic stages, when the cells take on other functions besides that of fat storage. The Organs of Reproduction. The intci’iial parts of the rejjroductive organs, in which the germ cells arc lodged, and in which these cells undergo their subsequent development into the spermatozoa or eggs, accordiiig to their sex, appear first as thickenings of the sjjlanchnic walls of the derm in the abdominal region of the bod}'. The cell groups that form these thickenings, or genital ridges, arc the rudiments of the tc.‘!(cs or ovaries (Fig. 22, Rep). From each a strand of cells continues rearward, which becomes the duct of the organ, a vas deferens in the male, an 40 PRINCIPLES OF INSECT MORPHOLOGY oviduct in the female. In a few of the lower insects each duct opens by an independent aperture near the posterior end of the body ; but in most insects the two vasa deferentia or the paired oviducts unite in a single median tube produced by an invagination of the ectoderm. The ecto- dermal tube thus formed becomes the ductus ejaculatorius in the male (Fig. 292 A, Dej), and the oviductus communis of the female (Fig. 284 A, Ode). The Tracheal System. ^The respiratory organs of insects are entirely ectodermal; they consist both of evaginations and of invaginations of the body wall. The latter, taking the form of branching tubes, or tracheae, ramifying minutely throughout the body cavity, constitute the usual respiratory system of all insects whether aquatic or terrestrial. Respira- tory evaginations form “blood gills” and occur only in a few aquatic larvae. The rudiments of the tracheae appear first at a comparatively late stage of embryonic growth as segmental pits in the ectoderm along the sides of the body (Fig. 19 B, Tra). The pits deepen into tubes, the tubes fork out into branches, the branches subdivide until the tracheae from each primitive pit, the opening of which is now a spiracle, form a finely branched system in the corresponding half of the body segment. By the union of some of the branches from successive and opposite spiracles, longitudinal trunks and transverse commissures are established. Since the tracheal system is evidently a comparatively late acquisition in insects, it is not necessary to assume that all the segments once bore spiracles. There is embryonic evidence, however, of the former existence of spiracles on at least each of the segments from the last head segment to the tenth abdominal segment, as will be shown in Chap. XV. The Definitive Body Form. ^The final external form of the insect may be traced through five theoretical evolutionary stages. First is the wormlike stage (Fig. 24 A) in which the animal consists of a long segmented part coextensive with the length of the alimentary canal and of a short unsegmented preoral part, or prostomium {Prst). The mouth {Mth) is situated ventrally between the prostomium and the first segment; the anus is terminal in the last segment, or periproct {Prpt). The prostomium contains the principal sensory ganglion and may be regarded as the primitive head, or archicephalon. In the second stage (Fig. 24 B) each body segment between the prostomium and the periproct acquires a pair of movable lateroventral appendages, and one or two pairs of antennal organs may be developed on the prostomium. The third stage (Fig. 24 C) is characterized by the union of the first postoral somite with the primitive head to form a composite head struc- ture, which, being the first stage in the evolution of the definitive head, may be termed the protocephalon (Prc). The somite involved is that GENERAL ORGANIZATION AND DEVELOPMENT 41 bearing the postantennal appendages, or second antennae (Fig. 23 B, Pnt), which in msects are reduced in the cmbrj'o and disappear in the adult, while the somite itself loses its identity after union with the procephalic lobes (C), though its ganglia become the tritoccrebral lobes of the brain. The protocephalon forms the procephalic part of the definitive insect head, but in some Crustacea it remains as a distinct cephalic structure bearing the e 5 '^es, the labrum, and the two pairs of antennae. The protocephalon, or the procephalic part of the definitive arthropod head, is commonly regarded as containing two primitive somites between the prostomium and the tritoccrebral segment, cor- responding to preantennal and the first antennal appendages, but there is Fig. 23. Differentiation of tno sections (tOKmata) of the insect trunk ciurinK embryonic development. A, young embryo of Leptinolarsa. (From Whedrr, ISSO.) 15, cmI)iyo of Anurida witli appendage rudiments. (From Whrclcr, IS93.) C, embryo of Xaucorin, with trunk differentiated into protocephalic, gnathal, thoracic, and abdominal sections. (From Heymons, 1S99.) reason to believe that the antennal and preantennal regions of the head are prostomial in origin, and that the postantennal, or tritoccrebral, region represents the first primitive .somite, which is postoral. This view is e.xpressed in the diagrams (Fig. 24) and will be amplified in subsequent chapters on the head and the nervous sA'sfem. The fourth stage in the develoinncnt of the bodj' form (Fig. 24 D) differentiates the bisects from all other arthropods. It is well shown in the embrj'o of many insects (Fig. 23 C). In this stage the trunk seg- ments back of the protocephalon {Pre) become segregated into three regions. The first may be called the gnathal region (Gn), since its append- ages are destined to become feeding organs; the second is the thoracic region (Th), set apart as the locomotor center of the insect in' the special development of its appendages as locoinotoiy organs; the third is the abdominal region (Ab), on which the appendages are reduced and mostly obliterated. 42 PRINCIPLES OF INSECT MORPHOLOGY In tlio fifth stage (Fig. 24 E) the body of the insect attains its final form. The gnathal segments are now united with the protocephalon in the definitive head (H), which thus comprises, the prostomium and four succeeding segments and carries the gnathal appendages as well as the protocephalic appendages. In the Pterygota the thorax acquires wing rudiments in the form of paranotal lobes, two pairs of which eventually ^ 1 eproauctive activities. a by wMcl aside a few cells to nrocrp!+ group of cells' like itself, sets soma from the others we ^ ^ ^ species and then forms a highly organized systems of organs ^ «tudy of the Lions entity and fulfills its destinv maintains itself as a living Before the insect ettoee^/XST-r,' “> “• out of the egg shell. functions, however, it must get GENERAL ORGANIZATION AND DEVELOPMENT 43 Hatching. A young animal enclosed in a hard-shelled egg is left to gain its liberty by its own exertions; with most insects maternal responsibility ends with the deposition of the eggs. Many insects at hatching, moreover, find themselves in a situation where it is yet impos- sible to begin their destined postembryonic life, for the eggs may be buried in the ground, contained in a chamber excavated in the wood of a tree by the mother, hidden in a deep crevice, covered with a protective substance, or enclosed in a horny case. Hence the young creature is often confronted with a hard task to be performed during the first few minutes or half hour of its active existence. Its escape from its first surroundings, however, is not made in a haphazard manner but is orderlj’' directed by “instincts,” and often the young insect is provided with special structures to lighten its task. The first adult function to become active in the hatching insect larva is tracheal respiration. The embryo within the egg makes use of the oxygen that diffuses through the egg shell and is dissolved in the body fluid; the entire tracheal system at this time is filled with a clear liquid. With the first movements of the young larva preparatory to hatching, however, the tracheal liquid is quickly replaced by free air, the liquid being drawn back from the main tracheal branches into the tracheoles, from which it finally disappears, while air simultaneously fills the tubes as the liquid retreats. So mysterious in aspect is this rapid aeration of the tracheal system that various impossible theories have been proposed to account for it on the assumption that the entering air drives out the liquid. It has been shown by Keilin (1924a), however, that the activating force must be the retraction of the liquid, and that the air automatically fills the emptied tracheae, since the walls of the latter are constructed to withstand atmospheric pressure. Sikes and Wiggles- worth (1931) point out that the mechanism that brings about the absorp- tion of the tracheal liquid at this time is probably the same as that which is functional after hatching, namely, that it is osmotic tension created in the blood by the discharge of metabolites resulting from the first muscular activities of the larva, which causes the tracheal liquid to be drawn into the blood or the tissues through the semipermeable walls of the tracheae, as in ordinary tracheole respiration (see page 459). If the larva has an open tracheal system, air can enter the tracheae through the spiracles, but if the latter are closed it must diffuse into the tracheae from the blood. The process of hatching, or eclosion from the egg, is accomplished by some insects simply by gnawing a hole through the chorion with the mandibles. With most insects, how^ever, the jaws are not free organs at the time of hatching, or they may not be of the biting type of structure. In such cases the insect must rupture the egg shell by body movements, and it olten gives itself for this purpose a closer contact against the chorion 44 PRINCIPLES OF INSECT MORPHOLOGY by swallowing the amniotic fluid, or air that has