Principles of Insect Morphology R. E. Snodgrass Principles of Insect Morphology R.E.Snodgrass CHAPTER I INTRODUCTION CHAPTER II GENERAL ORGANIZATION AND DEVELOPMENT Glossary of Embryological Terms CHAPTER III THE BODY WALL AND ITS DERIVATIVES 1. The Body Wall 2. External Processes of the Body Wall 3. Sense Organs 4. Ectodermal Glands, Corpora Allata, Oenocytes 5. Muscle Attachments on the Body Wall 6. Moulting Glossary of Terms Applied to the Body Wall CHAPTER IV BODY REGIONS, SCLERITES, AND SEGMENTATION Glossary of General Terms Applied to the Body Segments and the Skeletal Plates CHAPTER V THE SEGMENTAL APPENDAGES OF ARTHROPODS Glossary of Terms Applied to the Appendages CHAPTER VI THE HEAD 1. General Morphology of the Arthropod Head 2. Structure of the Definitive Insect Head 3. Special Modifications in the Structure of the Head Glossary of Terms Applied to the Head CHAPTER VII THE HEAD APPENDAGES 1. Preantennal Appendages 2. The Antennae 3. The Postantennal Appendages 4. The Mandibles 5. The Superlinguae 6. The Maxillae 7. The Labium 8. Glands of the Head Appendages Glossary of Terms Applied to the Head Appendages CHAPTER VIII THE THORAX 1. Evolution of the Thorax 2. The Neck 3. General Structure of the Thorax 4. The Prothorax 5. The Pterothorax 6. The Thoracic Muscles Glossary of Terms Applied to the Thorax CHAPTER IX THE THORACIC LEGS 1. The Structure of the Legs 2. Muscles and Mechanism of the Legs Glossary of Terms Applied to the Parts of an Insect's Leg CHAPTER X THE WINGS 1. Origin and Evolution of the Wings 2. Development of the Wings 3. Structure of the Wings 4. The Wing Muscles 5. The Wing Movements 6. Insect Flight Glossary of Terms Applied to the Wings CHAPTER XI THE ABDOMEN 1. General Structure of the Abdominal Segments 2. The Abdominal Musculature 3. The Abdominal Appendages CHAPTER XII THE ORGANS OF INGESTION 1. The Preoral Cavity 2. The Cephalic Stomadaeum 3. The Feeding Mechanism of Neuroptera and Coleoptera 4. The Feeding Mechanism of Hymenoptera 5. The Feeding Mechanism of Lepidoptera 6. The Feeding Mechanism of Diptera 7. The Mouth Parts of Siphonaptera 8. The Feeding Mechanism of Thysanoptera 9. The Feeding Mechanism of Hemiptera 10. The Feeding Mechanism of Anoplura CHAPTER XIII THE ALIMENTARY CANAL 1. Development of the Alimentary Canal 2. General Structure of the Alimentary Canal 3. The Stomodaeum 4. The Mesenteron 5. The Proctodaeum 6. The Filter Chamber Glossary of Terms Applied to the Alimentary Canal CHAPTER XIV THE ORGANS OF DISTRIBUTION, CONSERVATION, AND ELIMINATION 1. The Blood 2. The Organs of Circulation 3. The Fat-body 4. The Oenocytes 5. The Corpora allata 6. The Organs of Elimination Glossary of Terms Used in This Chapter CHAPTER XV THE RESPIRATORY SYSTEM 1. The Integument as a Respiratory Organ 2. Blood Gills 3. The Tracheal System 4. General Mechanism of Tracheal Respiration Glossary of Terms Applied to the Respiratory System CHAPTER XVI THE NERVOUS SYSTEM 1. General Structure, Organization, and Function of the Nervous System 2. The Central Nervous System 3. The Stomodaeal Nervous System 4. The Peripheral Nervous System Glossary of Terms Applied to the Nervous System CHAPTER XVII THE SENSE ORGANS 1. General Structure and Classification of Insect Sense Organs 2. The Hair Organs 3. The Campaniform Organs 4. The Plate Organs 5. The Scolopophorous Organs 6. The Eyes Glossary of Terms Applied to the Sense Organs CHAPTER XVIII THE INTERNAL ORGANS OF REPRODUCTION 1. The Female Organs 2. The Male Organs 3. General Morphology of the Reproductive Organs Glossary of Terms Applied to the Internal Reproductive Organs CHAPTER XIX THE ORGANS OF COPULATION AND OVIPOSITION 1. The Male Genitalia 2. The Female Genitalia Glossary of Terms Applied to the External Genitalia REFERENCES INDEX Last updated Dec. 17, 1997 Leo Schouest Principles of Insect Morphology R.E. Snodgrass CHAPTER I INTRODUCTION Chapter Outline Introduction 1. The Annelida 2. The Onychophora 3. The Arthropoda · The Trilobita · The Chelicerata 1. Eurypterida 2. Xiphosura 3. Pycnogonida or Pantapoda 4. Arachnida · The Mandibulata 5. Crustacea 6. Myriapoda 7. Hexapoda · Protura · Collembola · Diplura · Thysanura · Pterygota INTRODUCTION Morphology, in the biological sense, is the science of form in living organisms. Anatomy is the determination of structural facts. Morphology 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 living state. The various structural types of organisms are special ways of accomplishing these functions, that is, for doing the same things in different ways 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 working of the physical mechanisms with which he deals in his anatomical studies, he must look for the significance of structural modifications and innovations, 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 within 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. Structural modification has been carried to a high degree in all the arthropods, affecting some organs in one group, others in another group; and particularly is this true of the insects. Hence, in the discussion of the morphology of insect organs given in the following chapters, many references 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 system is well developed in some forms by enclosure of tracts of the haemocoele 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 the gonads (ovaries and testes). The ripening ova and spermatozoa are discharged from the gonads either into the general coelomic cavity, from which they escape through the nephridia or through pores of the body wall, or into special coelomic receptacles connected by ducts with the. exterior. The young annelid larva has a characteristic form and is known as a trochophore. THE ONYCHOPHORA The onychophorans, including Peripatus and related genera, are wormlike animals resembling the annelids in many respects. Though segmentation is not evident in the cylindrical body or in the somatic musculature, the presence of a series of paired lateroventral ambulatory appendages gives the animal a segmented appearance. The "legs" resemble the annelid parapodia in that each is a hollow evagination of the body wall movable by 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 simple eyes. An extraoral mouth cavity contains the true oral opening and a pair of strongly musculated mouth hooks.