SMITHSONIAN MISCELLANEOUS COLLECTIONS VOLUME 85, NUMBER b morpholo(;y of the insect abdomen FART I. GENERAL STRliCTllRr^ OE THli ABDOMEJ AND rrS APPENDAGES BY R. E. SNODGRASS Bureau of Entomology, U. S. Department of Agriculture (l'UBLlC\Tloy 3124) CITY OF WASHINGTON PUBLISHED BY THE SMITHSONIAN INSTITUTION NOVEMBER 6, 1931 BALTIMORE, MD., U. S. A. MORPHOLOGY OF THE INSECT ABDOMEN PART I. GENERAL STRUCTURE OF THE ABDOMEN AND ITS APPENDAGES By R. E. SNODGRASS Bureau of Entomology U. S. Dkpartment of Aoriculture CONTENTS Introduction i I. The abdominal sclerotization 6 II. The abdominal segments 14 The visceral segments 16 The genital segments i" The postgenital segments 19 III. Tlie abdominal musculature 28 General plan of the abdominal musculature 31 The abdominal musculature of adult Pterygota 42 The abdominal musculature of endopterygote larvae 48 The abdominal musculature of Apterygota 56 IV^. The abdominal appendages 62 Body appendages of Chilopoda 65 Abdominal appendages of Crustacea 68 The abdominal appendages of Protura 70 General structure of the abdominal appendages of insects 71 The abdominal appendages of Collembola 72 The abdominal appendages of Thysanura 74 The abdominal gills of ephemerid larvae 77 Lateral abdominal appendages of sialid and coleopterous larvae. ... 79 The abdominal legs of lepidopterous larvae 83 The gonopods 88 The cerci (uropods ) 92 The terminal appendages of endopterygote larvae 96 Terminal lobes of the paraprocts 107 Aforphology of the abdominal appendages loS Ablireviations used on the figures 122 l\cferences 123 INTRODUCTION The incision of the insect into head, thorax, and abdomen is in general more evident in the cervical region than at the thoracico- abdominal line ; but anatomically the insect is more profoundly divided between the thorax and the abdomen than it is between the head and Smithsonian Miscellaneous Collections, Vol. 85, No. 6 2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 85 the thorax. The series of segmental appendages is continuous, and uniform except for minor adaptational modifications, from the head to the metathorax, inclusive ; on the abdomen the appendages abruptly cease, or are greatly altered in form. The musculature of the abdomen, consequently, is sharply differentiated from that of the thorax. Even the respiratory mechanism attests that the thoracico-abdominal bound- ary marks some deep-seated change in the body organization, since the closing apparatus of the spiracles is almost invariably different in one way or another before and behind the intersegmental fold separating the thorax from the abdomen. The definitive head contains three segments that have been, comparatively speaking, but recently transferred from the body to the cephalic region ; the waist line of the insect has long been definitely established, and only in a single order has an abdominal segment been given over to the thorax. The abdomen is distinctly the visceral region of the body, and its major active functions in adult insects are those of respiration, copulation, and oviposition. Yet, notwithstanding the functional and structural differences that have come to separate the insect body into cephalothoracic and ab- dominal regions, we can not avoid the assumption that modern in- sects are derived from polypod ancestors, and that the abdominal segments at some time in the past history of the Hexapoda had the same essential structure as that of the primitive thoracic and gnathal segments. In studying the morphology of the abdomen and its ap- pendicular organs, therefore, we must attempt to find in the modern structure a basic plan of organization the same as that we are led to believe exists in the cephalic and thoracic regions from a* study of the segments, the appendages, and the musculature in the preabdominal parts of the body. Considering, then, the nature of the task, it is not surprising that students of insect morphology find in the abdomen problems far more difficult of solution than are those encountered in the head or thorax, and that there are many fundamental problems in the abdomen which are still unsolved. To the systematist in entomology the study of the abdomen, or particularly of the genital appendages, is becoming of ever increasing importance, and specialists are coming to feel acutely the need of a fundamental understanding of the organs that have been found in so many cases to give the best characters by which species may be dis- tinguished. Unfortunately, however, no investigator has yet discovered a means for determining with certainty the homologies of the organs most useful for diagnostic purposes. In truth, we may say that the : NO. 6 INSECT ABDOMEN—SNODGRASS 3 morphology of the insect abdomen is a field in which no angel yet has trod, and is, therefore, one in which the fool unhindered may achieve his destiny. However, it is reputed that there is some merit in knowing oneself to be a fool, and, if it is the wisdom of the wise to enter only where the foolish have sprung the trap, the fool at least has a mission to perform. Hence, the writer offers the generalizations contained in this and a second paper to follow, not with the conviction that they will prove infallible, but in the hope that others will try to disprove them—and thereby establish their value. The principal conclusions derived from the study of the abdomen and its appendages to be given later in detail may be itemized in advance as follows 1. The insect abdomen consists primarily of 12 segments, the first 1 1 of which are true somites, while the last is the periproct, or telson. 2. The twelfth segment is practically obliterated in all the true Insecta, except for possible remnants in larval Odonata. 3. The eleventh somite becomes the functional anal segment with the suppression of the twelfth segment. Its tergum forms the epiproct. The venter of the eleventh segment is distinct in some Thysanura, but it is usually reduced or obliterated, except for two terminal lobes, which are the paraprocts. The appendages of the eleventh segment are the cerci. 4. The tenth segment is usually distinct in generalized insects, Init it is often combined with the eleventh to form a composite terminal segment. The embryonic appendages of the tenth segment are sup- pressed in postembryonic stages of the more generalized insects ; they form the postpedes of holometabolous larvae, and perhaps the ap- pendicular processes of the proctiger, or tenth segment, of some adult Holometabola. 5. The eighth and ninth somites are the genital segments, which bear the gonopods. The median gonopore of the female is located typically behind the eighth sternum, that of the male behind the ninth. Deviations from these positions are secondary ; but the opening of the paired oviducts of Ephemerida between the seventh and eightli abdominal sterna probably represents a primitive condition, exhibited also by the embryos of certain insects (see Heymons, 1892, and Wheeler, 1893). 6. The abdominal appendages of adult and larval insects are serially homologous with the legs and mouth parts. Each consists of a basis, and of one or two movable appendicular processes. The basis appears to comprise the coxal and subcoxal regions of a typical ai)pendage, the two parts being either distinct or united. The coxal part often 4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 bears an eversible or retractile sac having various functions, repre- sented by the vesiculae of Thysanura, the gill-bearing lobes of certain neuropterous larvae, and the plantar lobes of caterpillars and sawfly larvae. The basis of an abdominal limb usually takes the form of a lobe or plate of the body wall, and in the pregenital segments the limb bases are generally united with the sterna in adult msects. The appendicular process of the basis more commonly present is the stylus of the lower insects, or its derivatives, including the clasper- like organs borne by the male gonopods of the higher insects. The other limb process is the gonapopJiysis, which occurs only on the gonopods. Both the styli and the gonapophyses may l^e movable by muscles arising within the supporting basal lobes or plates, or on segmental areas derived from the latter. 7. No positive evidence can be adduced from the known facts of anatomy or embryology to establish the homology of either the stylus or the gonapophysis. Many structural interrelations, however, sug- gest that the stylus is the telopodite of the appendage, and that the gonapophysis is an endite process of the basis. 8. The genital appendages, or gonopods, have the same essential structure as the appendages of the pregenital segments. Their dis- tinguishing feature is the presencq of the gonapophyses arising mesally from the bases. In the female, the gonapophyses of the two pairs of gonopods form the first and second pairs of valvulae of the ovipositor ; in the male the gonapophyses of the ninth segment be- come the parameres. The styli of the gonopods are usually suppressed in the female of pterygote insects ; those of the ninth segment of the male form the movable claspers. or Jiarpcs. of the copulatory apparatus in the Endopterygota. 9. The bases of the gonopods in adult female insects become plates supporting the first and second valvulae ; those of the second genital segment may form a third pair of valvulae. In the male the bases of the single pair of gonopods often form distinct pleural plates of the ninth segmental wall between the tergum and the sternum, or they may fuse with either the tergum or the sternum, or with both ; again they may unite with each other to form a plate either coalesced with the sternum or free and independently movable behind the latter. 10. The parameres of the male are associated with the median penis in the lower insects, generally uniting with the latter except in Thysanura ; but the penis may be suppressed, and the |)arameres then unite with each other -and inclose the terminal part of the ejaculatory duct to form the more complex copulatory organ known as the aedeagiis.