<P> The ground plan of the abdomen of an adult insect typically consists of 11--12 segments and is less strongly sclerotized than the head or thorax . Each segment of the abdomen is represented by a sclerotized tergum, sternum, and perhaps a pleurite . Terga are separated from each other and from the adjacent sterna or pleura by a membrane . Spiracles are located in the pleural area . Variation of this ground plan includes the fusion of terga or terga and sterna to form continuous dorsal or ventral shields or a conical tube . Some insects bear a sclerite in the pleural area called a laterotergite . Ventral sclerites are sometimes called laterosternites . During the embryonic stage of many insects and the postembryonic stage of primitive insects, 11 abdominal segments are present . In modern insects there is a tendency toward reduction in the number of the abdominal segments, but the primitive number of 11 is maintained during embryogenesis. Variation in abdominal segment number is considerable . If the Apterygota are considered to be indicative of the ground plan for pterygotes, confusion reigns: adult Protura have 12 segments, Collembola have 6 . The orthopteran family Acrididae has 11 segments, and a fossil specimen of Zoraptera has a 10 - segmented abdomen . </P> <P> Generally, the first seven abdominal segments of adults (the pregenital segments) are similar in structure and lack appendages . However, apterygotes (bristletails and silverfish) and many immature aquatic insects have abdominal appendages . Apterygotes possess a pair of styles; rudimentary appendages that are serially homologous with the distal part of the thoracic legs . And, mesally, one or two pairs of protrusible (or exsertile) vesicles on at least some abdominal segments . These vesicles are derived from the coxal and trochanteral endites (inner annulated lobes) of the ancestral abdominal appendages . Aquatic larvae and nymphs may have gills laterally on some to most abdominal segments . Of the rest of the abdominal segments consist of the reproductive and anal parts . </P> <P> The anal - genital part of the abdomen, known as the terminalia, consists generally of segments 8 or 9 to the abdominal apex . Segments 8 and 9 bear the genitalia; segment 10 is visible as a complete segment in many "lower" insects but always lacks appendages; and the small segment 11 is represented by a dorsal epiproct and pair of ventral paraprocts derived from the sternum . A pair of appendages, the cerci, articulates laterally on segment 11; typically these are annulated and filamentous but have been modified (e.g. the forceps of earwigs) or reduced in different insect orders . An annulated caudal filament, the median appendix dorsalis, arises from the tip of the epiproct in apterygotes, most mayflies (Ephemeroptera), and a few fossil insects . A similar structure in nymphal stoneflies (Plecoptera) is of uncertain homology . These terminal abdominal segments have excretory and sensory functions in all insects, but in adults there is an additional reproductive function . </P> <P> The organs concerned specifically with mating and the deposition of eggs are known collectively as the external genitalia, although they may be largely internal . The components of the external genitalia of insects are very diverse in form and often have considerable taxonomic value, particularly among species that appear structurally similar in other respects . The male external genitalia have been used widely to aid in distinguishing species, whereas the female external genitalia may be simpler and less varied . </P>

Small insect with a pair of claws on its abdoman