NO. 8 INSECT ABDOMEN SNODGRASS 7 



The gonopods in their more generaHzed form (fig. 2D) resemble 

 the pregenital appendages of the Thysanura (C), but they lack 

 eversible vesicles, and each is characterized by the presence of a 

 genital process, or gonapophysis (D, Gon) arising from the proximal 

 mesal angle of the basis. Typically the distal process, or stylus (Sty), 

 and the gonapophysis (Gon) are each movable by muscles arising in 

 the basis ; and the basis is movable on the body by muscles arising 

 on the tergum of the supporting body segment. In the female, the 

 bases of the gonopods (fig. i C) become the supporting plates of the 

 ovipositor (first and second valvifers), and retain their dorsal muscles 

 arising on the eighth and ninth abdominal terga. The female gonapo- 

 physes form the first and second valvulae {iGon, 2Gon), the basal 

 muscles of which persist in Thysanura, but are generally lost in 

 Pterygota. The distal processes (styli) of the gonopods are retained 

 in Thysanura and on the second gonopods in some of the more gen- 

 eralized Pterygota. 



The pygopods, or appendages of the tenth abdominal segment, are 

 always absent in postembryonic stages of exopterygote insects, though 

 they may be present in the embryo. In many endopterygote larvae, 

 however, they are well developed as the " anal legs ", or postpedes 

 (fig. 2E), which usually resemble the appendages of the pregenital 

 segments. The distal process (stylus) is retained on the pygopods 

 only in certain Neuroptera, but in some Neuroptera and in Lepidop- 

 tera and chalastogastrous Hymenoptera the vesicle {Vs) forms a 

 retractile lobe generally provided with claws. 



The uropods, or cerci (fig. 2 F, Cer), belong to the eleventh ab- 

 dominal segment. Each appendage arises typically from a membranous 

 area behind the tenth tergum between the basal angles of the epiproct 

 and the paraproct. In certain orthopteroid insects, the principal part 

 of the cercus is supported on a small but distinct basal segment {LB). 

 Most of the muscles that move the cercus are muscles of the tenth 

 segment, but in some insects a muscle of the cercus arises on the epi- 

 proct. The cerci never have muscles arising on the paraprocts. 



From this brief review of the various forms of appendages that 

 occur on the insect abdomen it will be seen that the appendages all 

 have a common type of structure, and that the gonopods differ from 

 the others only in the presence of proximal processes (the gonapo- 

 physes) arising from the mesal angles of their bases. The bases of 

 the abdominal appendages form a series of lobes, plates, or integu- 

 mental areas occupying the pleural region on each side of the abdomen, 

 which is continuous anteriorly with the regions of the limb bases on 

 the thorax and the head. 



