NO. 6 



INSECT ABDOMEN SNODGRASS 



75 



abdomen as a free flap with a iiieinbranous dorsal wall (fig. 32 B). 

 Both the stylus and the eversible sac are provided with muscles arising 

 within the basal plate (figs. 31 A, 32 B, siiicls, rvs). The muscles ot 

 the stylus are inserted on the base of the stylus ; the muscles of the 

 vesicle traverse the latter, when the vesicle is everted, to be inserted 

 within its distal extremity. 



The appendages of the pregenital segments are never developed 

 into any other form in the Thysanura than that which they have 

 in the Machilidae, but they may be variously reduced, or united with 

 the sternum. The styli and eversible sacs are sometimes absent, or 

 either organ may occur alone (fig. 4. I, VIII). A pair of vesicles 

 is frequently present on each basal plate, but the stylus never occurs 



Fig. 32. — Appendages of Thysanura and Dicellura. 



A, base of metathoracic leg of Ncsoiiiachilis maoricus, showing styliform spur 

 on coxa. B, typical structure of a pregenital appendage of same, dorsal view. 

 C, posterior lateral part of sternum of Hcfcrojapyx yallardi with united limb 

 basis bearing the stylus. 



in duplicate. In Lepismatidae the basal plates of the appendages in 

 each segment are fused with the primary sternum to form a large 

 zygosternum, which is the definitive sternal plate of the segment. 

 The same is true in the Dicellura, though the regions of the limb 

 bases may remain partially separated from the region of the primary 

 sternum (fig. 32 C). 



The basal plates of the thysanuran appendages are commonly called 

 "coxae" (or "coxites") by American entomologists, while certain 

 European entomologists call them " subcoxae." The idea that the 

 plates are coxae is based chiefly on the fact that in the Machilidae 

 stylus-like spurs occur on the coxae of the second and third pairs of 

 thoracic legs (fig. 32 A, Sty?), which appear to be homologues of 

 the abdominal styli. The question of the possible identity of the 



