Il6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



apophyses are serially homologous with the eversible sacs of the pre- 

 genital segments. Silvestri argues as follows : On the first abdominal 

 segment of Projapyx and Anajapyx there are borne on each sub- 

 coxa (stylus-bearing plate) a typical stylus and, mesad of it, a 

 cylindrical or conical process. In Machilis and Nicoletia each sub- 

 coxa of the first segment has a retractile vesicle, and in Campodca 

 only a cylindrical process. The following six segments of Machilidae 

 bear on the subcoxae both styli and vesicles, but on the next two, the 

 genital segments, each subcoxa has a stylus and, in some genera, mesad 

 to it a genital process. Thus Silvestri contends that the gonapophysis 

 is evidently an eversible sac permanently everted. Muscles he ob- 

 serves are attached to each appendage, though he does not point out 

 that those of the gonapophysis are inserted on the base of the process, 

 while those of the vesicles traverse the organ to be inserted in its 

 extremity. 



The abdominal appendages of insects have not lacked attention 

 from students of arthropod phylogeny, because their several parts 

 make up a composite limb pattern that may be supposed to conform 

 with the biramous structure of crustacean appendages, and thus 

 indicate either that insects are closely related to the Crustacea, or that 

 the primitive arthropod limb was a biramous structure. Writers who 

 espouse the idea of a crustacean ancestry for insects, following 

 Wood-Mason (1879), interpret the stylus as the exopodite, and find 

 the homologue of the endopodite in the gonapophyses of the genital 

 appendages. The theory must assume that the endopodites have been 

 suppressed on the pregenital segments, since the eversible vesicles 

 are eliminated as possible telopodite homologues by the fact that 

 they sometimes occur in duplicate. 



The theoretical possibilities of aligning the appendages of insects 

 with those of Crustacea have been exhaustively searched by Cramp- 

 ton. In a study of the terminal appendages of the tridactylid orthop- 

 teron, Ellipes, Crampton (1921) adduces evidence that he takes 

 to be conclusive of the biramous nature of insect appendages. The 

 dorsal pair of terminal appendicular processes in the Tridactylidae 

 are undoubtedly the cerci (fig. 45 A, B, Cer) ; the ventral pair (paptl) 

 are the lobes of the paraprocts (" paraprocessi "). After removing 

 the end of the abdomen and spreading the parts out from below until 

 they lie in one plane, Crampton makes a comparison of the tridactylid 

 appendages in this position with the uropods of an isopod crustacean 

 in the normal position (fig. 45 C, D), and arrives at the conclusion 

 that the cerci of the former correspond with the endopodites of the 

 latter, and that the paraproct lobes of Ellipes represent the exopodites 



