NO. 6 INSECT ABDOMEN — SNODGRASS 89 



the gonapophysis may be movable on the basis by muscles arising in 

 the latter and inserted on their bases (suicls, gincls). So far as has 

 been ascertained neither of the appendicular parts of a gonopod ever 

 contains intrinsic muscles. In the males of some Ephemerida the 

 genital styli are distinctly jointed and thus separated into apparent 

 segments, though the latter are not independently movable. In the 

 Thysanura the gonapophyses are marked by circular constrictions, 

 varying from a few to many according to the length of the organs 

 (fig. 33 A, B), but the resulting subdivisions have none of the charac- 

 ters of true segments, and are entirely comparable with the annulations 

 of the caudal filament and cerci (C; cf, Cer). 



In female insects the gonopods form the ovipositor, when this organ 

 is present, and both pairs of appendages enter into its composition. 

 In the Thysanura the gonapophyses only are involved in the ovipositor, 

 the basal plates and the styli retaining the structure typical of these 

 parts in the pregenital segments of Machilidae. Evidently the condi- 

 tion here represents a primitive stage, in which two pairs of median 

 apophyses of the appendages of the eighth and ninth abdominal seg- 

 ments form a simple egg-laying organ. In female Pterygota the styli 

 of the gonopods are usually lost, those of the first pair being always 

 absent, and the basal plates are transformed into a suspensory appara- 

 tus for the gonapophyses. The basal plates of the first gonopods (fig. 

 39 A) evidently become the small sclerites known as the valvifers 

 (B, Vlf), which support the first gonapophyses (iVl), though there 

 is a difi^erence of opinion on this point. Those of the second gonopods 

 form lobes (IXLB) supporting the second gonapophyses {2VI), or 

 they are drawn out into long processes that become a third pair of 

 blades in the ovipositor (C, jF/). The component blades of the adult 

 ovipositor are commonly called valvulae. It is to be observed that only 

 the first and second pairs of valvulae (B, C, iVl, 2VI) represent the 

 gonapophyses (A, iGon, 2Gon), those of the third pair (C, jF/) 

 being derived directly from the basal plates of the second gonopods, 

 the styli of which are apparently lost. 



The ovipositor is absent or rudimentary in many groups of insects, 

 but its wide distribution throughout the orders leaves little doubr 

 of its being a primitive structure of the Insecta. It has no homo- 

 logue in other Arthropoda, and it is doubtful if the rami of the gono- 

 pods in the Crustacea are homodynamous with the gonapophyses 

 of insects. The various theories concerning the possible homologies 

 of the genital processes of the gonopods in insects will be considered 

 in the closing discussion of this paper. 



