74. SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



The insects are thus prepared to complete the reproductive function 

 during the brief period of imaginal Hfe. (See Needham, Traver, and 

 Hsu, 1935.) 



The genital claspers of male Ephemeroptera are movable appen- 

 dicular organs structurally comparable with the abdominal appendages 

 of Thysanura, and the same terminology may be used in each case 

 with the understanding that the implied homologies may still be ques- 

 tioned for lack of positive evidence. Each clasper of the male mayfly 

 appears to consist of two principal parts, a distal stylus (fig. 25 G, 

 Sty), and a proximal coxopodite (Cxpd). The stylus is movable on 

 the coxopodite by muscles arising in the latter, and hence its identity 

 can be determined by finding the muscles inserted on its base. The 

 identity of the coxopodite, on the other hand, is not so easily estab- 

 lished, since, though the stylus muscles arise in the coxopodites, the 

 latter may become united with each other or assume various relations 

 with the supporting sternal plate of the ninth abdominal segment. 



If the genital claspers of the mayflies are homologous with the 

 thysanuran abdominal appendages, a generalized structure of the 

 former should be something like that shown in Blasturus nehulosiis 

 (fig. 25 G), in which the coxopodites are borne on the posterior 

 margin of the ninth abdominal sternum (IXStn) and are partially 

 united with each other medially (cf. fig. 24 A). The stylus muscles 

 here arise within the coxopodites, and the coxopodites themselves are 

 movable by muscles {cxmcl) arising on the sternal plate. More gen- 

 erally, however, the stylus muscles take their origin from a broad 

 plate continuous across the posterior margin of the sternum (fig. 25 A, 

 H, L, M, Cxpd), consisting of a median area (c) and often a pair 

 of lateral stylus-bearing lobes (H, h). It is suggestive, therefore, that 

 the stylus-bearing plate, or styliger, is a product of the united cox- 

 opodites. If so, the lateral parts of the styliger may become secon- 

 darily completely separated from the median part (I, J, c), forming 

 a pair of distinct stylus-bearing lobes ( b) containing the stylus muscles. 

 This last condition, well shown in Ephoron leukon (J), might there- 

 fore be taken as primitive, and the evidence would then seem to show- 

 that the styliger has been evolved by the union of a median part of 

 the sternum with the coxopodites (J, I, H, G), and separated from 

 the rest of the sternum as a stylus-bearing plate movable on the latter. 

 The principal weakness of this second view is that it does not account 

 for the original presence of the muscles that move the styliger (A, G, 

 cxmcl), which according to the first view are assumed to be sterno- 

 coxal muscles. The development of the claspers sheds no light on 

 the morphology of the organs. As shown by Speith (1933) the 



