46 



SOME ASPECTS OF MORPHOLOGY 



on definite basal plates, or coxites, while in others the coxites are 

 amalgamated with the ninth sternum to form a compound plate 

 or coxosternum. The claspers are each provided with muscles 

 arising either from the coxites or from the coxosternum, as the 

 case may be, and inserted into their bases. It appears probable 

 that in cases where the coxites are fused with the adjacent sternum 

 the styli alone form the claspers, as Snodgrass maintains. In 

 other cases where the coxites are free, they themselves may be 

 movable by muscles arising from the sternum of their segment, 



Fig. 28. Morphology of male external genitalia. A, generalised 

 condition. B, specialised condition (penis and parameres 

 united), a, aedeagus ; c, coxite or limb base ; c^, fused coxites ; 

 m, muscle of stylus ; p, parameres ; pe, penis ; s, ninth sternum ; 

 st, stylus. (From Snodgrass, 1931.) 



and in such cases the functional clasping organs are formed by 

 the coxites together with their styli. 



Among Orthoptera the claspers are not developed. In 

 Grylloblatta, Walker (1922) has shown that the coxites are free 

 and bear but little modified styli at their apices (Fig. 29). This 

 condition is evidently a primitive one and does not differ in 

 essentials from that prevalent in the Machilidge among the 

 Thysanura. In other Orthoptera the coxites are fused with the 

 sternum to form a coxosternum : in the Blattidae and some 

 other families unmodified styli are retained, but they perform no 

 copulatory function. The same condition is found among 

 Isoptera. Among the Ephemeroptera, Mecoptera and Trichoptera 

 the coxites are large and free and these plates, along with their 

 styli, are to be regarded as forming the functional clasping organs. 



