262 SEXUAL DIMORPHISM 



Slight differences of colour in the sexes of Hymenoptera 

 are of less importance, and are probably associated in all 

 cases with corresponding differences in the conditions of life 

 or development. 



Orthopteka. In this Order the males nearly all possess a 

 stridulating apparatus which is wanting in the female. The 

 structure of the apparatus is different in the different families, 

 but in all cases consists of toothed or ridged surfaces of the 

 cuticle, which are rubbed against other parts. Here the 

 movement of the parts under the influence of sexual excite- 

 ment, whether originally producing a sound or not, must, if 

 stimulation affects growth at all, have produced a modifica- 

 tion of structure, and on the hypothesis that such effects 

 become in time hereditary, the present condition is explained. 

 On the other hand, the selection theory merely assumes that 

 the variations occurred, and survived, but fails to account for 

 the absence of such variations in most cases in the female. 



In some species of Orthoptera, as in the Pneumora of 

 South Africa mentioned by Darwin, the males are winged 

 and the females wingless, and the same is the case in the 

 common European cockroach. This condition occurring in so 

 many orders of insects may be explained by disuse of the 

 wings in the females in all cases. 



In Neuroptera, differences between the sexes in habits and 

 structure seem to be but slightly developed, although there 

 are often considerable differences in colour. 



In the Homoptera, Hemiptera, and Diptera, a few cases of 

 sexual dimorphism are mentioned by Darwin, such as the 

 organ of sound in the male Cicadge, wingless females in the 

 Hemiptera, and horn-like outgrowths on the head of a species 

 of the Diptera. They all seem to me to be explicable by the 

 views I have maintained, but I am not able to compare in 

 detail the structures and corresponding habits or conditions. 



