372 SEXUAL SELECTION. [Part H. 



surface of this segment, when held in the proper light, is 

 seen to be clothed with hairs, wliich are absent or are rep- 

 resented by excessively fine down in the males. It should 

 be noticed that, in all Coleoptera, the eflfective part of the 

 rasp is destitute of hairs. In 0. senegalensis the differ- 

 ence between the sexes is more strongly marked, and this 

 is best seen when the proper segment is cleaned and 

 viewed as a transparent object. In the female the whole 

 surface is covered with little separate crests, bearing 

 spines ; while in the male these crests become, in proceed- 

 ing toward the apex, more and more confluent, regular, 

 and naked ; so that three-fourths of the segment is covered 

 with extremely fine parallel ribs, which are quite absent 

 in the female. In the females, however, of all three spe- 

 cies of Oryctes, when the abdomen of a softened specimen 

 is pushed backward and forward, a slight grating or strid- 

 ulating sound can be produced. 



In the case of the Heliopathes and Oryctes there can 

 hardly be a doubt that the males stridulate in order to 

 call or to excite the females ; but with most beetles the 

 stridulation apparently serves both sexes as a mutual call. 

 This view is not rendered improbable from beetles stridu- 

 lating under various emotions ; we know that birds use 

 their voices for many jDurposes besides singing to their 

 mates. The great Chiasognathus stridulates in anger or 

 defiance ; many species do the same from distress or fear, 

 when held so that they cannot escape : Messrs. Wollaston 

 and Crotch were able, by striking the hollow stems of 

 trees in the Canary Islands, to discover the presence of 

 beetles belonging to the genus Acalles by their stridula- 

 tion. Lastly, the male Atcuchus stridulates to encourage 

 the female in her work, and from distress when she is re- 

 moved." Some naturalists believe that beetles make this 



"* M. P. de la Brulerie, as quoted in ' Journal of Travel,' A. Murray, 

 vol. i. 1868, p. laS. 



