346 THE DESCENT OF MAN. 



tes, we have a nearly parallel case. In the females of 0. 

 gryplms and nasicornis the ribs on the rasp of the pro- 

 pygidium are less continuous and less distinct than in the 

 males; but the chief difference is that the whole upper sur- 

 face of this segment, when held in the proper light, is seen 

 to be clothed with hairs, which are absent or are repre- 

 sented by excessively fine down in the males. It should be 

 noticed that in all Coleoptera the effective part of the rasp 

 is destitute of hairs. In 0. senegalensis the difference be- 

 tween the sexes is more strongly marked, and this is best 

 Been when the proper abdominal 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, in proceeding toward the 

 apex, become more and more confluent, regular and naked; 

 so that three-fourths of the segment is covered with ex- 

 tremely fine parallel ribs, which are quite absent in the 

 female. In the females, however, of all three species of 

 Oryctes, a slight grating or stridulating sound is produced 

 when the abdomen of a softened specimen is pushed back- 

 ward and forward. 



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 stridu- 

 lation apparently serves both sexes as a mutual call. 

 Beetles stridulate under various emotions, in the same 

 manner as birds use their voices for many purposes besides 

 singing to their mates. The great Chiasognathus stridu- 

 lates in anger or defiance; many species do the same from 

 distress or fear, if held so that they cannot escape ; by 

 striking the hollow stems of trees in the Canary Islands, 

 Messrs. Wollaston and Crotch were able to discover the 

 presence of beetles belonging to the genus Acalles by their 

 stridulation. Lastly, the male Ateuchus stridulates to en- 

 courage the female in her work, and from distress when she 

 is removed.* Some naturalists believe that beetles make 

 this noise to frighten away their enemies; but I cannot 

 think that a quadruped or bird, able to devour a large 

 beetle, would be frightened by so slight a sound. The 

 belief that the stridulation serves as a sexual call is sup- 



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

 ray, vol. i, 1868, p. 135. 



