32-3 THE DESCENT OF MAN. 



from 131 to 138 sharp, transverse ridges or teeth {st) on 

 the under side of one of the nervures of the wing-cover. 

 This toothed nervure is rapidly scraped across a projecting, 

 smooth, hard nervure (r) on the upper surface of the oppo- 

 site wing. First one wing is rubbed over the other, and 

 then the movement is reversed. Both wings are raised a 

 little at the same time, so as to increase the resonance. In 

 some species the wing-covers of the males are furnished at 

 the base with a talc-like plate.* I here give a drawing 

 (fig. 12) of the teeth on the under side of the nervure of 

 another species of Gryllus, viz., G. domesticus. With 

 respect to the formation of these teeth. Dr. Gruber has 

 shown t that they have been developed by the aid of 

 -_^ selection, from the minute scales and hairs 

 with which the wings and body are covered, 

 and I came to the same conclusion with 

 respect to those of the Ooleoptera. But Dr. 

 Gruber further shows that their development 

 is in part directly due to the stimulus from 

 the friction of one wing over the other. 



In the Locustidae the opposite wing-covers 



differ from each other in structure (fig 13), 



^Nerv'iire of Gryi- ^^^^ ^^ action cannot, as in the last family, 



1 u s domesticus be reversed. The left wing, which acts as the 



(from Landois). |^^^^ j-^^ ^^^^ ^j^^^ right wing which serves as 



the fiddle. One of the nervures (a) on the under surface of 

 the former is finely serrated, and is scraped across the 

 prominent nervures on the upper surface of the opposite or 

 right wing. In our British Fhasgonura viridissima it 

 appears to me that the serrated nervure is rubbed against 

 the rounded hind-corner of the opposite wing, the edge of 

 which is thickened, colored brow^n, and very sharp. In 

 the right wing, but not in the left, there is a little plate, as 

 transparent as talc, surrounded by nervures, and called the 

 speculum. In EpMppiger vitium, a member of this same 

 family, we have a curious subordinate modification; for the 

 \ving-cover& are greatly reduced in size, but '' the posterior 

 part of the pro-thorax is elevated into a kind of dome over 



* Westwood, " Modern Class, of Insects," vol. i, p. 440. 

 f " Ueber der Tonapparat der Locustiden, ein Beitrag zum Darwin- 

 ismus," "ZeitscU. fiir wissenscb Zoolog.," B. xxii, 1872, p. 100, 



