322 



NOISES OF INSECTS. 



several prominent nervures answering to the margin of the 

 membrane or ocellus. There is," he further remarks, " every 

 reason to believe that the brisk movement with which the 

 grasshopper rubs these nervures against each other produces 

 a vibration in the membrane augmenting the sound. The 

 males in question sing continually in the hedges and trees 

 during the months of July and August, especially towards 

 sunset and part of the night. When any one approaches, 

 they immediately cease their song." 1 In these insects, as in 

 the crickets, M. Goureau has detected in the strong horny 

 ridge immediately behind the mirror or tympanum, near the 

 base of the upper surface of the left elytrum, the same trans- 

 verse notches as in Acheta and Gryllotalpa, while on the 

 under surface of the right elytrum a similar but less strongly 

 notched file-like ridge is found ; and it is obviously by the 

 rubbing of these rasps against the projecting nervures of the 

 borders of the wings, that the sounds resulting from the 

 brisk friction of the elytra proceed. Dr. Burmeister con- 

 ceives that they are chiefly caused by the forcible expiration 

 of air from the thoracic tracheae and spiracles, first driven 

 against the inflected external margin of the wing, and subse- 

 quently against the tympanum, which is thus caused to 

 vibrate and resound ; but Mr. Newport has pointed out that 

 this cannot be the cause, because in Acrida brachelytra the 

 elytra are so exceedingly short and narrow that they do not 

 cover, nor are near, any part of the spiracles, so that the air in 

 passing from these orifices cannot possibly be driven against 

 the tympanum ; which, however, being accompanied by 

 notched nervures, as in A. viridissima, though differently ar- 

 ranged, produces similar sounds. A still farther proof that 

 these notched nervures or files are the main agents in pro- 

 ducing the sounds, is afforded by the facts that their notches 

 are more distinct in newly disclosed specimens, especially of 

 Acrida viridissima, than in older individuals, in which they 

 have been partially obliterated by use ; and that the sounds, 

 as M. Goureau has remarked, may be readily produced in the 

 dead insect by gently rubbing the bases of the elytra together, 



1 De Geer, iii. 429. 



