INSECTA. 



929 



in newly developed specimens, particularly in 

 Acric/a vtridissima, the teeth or markings on 

 the file are more distinct than in those which 

 have been a longer time in the perfect state, in 

 which the teeth appear as if partially oblite- 

 rated by use. The sounds, as remarked by 

 M. Gpureau, may be readily produced in the 

 dead insect by gently rubbing the bases of the 

 wings together, a further proof that the rushing 

 of air from the spiracles is totally unconnected 

 with their production. A similar structure 

 exists in the wings of Acrida grisea and others 

 of the same genus. In the Achetida the parts 

 for stridulation are somewhat differently con- 

 structed. The wing of the common house- 

 cricket, Achetu domestica, differs materially in 

 the two sexes. In the male (Jig. 395) the two 



Fig. 395. 



Fig. 396. 



Wing of the male House- Cricket, Acheta dome&tica, 

 shewing thejile, b, and tympanum, a. 



wings exactly resemble each other, and the 

 nervures are more irregularly disposed than in 

 the female, in which they are arranged either 

 longitudinally or diagonally with but very few 

 that run in a transverse direction. When the 

 wing of the male is attentively examined, 

 nearly one-half of its surface is found to be 

 adapted to perform the office of a tympanum. 

 This part is more transparent and elastic than 

 the other, and is crossed by many nervures in a 

 manner somewhat similar to the tympanum in 

 Acrida brachelytra. Besides these there is on 

 the under surface of each wing a large nervure, 

 which is curved and placed somewhat trans- 

 versely near the base of the wing, as in Acrida. 

 It is the file or bow, and is covered by a vast 

 number of minute, but freely elevated, semi- 

 circular teeth, which gradually decrease in size 

 as they approach the external angle of the 

 wing fjig. 396). The smallness of the teeth, 

 and the extent of surface over which they are 

 passed when the two wings are rubbed briskly 

 across each other, is probably the cause of the 

 very acute sounds produced by this insect. In 

 Gryllotalpa, which is said to produce a hoarse 

 croaking sound, the two wings exactly resem- 



VOL. II. 



The round file of Acheta domestica. 



ble each other, as in Acheta. The nervures 

 are thick and strong, and there is no distinct 

 vibratory membrane, but on the under surface 

 of each wing are a vast number of minute 

 sharp-pointed teeth arranged closely together 

 along the middle of the nervures, not only 

 upon that one which is analogous to the file in 

 Acrida and Acheta, but also upon three others 

 which run in a parallel direction with it, as well 

 as on their transverse or connecting branches, 

 so that the whole of the nervures at the base 

 of each wing are covered with files, which, 

 when the two wings are rubbed across each 

 other, produce, owing to the shortness of the 

 nervures, a low grating sound. We do not at 

 first perceive the necessity for a stridulatory 

 apparatus on the under-surfaces of both wings, 

 if the sounds produced result simply from the 

 attrition of the wings against each other, and 

 the wings have always the same relative posi- 

 tion. But on close examination it is found 

 that, although in the Gryllida the right wing 

 either constantly overlaps the left or the left the 

 right, in the Achetida this is not the case, but 

 that sometimes one wing and sometimes the 

 other in the same insect is the supenor. With 

 regard to the acuteness of the sounds produced 

 by the house-cricket, it probably depends 

 much upon the length of the vibrating nervures 

 on the large tympanum, as well as the small- 

 ness of the teeth in the file, as the hoarse 

 sounds do, perhaps, upon the shortness of the 

 nervures in Gryllotalpa. In Locustida: the 

 stridulation is not connected with the structure 

 of the wings. 



Besides these various parts for the produc- 

 tion of sounds, the wings of some insects are 

 furnished with others equally remarkable, but 

 designed fora different purpose. These con- 

 sist of certain little hooks and foldings on the 

 margins of the wings, by means of which in 

 some families the two pairs are united during 

 flight, in order that the motions of these organs 

 may be in perfect unison with each other. In 

 some genera, as in the Lepidoptera, the males 

 alone are provided with these booklets, as was 

 formerly noticed by Mr. Haworth* in Apatura 

 Iris, in which the wings of the male are con- 

 nected at their base by means of a strong elastic 

 spring, which arises from the base of the costal 



* Lepidoptera Britannica, 8vo. Londini, 1803. 

 3 P 



