FLIGHT OF INSECTS. 300 



Their moisture rapidly evaporates, leaving the deU- 

 cate film dry and firm, so as to be ready for imme- 

 diate action. Fibres, or nervures, as they are called, 

 are generally apparent on their surface, and form 

 a delicate network for the support of this fine mem- 

 brane ; like the frame of the arms of a windmill, 

 which supports the canvass spread over them. The 

 microscope shows that these fibres are tubular, and 

 contain air ; a structure the most eftectual for con- 

 joining lightness with strength ; and many entomo- 

 logists are of opinion that the insect has the power, 

 during the act of flying, of directing air into the 

 nervures, so as to dilate them to the utmost, and 

 render them quite tense and rigid.* 



In the great majority of insects the wings are 

 four in number ; of which the first pair are, as we 

 have seen, affixed to the mesothorax, and the second 

 to the metathorax. These two segments of the 

 thorax, composing what has been termed the ali- 

 Irunk, constitute the most solid portion of the ske- 

 leton, and are frequently strengthened by ridges, 

 and other mechanical contrivances for support. 

 The superior extremities of these supports, which 

 have been compared to the clavicles, or furcular 

 bones of birds, are always curved inwards. This 

 part of the trunk requires to be alternately dihited 

 and contracted during flight ; and hence the several 

 pieces of which its dorsal portion is composed are 

 loosely connected together by ligaments. f 



* The wings of many insects present no nervures. This is the 

 case with all the smaller Hymenoptera, 



t See Chabrier's " Essai sur le Vol des Insectes," Memoires du 

 Museum d'Histoire Naturelle; vi. 410, vii. 297, and viii. 47 and 

 349. See also Zoological Journal; i. 391. 



