590 EXTERNAL ANATOMY OF INSECTS. 



have a large prothorax, as the Coleoptera, it may, indeed, 

 be useful in flight as a counterpoise to the abdomen ; 

 and since when the wings descend it rises, and mm versa, 

 it may be of some service by its vibrations a ; but for this 

 it required no complexity of structure. But not so the 

 alitrunk : it consists of parts much more numerous, and 

 this number of parts is of great importance to the animal 

 in its flight. All of them are so put together, being lined 

 by a common elastic ligament b , as to be capable of a 

 certain degree of tension and relaxation, which enables 

 the animal to compress or dilate the trunk as its ne- 

 cessities require. To cause the elevation of the wings, 

 it must be compressed or have its longitudinal diameter 

 increased, and its vertical and transverse diminished: 

 this compression is produced by the condensation of 

 the internal air, which parts with some of its caloric, and 

 by the action of the levator muscles. To cause the de- 

 pression of the wings, it must be dilated, or have its longi- 

 tudinal diameter diminished, and its vertical and trans- 

 verse increased, which is effected by the rarefaction of 

 the internal air, and the action of the depressor muscles c . 

 In some Orders, the Coleoptera, &c, this effect is pro- 

 moted by the segments of the trunk, which are attached 

 by loose ligamentous membranes, and received, one or 

 more of them, into each other, which facilitates the 

 above action d . Thus much for the general use of these 

 parts. I shall further here mention a partial one of 

 two of them which seems indicated by a particular cir- 



a Chabrier Surle Voldes Ins. c. i. 413 — . 



b See above, p. 402. 



Chabrier Sur le Vol des Ins. c. i. 446, 448, 451—. 



d Ibid. 412. 



