/ 
588 EXTERNAL ANATOMY OF INSECTS. 
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*, 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 Jevator muscles. To cause the de- 
pression of the wings, it must be dilated, or have its longz- 
tudinal diameter diminished, and its vertical and trans- 
werse increased, which is effected by the rarefaction of 
the internal air, and the action of the depressor muscles ». 
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®. 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- 
cumstance, and upon which a theory may be built. In 
some insects the primary and secondary wings or their 
analogues are placed before the legs, in others over the 
legs, and in others behind the legs: but whatever their 
position, the pieces which I have named the scapularia 
* See above, p. 401. 
> Chabrier Sur le Vol des Ins. c. i. 446, 448, 461-~. 
© Ibid. 412. 
