333 



SECT. IV.— PERFECT INSECTS. 



Chapter XIV. 



Expansion of the Body and Wings in Insects newly transformed. 



The mechanism by which winged insects, as well as 

 birds, are enabled to support themselves in the air, is 

 one of the most admirable instances of providential 

 wisdom, to facilitate the locomotion and the distribu- 

 tion of the smaller animals. The great agent em- 

 ployed for this purpose is air, which is made to 

 serve the double purpose of assisting in the assimi- 

 lation of nutriment by the supply of oxygen, and the 

 removal of carbon, and of diminishing the weight of 

 the body in order to render it buoyant. In birds, 

 the lungs have several openings communicating with 

 corresponding air-bags or cells which fill the whole 

 cavity of the body from the neck downwards, and 

 into which the air passes and repasses in the pro- 

 cess of breathing. This is not all : the very bones of 

 birds are hollowed out with the design of receiving 

 air from the lungs, from which air-pipes are con- 

 veyed to the most solid parts of the body, and even 

 into the quills and plumelets of the feathers, which 

 are hollow or spongy for its reception. As all these 

 hollow parts, as well as the cells, are only open on the 

 side communicating with the lungs, the bird requires 

 only to take in a full breath to till and distend its 

 whole body with air, which, in consequence of the 

 considerable heat of its body, is rendered much lighter 

 than the air of the atmosphere. By forcing this air 

 out of the body again, the weight becomes so much 



u 3 



