ii8 THE PHILOSOPHY 



their gills ; but Nature has provided moft of them with air-bags or 

 bladders, which may anfwer the double purpofe of enabling them 

 to afcend and defcend in the water, and to communicate a vital prin- 

 ciple to their whole fyftem. 



We (hall conclude this fubje£t with an account of the modes em- 

 ployed by Nature for tranfmitting air into the bodies of insects. 



In this feemingly contemptible, and often noxious clafs of ani- 

 mals, Nature has exhibited a wonderful diverfity of form, of man- 

 ners, of inftinds, of deformity, and of beauty. But, however in- 

 fignificant thefe creatures may appear to inattentive obfervers, Na- 

 ture has been equally provident in the formation of their bodies, 

 and in the means of preferving the different individuals, according 

 to their kinds, as in the larger animals, which have the appearance 

 of more importance in the fcale of being. To infedts fhe has denied 

 lungs fimilar to thofe of men, quadrupeds, birds, and filhes ; but, as 

 the tranfmiflion of air into their bodies was neceffary to continue 

 'the principle of life, fhe has furnifhed them with peculiar inftru- 

 ments and apparatus for accomplilhing this indifpenfible purpofe. 



Air is conveyed into the bodies of infeds by inftruments called 

 tracheae oxjligmata. The tracheae, or wind-pipes, are, in many 

 infeds, long tubes protruding externally from different parts of the 

 body. In fome, they proceed from the pofterior part, and have the 

 appearance of one, two, or three tails ; in others, they arife from 

 the back or fides. The Jligmata are fmall holes, generally of a dif- 

 ferent colour from the reft of the body, and run along the fides of 

 many caterpillars in regular and beautifully dotted lines. That thefe 

 tracheae and ftigmata are deftined for the tranfmifTion of air, has 

 been proved by repeated experiments ; for, when flopped up by the 



application 



