THE FLIGHT OF INSECTS 315 



Another possibility, however, must be borne in mind. The 

 wings of an insect are traversed by air-tubes, with which blood- 

 spaces are always associated in early stages of development. And 

 there is reason to think that even in some adult insects the blood 

 circulates in the wings to some extent. It is therefore quite 

 possible that the original folds had reference to breathing as well 

 as to parachuting. And this has led some authorities to surmise 

 that wings have been evolved from tracheal gills possessed by 

 primeval aquatic forms (see vol. ii, p. 463). But the evidence in 

 support of this view is of the scantiest description, and it is in the 

 highest degree improbable that the members of the original insect 

 stock were of aquatic habit. 



