46 THE BIOLOGY OF INSECTS 



springtails (Collembola). These live mostjjr in sheltered 

 situations, under bark, in soil, beneath stones, where they 

 can effect directly through the skin and cuticle gaseous 

 exchange with the surrounding moist air. The whole body 

 in such cases appears to act as a blood-gill. But we have 

 still much to learn as to the method by which these exchanges 

 are brought about. 



Enough has perhaps been stated to suggest how the 

 living insect builds its tissues and obtains its sources of 

 energy out of the materials supplied in its food, how it draws 

 directly or indirectly on the atmosphere for its needed 

 oxygen, and how it gives back to its surroundings the com- 

 paratively simple waste substances such as carbon dioxide, 

 water, urates — the end-products of those internal changes 

 that are a necessary accompaniment of its life. 



