ADAPTATIONS IN INSECTS 189 



The internal organs of insects are especially adapted for an 

 aerial mode of life. The most characteristic are adaptations 

 for breathing air. There are, obviously, two possible different 

 ways in which organs, tissues, and cells of the body may obtain 

 fresh oxygen ; each tissue may get it directly from the outside 

 by osmosis, as in the coelenterates and earthworms, or each cell 

 may get it from some specially modified oxygen-carrying agent. 

 The blood vascular system forms the agent in the majority 

 of higher types, but in the insects the blood system has no such 

 functions, and in many cases is absent altogether. Air is car- 

 ried from the atmosphere directly to the tissues and cells by 

 special tubes called tracheae, which form a complicated system 

 or branching system of vessels distributed throughout the body. 

 The main trunks end in external openings termed spiracles, 

 which may be variously placed in different types of insects. In 

 some cases, there is only one pair of such openings; again there 

 may be a pair to each somite of the abdomen and thorax 

 (cockroach, grasshopper), or many openings may be distributed 

 about the body. 



