CHAPTER III 



The Internal Structure of Insects 



How Insects Breathe 



All insects, even those living in water, need air. But their method 

 of obtaining it is entirely different from that developed in higher 

 animals. No insect has 

 nostrils, or any opening in 

 its head through which it 

 breathes. Instead, there is 

 a row of small apertures, 

 called spiracles, down each 

 side of its body, one on 

 each segment, beginning 

 with the second or third 

 segment of the thorax and 

 extending back along the 

 rings of the abdomen. The 

 spiracles of each side open 

 into an air tube running 

 lengthwise of the insect, 

 just within the body wall. 

 From these main tubes 

 smaller tubes diverge, and 

 these in turn branch and re- 

 branch, growing constantly 

 smaller, until the finer tubes 

 permeate every part of the 

 insect, even to the tips of 



the antennae and the joints of the feet. The tubes are known as 

 tracheae and the entire group as the tracheal system. The smaller 



11 



Fig. 9. 



Tracheal system of an insect. (Dia- 

 grammatic.) Original. 



