WAYS AND MEANS OF LIVING 



along the sides of the body (Fig. 70), and from these 

 trunks are given off branches into each body segment and 

 into the head, which go to the alimentary canal, the heart, 

 the nervous system, the muscles, and to all the other 

 organs, where they break up into finer branches that 

 terminate in minute end tubes going 

 practically to every cell of the body. 



Many insects breathe by regular 

 movements of expansion and contrac- 

 tion of the under surface of the abdo- 

 men, but experimenters have not yet 

 agreed as to whether the air goes in 

 and out of the same spiracles or 

 whether it enters one set and is ex- 

 pelled through another. It is probable 

 that the fresh air goes into the smaller 

 tracheal branches principally by gas 

 diffusion, for some insects make no 

 perceptible respiratory movements. 



The actual exchange of oxygen from 

 the air and carbon dioxide from the 

 tissues takes place through the thin 

 walls of the minute end tubes of the 

 tracheae. Since these tubes lie in im- 

 mediate contact with the cell surfaces 

 the gases do not have to go far in 

 order to reach their destinations, and 

 the insect has little need of an oxygen 

 carrier in its blood — its whole body, 

 practically, is a lung. And yet some 

 investigations have made it appear 

 likely that the insect blood does con- 

 tain an oxygen carrier that functions 

 in a manner similar to that of the 

 hemoglobin ot vertebrate blood, 

 though the importance of oxygen 

 transportation in insect physiology has 



Fig. 70. Respiratory 

 system of a caterpillar. 

 The external breathing 

 apertures, or spiracles 

 (Sp, Sp), along the 

 sides of the body open 

 into lateral tracheal 

 trunks (a, a), which 

 are connected crosswise 

 by transverse tubes 

 (b, b) and give off mi- 

 nutely branching tra- 

 cheae into all parts of 

 the head (H) and body 



[115] 



