TRACHEAL RESPIRATION 



115 



(Maluf, 1939), and the integument is responsible in many 

 insects for a definite fraction of the C0 2 elimination amounting 

 sometimes to 1/4, even in 

 forms with a highly de- 

 veloped tracheal system. 



The structure of tracheal 

 systems shows a very wide 

 range of variations, and 

 many structural peculiarities 

 are far from being under- 

 stood from an cecological or 

 physiological point of view. 

 It is possible, however, to 

 correlate certain broad struc- 

 tural features with the func- 

 tion and to arrange the 

 tracheal respiratory mechan- 

 isms according to a sys- 

 tem, however provisional. 

 Tracheal respiration must 

 have been evolved on land 

 and the tracheates are 

 mainly terrestrial, but quite 

 a number of forms have sec- 

 ondarily adopted an aquatic 

 existence and have had their 

 respiratory mechanisms 

 modified in various ways. 

 In the terrestrial forms we 

 can make a distinction be- 

 tween those forms for which an exchange of gases by diffusion 

 is sufficient and those which have recourse to mechanical 

 ventilation. 



Tracheal respiration by diffusion. In a large number of 

 tracheates the tracheal system is arranged (at least in principle) 

 on the system shown in Fig. 64. From a number of spiracles 

 symmetrically arranged along the sides of the body a number 



Fig. 64. Injection preparation of 

 tracheal system of Cossus larva. About 

 natural size. (Krogh.) 



