132 



RESPIRATORY MECHANISMS 



that this increase is instantaneous and that the resting fre- 

 quency and depth is restored immediately when the flight 

 stops, and concluded, no doubt rightly, that no chemical but 

 a purely reflex mechanism must be involved. 



Adaptation of tracheal respiration to an aquatic existence. Among 

 the higher vertebrates respiring by lungs we found a small 

 number of forms which had secondarily taken to the water and 

 developed the necessary adaptations, all of them practically 

 along the same line, returning to the surface at intervals to 

 breathe. Among the tracheates a fairly large number have 

 become aquatic, but, partly owing to their small size, they 

 have been able to solve the respiratory problems in the most 

 diverse ways. In one point, however, they have proved in- 

 ferior to the aquatic vertebrates, they have not been able to 

 invade the oceans, and only very few can exist in brackish 

 water. 



Respiration at the surface by diffusion. In the simplest type of 

 adaptation — from a physiological point of view — air-breathing 

 through spiracles is retained. The functional spiracles are 



Fig. 75. A, hydrophile hair in the water surface, seen in cross-section. 

 A', position of equilibrium; B, hydrophobe hair in surface; B', equilibrium; C, 

 spiracle surrounded by hydrophobe hairs (Stratiomys), position of equilibrium 

 below surface; D, reaching surface; E, in equilibrium; F, short hydrophobe hairs 

 on ventral cuticle of Dytiscus; G, curved hydrophobe hairs in Elmis and Harmonia. 

 (Wigglesworth.) 



