IX] THE RESPIRATORY SYSTEM 201 



that air can, if necessary, be prevented from escaping from the 

 tracheae by the action of the combs of bristles, which lie just 

 inside the spiracles, and close them from within, whenever the 

 pressure is in that direction. Also, we know that the tracheae in 

 the imago maintain an internal pressure greater than that of the 

 atmosphere, since they collapse almost at once when the animal 

 is dissected, whereas those of the larvae do not. Such a pressure 

 could only be maintained, provided that the spiracles can remain 

 closed during the movement of expiration. 



We do not know for certain how insects get rid of their carbon 

 dioxide. It is supposed that a great deal escapes through the 

 integument. In the larva, the act of ecdysis may be connected 

 with the removal of this and other impurities, which may help in 

 the formation of the cuticle. In the larva also, rectal respiration 

 must remove a great deal of this gas, by the ordinary processes of 

 diffusion. 



In the Anisopterid larva, the respiratory movements of the 

 rectum are easily watched. A large volume of water is drawn in 

 rather quickly. This can be let out slowly, with no apparent 

 effort, or it may be shot out with great force so as to propel the 

 larva forward. 



In the Zygopterid larva, no definite respiratory movements 

 can be seen with the naked eye, beyond an occasional waving of 

 the gills to and fro. In larvae which have lost their gills, the 

 abdomen is often strongly arched; or it may be jerked quickly 

 to and fro. Rectal respiration in these larvae can be watched 

 by a careful use of carmine particles. The vigorous rectal respira- 

 tion which takes place in newly-hatched Zygopterid larvae can be 

 followed with ease under a low power, owing to the transparency 

 of the integument. The whole problem of the respiration of these 

 larvae is, at the present time, much in need of further investiga- 

 tion. 



The degree to which the spiracles are functional in Dragonfly 

 larvae was investigated by Dewitz [49] in a deeply interesting and 

 well-known paper. (See also Miall[94].) 



