TRACHEAL RESPIRATION 



141 



The working of tracheal gill mechanisms is subject to certain 

 physical conditions which have been clearly recognized and 

 ably discussed by H. Koch (1936). 



Fig. 82. 



Dissection of rectum of Libellula depressa showing 6 double rows of 

 tracheal gills. Ca. 8/1. (Koch.) 



Just as in the air-gills described above, the nitrogen per- 

 centage in the enclosed air will practically always be higher than 

 in the atmosphere, and the nitrogen would have the same 



One branchial leaf with tracheoles. Ca: 200/1. (Koch.) 



tendency to diffuse away and cause the ultimate disappearance 

 of the whole of the air in the system, except for the fact that 

 the walls of tracheal gills are rigid enough to stand a difference 



