THE GILL-CHAMBER OF DRAGONFLY NYMPHS 345 



PHYSIOLOGICAL CONSIDERATIONS 



Dissolved in the water which enters the rectum are a num- 

 ber of gases, each exerting its partial pressure independent of 

 the others. To some, as oxygen and carbon dioxide, the lining 

 of the rectum is a permeable membrane; they pass through it. 

 Dewitz ('90) has shown that these gases pass through chitin. If 

 the pressure of the gas in question be greater in the air within 

 the tracheae, no inward passage will take place, but the reverse. 

 The question of whether or not one side of the rectal wall differs 

 from the other in having the gas in the form of a solution or a 

 free gas, does not make any difference here, for the wall is im- 

 pervious to the solvent. 



As the activities of the nymph use up a part of the oxygen 

 in the air within the tracheae, more of this gas will diffuse in 

 through the gills from the amount present in the water, whose 

 pressure is thus greater than that of the oxygen within the 

 tracheae. Similarly, the activities of the nymph soon cause 

 enough carbon dioxide to accumulate within the tracheae to 

 cause diffusion outwards through the walls of the gills. 



The question of circulation of air in the tracheal system does 

 not seem either important or difficult to me. The active con- 

 traction of the abdomen in the working of the rectum for swim- 

 ming or respiration ought certainly to produce a considerable 

 amount of movement of air in the main tracheal trunks. Given 

 this, the immense amount of area for diffusion which the gills 

 provide certainly suffices for gas exchange, with only the diffusion 

 of the gases within the tracheae to bring carbon dioxide to the 

 seat of exchange and carry away the oxygen. What eddy cur- 

 rents may exist and aid this, I cannot say; it seems likely that 

 some do exist. 



SUMMARY 



1. The rectum in all anisopteran nymphs possesses six longi- 

 tudinal rows of respiratory structures, each of which is divided 

 symmetrically into halves. The six rows are situated, three 

 on the dorsal and three on the ventral walls in the Aeschnidae, 



