TRACHEAL RESPIRATION 



137 



by working the tail as a screw, is driven into the tissue until 



connection is established with the intercellular air spaces. 



The structure of the tracheal system 



showing an oval cross section of the main 



trunks and two air sacs would seem to 



invite mechanical ventilation, but the 



experiments failed to show anything but 



diffusion. In one of the experiments a 



larva was enclosed in a piece of glass 



tubing as shown in Fig. 79 with a root 



from an aquatic plant, mounted so that 



any desired gas mixture could be blown 



through the intercellular spaces. The 



larva very soon found and pierced the 



root, and the tracheal system could now 



conveniently be observed through a 



microscope. No respiratory movements 



took place while atmospheric air was 



passed through the root. 



The oxygen percentage in the sub- 

 merged roots is normally below 10% and 

 may fall to 4%, and in winter even lower 

 (Ege, 1915), and one might expect that 

 low percentages were necessary to induce 

 ventilation. Accordingly a gas mixture 

 containing only 3.4% 2 was passed 

 through. This did not impair the animal's vitality and did 

 not induce any ventilation. Pure nitrogen paralyzed the 

 animal, but likewise failed to induce respiratory movements. 



Fig. 78. Tracheal 

 system of Mansonia lar- 

 va. (Wesen berg- Lund.) 



Fig. 79. Experimental arrangement to study ventilation in Mansonia larva. 



(Krogh.) 



