TRACHEAL RESPIRATION 



143 



sects, of which the pupal stage of the mosquito Simulium 

 subornatum is a good example, a respiratory gill mechanism 

 has been evolved which can be used both in dry air and in 

 water (Nadine Pulikovsky, 1927). The pupae are fixed to 

 stones in mountain streams which dry up 

 often enough. Outside the spiracles we find 

 the hollow air-filled cuticular appendage 

 shown in Fig. 84 into which oxygen can dif- 

 fuse with equal facility from air and from water. 



The regulation of respiration in aquatic insects 

 has not been studied in any exhaustive or 

 comprehensive way. In several of the in- 

 sects carrying a supply of air down with 

 them, the decrease in buoyancy appears to 

 be the normal stimulus for returning to the 

 surface (Corixa, Notonecta), but lack of oxy- 

 gen will no doubt turn out to be effective 

 in almost all cases and has been shown to 

 regulate the movements by which Phryganea 

 larvae ventilate their tubes (van Dam, 1938), 

 the rhythmic vibrations of the gills in ephem- 

 erid larvae and the ventilation of the gills in 

 Odonata. In these cases the supply of oxygen-saturated water 

 produces a long continued apnoea (Babak and Foustka, 1907). 



Botjes (1932) made experiments to show that the move- 

 ments of the hind legs of Corixa, which apparently serve to 

 renew the water along the air store in the abdomen, are 

 initiated by high C0 2 tensions. The observation that such 

 movements are induced by C0 2 tensions of 40-45 mm (6%) 

 is no doubt correct, but such high tensions must be rare in the 

 natural habitat of Corixa, and^ no attempt has been made to 

 show that they are ever met with. On the other hand Botjes 

 was unable to induce the movements by oxygen lack which 

 will bring the animal to the surface. This problem, like so 

 many others concerning the respiratory mechanisms in the 

 animal kingdom, will have to be studied again by improved 

 methods. 



Fig. 84. Simu- 

 lium subornatum pupa 

 with cuticular ap- 

 pendages. Ca. 7/1. 

 (Pulikovsky.) 



