TRACHEAL RESPIRATION 



133 



placed at one end of. the body which is 

 brought in contact with the surface, and in 

 most cases the surface film is utilized for 

 support by hydrofuge hairs or other struc- 

 tures (Fig. 75). It is essential that the 

 spiracle opening itself is hydrophobe, so 

 that water cannot possibly penetrate into 

 the tracheal system. 



In many small forms, of which the com- 

 mon mosquito larvae (Culex) can be taken as 

 typical, there is no mechanical ventilation, 

 but the air in the system is renewed by 

 diffusion only. This was shown in unpub- 

 lished experiments by the writer as illus- 

 trated in Fig. 76. The larva is enclosed in 

 the funnel of a micro gas-analysis tube, a 

 small air-bubble is driven down so far that 

 the spiracles can come in contact with it and 

 focused upon by a horizontal microscope. 

 When the larva comes in contact with the 

 bubble after being submerged for several 

 minutes a sudden upward jump of the me- 

 niscus is observed corresponding to a vol- 

 ume of 0.2-0.25 mm 3 , but thereafter it does 

 not indicate any respiratory movement 

 whatever. During the period of submer- 

 gence most of the oxygen in the tracheal 

 system is used up and only to a slight extent 

 replaced by C0 2 , and the initial "inspira- 

 tion" is simply the filling up of the partial 

 vacuum created. The total volume of the 

 tracheal system is about 1.5 mm 3 , and the 

 oxygen in it will suffice for the normal 

 metabolism in 5-10 minutes. 



As in the terrestrial insects, the diffusion 

 type of ventilation imposes a size limit 



Fig. 76. Experi- 

 mental arrangement 

 to study ventilation 

 in mosquito larva. 

 (Krogh.) 



