136 



RESPIRATORY MECHANISMS 



sion could take place, and the animal became heavier than 

 water in 14 minutes and died in 35 minutes, although when 

 breathing ordinary air at the same temperature it could 

 subsist for 6 hours Without access to the surface. 



This respiration mechanism is essential for several small 

 aquatic insects belonging to different groups and also for some 

 larger ones in winter when the metabolism is low. Several 

 species of Dytiscidce move about vigorously under ice and 

 supplement their store of air by catching bubbles rising from 

 plants, or from the mud. Even if these latter contain very 

 little oxygen they may be essential for keeping up the quantity 

 of gas on which the uptake of oxygen from the water depends. 



The animal which is prob- 

 ably most definitely adapted 

 to obtain oxygen by diffu- 

 sion into a layer of air is the 

 spider Argyroneta which al- 

 ways carries a layer of air 

 round the whole of the abdo- 

 men and part of the thorax. 

 In this case the air sometimes 

 lasts for several days even 

 in summer (Braun, 1931). 



Respiration from aquatic 

 plants. A small number of 

 beetle and Diptera larvae are 

 able to obtain air for their 

 respiration from aquatic 

 plants, not only by collect- 

 ing bubbles, but directly 

 from the intercellular spaces. 

 I have had an opportunity to make some observations and 

 experiments on the larva of the mosquito Mansonia shown in 

 Figs. 77 and 78. The atrium of the spiracle is drawn out into 

 a tube of hard chitin with a sharp cutting edge. This is 

 placed against the root or rhizoma of an aquatic plant and, 



Fig. 77. Mansonia larva on plant root. 

 Ca. 10/1. (Wesenberg-Lund.) 



