16 INSECT PHYSIOLOGY 



will enter by these routes. It is probably safe to say that in most ter- 

 restrial insects almost all the oxygen is taken in by the tracheae; in the 

 case of Carausius, it has been shown that about 25 per cent, of the 

 carbon dioxide escapes through the skin, and in larvae of Dytiscus 

 and Eristalis somewhat less. 



The function of the spiracles 



This simple conception of tracheal respiration is complicated by the 

 need for retaining water; for most of the evaporation from an insect 

 takes place through the spiracles. Burmeister (1832) discovered that 

 each spiracle is provided with an occlusive mechanism, the main 

 function of which is probably to protect the insect from loss of water : 

 the adult flea, which has well-developed sphincters in the spiracles, is 

 far more resistant to desiccation than the larva which has none. The 

 bug Rhodnius will survive for some weeks in a desiccator over sul- 

 phuric acid; but if it is caused to keep its spiracles open by the addi- 

 tion of 5 per cent, of carbon dioxide to the air it is completely dried 

 up and dead within forty-eight hours. 



Normally the spiracles are kept closed, being opened only just 

 often enough to keep the insect supplied with oxygen. In the flea, 

 for example, while the insect is at rest all the respiration is effected 

 through the first and eighth spiracles of the abdomen. These show a 

 regular rhythm of opening and closing every five or ten seconds. If 

 the insect struggles they remain open a little longer. If the rate of 

 metabolism is increased by raising the temperature they open more 

 frequently and other spiracles are brought into operation. And at the 

 height of digestion and egg production in the female all the spiracles 

 may be kept open for quite long periods. This necessarily results in 

 an increased rate of water loss. We can see now how necessary it is 

 for the tracheae to anastomose: while the spiracles are closed any 

 organ or tissues which is active can draw oxygen from any part of 

 the system; and the oxygen throughout the system can be renewed by 

 the opening of a single pair of spiracles. 



This function of the spiracles has been termed the 'diffusion con- 

 trol' of insect respiration. It is regulated by respiratory centres in the 

 ganglia of the ventral nerve cord and in the brain. These centres are 

 stimulated by carbon dioxide: exposure to 2 per cent, carbon dioxide 



