RESPIRATION 21 



of carbon dioxide: oxygen uptake through the spiracles is more or 

 less continuous, for oxygen want causes the spiracles to open at fre- 

 quent intervals for very brief periods; but carbon dioxide output 

 takes place in 'bursts' which may happen only once in twenty-four 

 hours. At these times the large accumulation of carbon dioxide 

 causes a prolonged opening of the spiracles. This phenomenon is, 

 indeed, the result of the control of the opening and closure of the 

 spiracles by the combined action of oxygen want and carbon di- 

 oxide accumulation in the blood - acting in large insects with a very 

 low rate of metabolism. 



We have seen (p. 15) that in thin-skinned insects much of the 

 carbon dioxide diffuses through the general surface of the body, and 

 therefore cannot serve as a respiratoiy stimulus at all. Thus we find 

 that mosquito larvae, and the larvae of Aesclma and Dytiscus, are 

 driven to seek air at the water surface by oxygen want and not by 

 carbon dioxide excess. On the other hand, the leg movements in the 

 aquatic bug Corixa, which direct a stream of water over its air store 

 (p. 23), are called forth by the accumulation of carbon dioxide; 

 while the same insect is caused to rise to the surface when the air 

 store is reduced to a given size. 



We are now in a position to revert to the question of the stream of 

 air directed through the tracheal system by the co-operation of 

 ventilatory and spiracular movements. We have seen that in the in- 

 sect at rest the supply of oxygen is effected solely by diffusion through 

 one or two pairs of spiracles. As the demand for oxygen increases, 

 more spiracles come into operation andthey remain open for longer 

 periods. When the oxygen requirements are still greater ventilatory 

 pumping movements begin and then the rhythm of opening and 

 closure of the spiracles is changed in such a way as to cause a directed 

 stream of air through the system. All this implies a most intricate 

 nervous co-ordination between the control of the pumping move- 

 ments and the regulation of the spiracles; and the rhythm may 

 change completely as the intensity of stimulus increases. 



Regulation of tracheal supply 



The general arrangement of tracheae is part of the inborn pattern 

 of growth in the insect, but this pattern is subject to modification in 



