RESPIRATION 17 



causes the spiracles to remain permanently open. They arc also 

 stimulated by the acid metabolites which result from oxygen lack : 

 traces of lactic acid injected into the flea will cause the spiracles to 

 remain widely open even in the absence of carbon dioxide. In normal 

 respiration oxygen want and carbon dioxide accumulation co-oper- 

 ate in the control. There appears to be some degree of control within 

 the muscles of the spiracles themselves, for even after complete re- 

 moval of the central nervous system weak responses remain. 



Ventilation of the tracheal system 



This type of respiratory system, dependent solely upon gaseous 

 diffusion, is adequate for most small insects, and even large insects if 

 they are relatively sluggish; but it is insufficient for actively running 

 or flying species with a high rate of metabolism and a massive de- 

 mand for energy. In these, in the larger Orthoptera and beetles, in 

 bees, wasps and flies, a greater or less degrees of mechanical ventila- 

 tion of the tracheal system is superadded. We have seen that the 

 spiral folding of the typical tracheae renders them resistant to col- 

 lapse under pressure; but, in some cases at least, they can be ex- 

 panded and contracted in their long axis like an accordion, and their 

 capacity reduced by as much as 20 to 30 per cent. In other cases they 

 are not round in cross-section but elliptical {Dytiscus larva) or even 

 ribbon-like (in the thorax of Muscid flies) and will then readily col- 

 lapse when the pressure around them is increased. Or they may have 

 thin-walled cavities at intervals along their course (as in Mclolontha), 

 or large dilatations ('air sacs') which occupy a great part of the body 

 cavity (as in many Orthoptera, Hymenoptera, &c.) (Fig. 5, B). 



There is little doubt that, as Treviranus suggested, the main func- 

 tion of these collapsible tracheae or air sacs is the ventilation of the 

 respiratory system. Like the ventilation of the lungs of vertebrates, 

 the filling and emptying of these air sacs is secondary to the respira- 

 tory movements of the rigid body wall which encloses them. When 

 the body wall is in the inspiratory position they are widely open; in 

 the expiratory position many of them are collapsed and empty. In 

 addition they are doubtless ventilated to some extent by general 

 movements of the body - for example, during flight in the locust 

 Schistocerca, automatic ventilation of the intermuscular air sacs is 



