RESPIRATION 19 



being passive. In most cases expiration is active and inspiration 

 passive, though in the larva of Aeschna and in the grasshopper in- 

 piratory muscles also are present. The movements may consist of 

 dorso-ventral flattening movements (Orthoptera, Coleoptera) or 

 longitudinal telescoping movements (Diptera, Hymenoptera). 



The simplest interpretation of these movements is that they are 

 ventilating all parts of the tracheal system alike. But if, during the 

 respiratory contraction, the spiracular valves in different parts of the 

 body open and close independently of one another, it is possible that 

 a directed stream of air will be driven through the tracheal system - 

 certain spiracles serving always for inspiration, others for expiration 

 - an arrangement which would serve to enrich the air in the inspira- 

 tory section of the system. The idea that such a circulation of air does 

 occur has often been put forward, but only in recent years has it been 

 convincingly demonstrated. Thus, it has been shown that if the four 

 anterior pairs of spiracles in the locust {Schistocerca) are enclosed in 

 one gas chamber, and the six posterior pairs in another, air is actively 

 transported during respiration from the anterior to the posterior 

 chamber. The underlying principle of this mechanism is that the 

 rhythmical nervous impulses which control the ventilating move- 

 ments are co-ordinated with impulses which bring about the syn- 

 chronous movement of the spiracles - some of these opening during 

 inspiration, other during expiration. But before looking further into 

 this point, we must consider the control of the ventilating movements 

 themselves. 



The regulation of respiration 



The respiratory movements are effected by impulses from nerve 

 centres. These centres are of two kinds: so-called primary respiratory 

 centres which lie in the segmental ganglia and control the movements 

 of their own segments (for the isolated segments of the abdomen may 

 perform respiratory movements), and secondary centres which have 

 an overriding action and control the movements in the whole insect. 

 One might expect the secondary centre to lie in the head, but this 

 seems never to be the case: decapitation produces only a temporary 

 and uncertain effect on respiration. In the few insects that have been 

 studied, it is situated in one or other of the thoracic segments. 



