20 INSECT PHYSIOLOGY 



The isolated ganglia of insects which possess a mechanical venti- 

 lation of the tracheal system (Dytiscus, Aeschnd) show rhythmical 

 changes in electrical potential of the same frequency as the respira- 

 tory movements of the intact insect. There must, therefore, be a 

 spontaneous respiratory rhythm inherent in the nerve centres. But 

 the activity of the centres, both in rate of rhythm and depth of move- 

 ment evoked, may be influenced by external stimuli, either nervous 

 or chemical; and during times of rest the ventilating respiratory 

 movements may cease altogether. There is evidence that during the 

 onset of activity - flight, for example - the respiratory movements 

 may sometimes be initiated by nervous stimuli alone (for any stimu- 

 lus will cause an increase in respiration in insects) ; but they can also 

 be initiated by chemical stimuli alone; and, although there must be 

 endless variation in different species, perhaps these play the greater 

 part in the normal life of the insect. 



Either a lack of oxygen or an excess of carbon dioxide may stimu- 

 late the respiratory centres and cause hyperpnoea; in some insects an 

 excess of oxygen may cause a prolonged apnoea. Now excess of 

 oxygen will not diminish the production of carbon dioxide; nor will 

 a lack of oxygen increase it. If, therefore, carbon dioxide were the 

 sole respiratory stimulant, respiration should not be influenced by 

 the tension of oxygen. But there is no doubt that carbon dioxide can 

 control respiration: in Carausius, the activity of the secondary or 

 prothoracic centre is stimulated by carbon dioxide tensions of 0-2 

 to 3 per cent. ; the primary centres respond to 12-15 per cent. ; and in 

 the Periplaneta and other insects, ventilation begins, even in the 

 insect at rest, at a carbon dioxide tension of 10 per cent. In Schisto- 

 cerca carbon dioxide is normally more important in control than 

 oxygen lack. Since it is reasonable to assume that the chemical 

 factor at work is the same in all these cases, it seems probable that 

 this factor is the acidity in the nerve centres - due either to an excess 

 of carbon dioxide, or to acid metabolites accumulating in the absence 

 of oxygen. 



During the winter months the pupae of many insects go into a 

 state of dormancy (p. 94) : the rate of metabolism falls, and the res- 

 piratory centres become rather insensitive to carbon dioxide. In this 

 state of dormancy large insect pupae show a curious cyclic discharge 



