THE PHYSIOLOGY OF RESPIRATION IN INSECTS. 375 



minute, and on these bases the eighteen spiracular sacs would 

 drive 7,200 cubic millimetres of air into the tracheae per hour. 

 It may be assumed, I think, that during flight an even higher 

 respiratory energy may exist ; but even with 240 respiratory 

 acts in a minute, a number which may probably be excessive, 

 only 14,400 cubic millimetres of air could be taken into the 

 tracheae. On these grounds it may be safely concluded that 

 the whole of the oxygen is taken from the inspired air. 



Of course the above must be regarded only as an approxi- 

 mation to the truth, but it cannot be very far wrong. 



Peyron [158J showed by actual analysis that the quantity 

 of oxygen in the gases obtained from the interior of the 

 bodies of insects does not exceed 8 or 9 per cent., whilst 70 

 per cent, is nitrogen, and the rest carbon dioxide the latter 

 is probably in solution in the blood, and if this is so the 

 tracheal air would consist of about 10 per cent, oxygen, and 

 go per cent, nitrogen. 



In the Blow-fly similar conclusions ma}- be arrived at on 

 the same bases, but the numbers cannot be so accurately 

 estimated. I think it is probable that the quantity of air 

 capable of being inspired is proportionately greater than in 

 the Cockchafer, perhaps even twice or three times as great, 

 weight for weight ; but a large quantity is expended in pro- 

 ducing the buzzing sound in flight. This excess is probably 

 supplied by the greater proportionate size of the thoracic 

 spiracles and spiracular sacs. The small size of the abdominal 

 spiracles and spiracular sacs must be remembered, and I do 

 not think it possible that more than three or four cubic centi- 

 metres of air can enter the respiratory sacs and tubes per 

 hour, and I suspect that considerably less is really inspired. 



The inspired air gives up its oxygen entirely or partially to 

 the blood, from which it is removed by the tissues, and stored 

 by them as intra-molecular oxygen. That the tissues are 

 richly supplied with oxygen in insects is evident from the length 

 of time they survive when deprived of air. 



Received View. It is usually held that the spiracles act alter- 

 nately as inspiratory and expiratory orifices, and that the influx 



