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 trachez per hour. 
{t 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 
trachez. 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 [158] 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 g 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 may 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 
