NUTRITION AND METABOLISM 75 



while others regard such curves as being really composed of a num- 

 ber of straight lines which intersect at 'critical points 1 where new 

 metabolic processes are supposed to supervene. 



The energy metabolism of such an insect as the honey-bee, when 

 at rest at 18° C, requires an oxygen consumption of about 30 c.mm. 

 per gm. per minute; during flight this may rise to 1,450 c.mm. per 

 gm. per minute, a 48-fold increase. Whereas in man, doing the 

 maximum amount of work possible, the respiratory metabolism is 

 increased only some 10 or 12 times. 



The honey-bee, the higher flies and some other insects burn only 

 carbohydrates during flight: they have a respiratory quotient (the 

 ratio of CO 2 evolved to 2 consumed) equal to one. But other insects 

 such as Aphids and locusts, which consume mainly glycogen in the 

 early stages of flight, rely wholly on fats for energy production during 

 prolonged flights. And surprisingly enough, the Lepidoptera, al- 

 though they feed solely on nectar, are unable to use carbohydrates 

 directly in their flight muscles: they must first convert the sugar into 

 fat; during flight the R.Q. is always about 0-75. In most insects, 

 during starvation, glycogen is first consumed, to be followed by 

 protein and fat. 



The biochemical processes of energy production from stored re- 

 serves are in general outline the same as in other animals. The citric 

 acid cycle occurs in insects as a main pathway for the liberation of 

 the hydrogen which serves as fuel for the cytochrome system. The 

 series of haemochromogen compounds that make up the cytochrome 

 system were originally discovered by David Keilin when he was 

 working with insects. 



Anaerobic metabolism 



Insects are remarkably resistant to lack of oxygen. Down to a low 

 level of oxygen tension, the precise level varying in different insects, 

 the oxygen uptake remains unchanged. Below this level it falls off; 

 but many species can survive for long periods in the complete ab- 

 sence of air. The extra metabolism of activity very quickly ceases, 

 and the insect becomes quiescent; the basal metabolism continues 

 anaerobically. The type of chemical change does not alter: the un- 

 oxidized metabolites simply accumulate. Thus, by the time the insect 



