NUTRITION AND METABOLISM 73 



hibernation and this reduction may be one factor in the increased 

 resistance of insects to cold during the winter. But the chief factor in 

 cold resistance is a lowering of the freezing point and undercooling 

 point (the lower limit of supercooling) of the whole insect. One cause 

 of this is an accumulation of glycerol in the haemolymph during dia- 

 pause, when it may reach a level as high as 9 per cent, (occasionally 

 even as high as 40-50 per cent.). At the same time there is a change of 

 unknown nature in the haemolymph which leads to the elimination 

 of the undefined 'nucleating agents' needed for the initiation of freez- 

 ing. In some insects, notably in certain caterpillars, the body fluids 

 may freeze solid in the winter; 'cold hardiness' then depends on the 

 prevention of ice formation extending to the intracellular water of 

 the tissues. Here again an accumulation of glycerol seems to be 

 involved. 



The temperature of insects 



We have seen that at high temperatures, insects above a certain size 

 can keep themselves cool for short periods by evaporation of water 

 from the body, or by making use of evaporating water in the en- 

 vironment. At low temperatures, their bodies are often kept warmer 

 than the surrounding air by the chemical changes going on within 

 them. And both these methods of temperature control are used by 

 the social Hymenoptera in maintaining the temperature of their nests 

 or hives. It seems that for the intense muscular activity of flight, a 

 high body temperature is necessary: the large Sphingid moths are 

 apparently unable to take flight immediately from rest; they first 

 stand with the wings vibrating - shivering, as it were - until the 

 temperature in the thorax has risen above 30° C. Only then can they 

 fly; and during flight the temperature will exceed 40° C. Similar 

 changes occur in other insects during activity; but whether insects at 

 rest will increase their metabolism in order to maintain the body 

 temperature is at present uncertain. The scales and hairs which invest 

 the body of moths and bees play an important part in insulating the 

 insect and thus helping to maintain the high temperature during 

 flight. In a large Sphingid moth they may serve to raise the tempera- 

 ture in the thorax by 8-9° C. The subcuticular air sacs play a similar 

 role in Odonata, &c. 



