74 INSECT PHYSIOLOGY 



Besides generating heat by muscular activity, many insects make 

 use of the sun's rays to warm their bodies to the optimum tempera- 

 ture, orientating themselves differently according to the temperature 

 attained : butterflies spread their wings in the sun, and locusts turn 

 themselves broadside on towards the sun when their temperature is 

 too low. It has often been suggested that the tendency of insects in 

 cold climates to be dark in colour is related to the absorption of 

 radiant heat from the sun - though it is sometimes forgotten that this 

 same dark colour will also favour the loss of heat during the night. 



Apart from these physiological regulations there are wide dif- 

 ferences in the temperatures to which different insects are adapted. 

 To quote two extreme cases: the firebrat Thermobia is active over 

 the range 12-50° C; it suffers heat injury a little above 51° C. 

 Grylloblatta, which is found in the high mountains of North Ameri- 

 ca, is normally active from —2-5-1 1-5° C. ; it is irreversibly damaged 

 by heat at 20-5° C. The physiological basis of these differences is not 

 known. 



Besides such specific differences in resistance to abnormal tem- 

 peratures there are individual differences resulting from adaptation : 

 the cockroach Blatta, for example, if it has been kept at 36° C, goes 

 into a state of 'chill coma' when the temperature is lowered to 9-5° 

 C. ; but if it has been kept, for no more than twenty-four hours, at 

 15° C, it does not go into chill coma until the temperature has fallen 

 to 2° C. And there is a similar adaptation at the upper end of the 

 temperature range. 



Respiratory metabolism 



The energy metabolism of insects varies, naturally, with the degree of 

 muscular exertion. It therefore varies enormously in different species, 

 and is much greater in the larva than in the pupa, and greater again 

 in the adult. It depends also upon the temperature; but a rise in 

 temperature not only increases the resting or 'basal' metabolism, but 

 usually leads to greater activity. Only in very exceptional circum- 

 stances, therefore, is any simple mathematical relationship likely to 

 be found between these two factors. Curves of various types, re- 

 lating temperature with oxygen uptake, notably during pupal de- 

 velopment, have been described mathematically by some authors; 



