222 SOME ASPECTS OF ECOLOGY 



temperatures that descend below the threshold of development, a 

 point is reached where cold becomes fatal. This point varies 

 enormously in different species, but is lowest in those which 

 hibernate and is also dependent on several factors. Certain over- 

 wintering species are well capable of withstanding a temperature 

 of — 50°. Remarkable as this fact is, it seems to fade into 

 insignificance when compared with the resting phase in rotifers 

 and tardigrades which have been recorded by Rahm (1923) in 

 Germany to be able to withstand a temperature of 7° absolute 

 (—272°) for several hours. The researches of Robinson and 

 Payne in America, and of Sacharov in Russia, have shown that 

 cold resistance in insects is explainable on certain biophysical 

 principles. Robinson's experiments showed that cold hardiness 

 cannot be acquired by an insect rapidly — it requires a falling 

 temperature. As the temperature sinks below freezing-point the 

 colloid particles in the tissues and body fluids withdraw and bind 

 free water as films around themselves. This so-called bound or 

 adsorbed water ceases to behave as ordinary free water since it 

 does not freeze even at — 20°. It is the ratio of bound to free 

 water which, when sufficiently high, protects insects against 

 freezing temperatures. In pupae of the moth {Telea polyphemus) 

 Robinson found that the bound water increased 48 to 50 per 

 cent, when the insects had become fully cold hardened. Payne 

 (1929) has stressed the importance of humidity, and her numerous 

 experiments indicate that cold hardiness bears an inverse relation 

 to absolute humidity, regardless of temperature. For a given 

 temperature insects are acclimatised to, their survival at lower 

 temperatures depends upon the amount of humidity present, as 

 is shown in the accompanying table (Table VII.). In the case 

 of the beetle {Synchroa punctata), Payne, by using thermo-electric 

 methods, found that when the insects were placed in a desiccator 

 at 15° their freezing and undercooling points were greatly lowered. 

 Thus, in the normal larvae, these points were — 3-04° and — 6-71° 

 respectively, whereas in the desiccated larvae these same points 

 were — 71-4° and 23-26°. It therefore appears that the moisture 

 content of the insect itself is also an important factor in the process. 

 Now the water content of insects is known to have a direct relation 



1 



