226 SOME ASPECTS OF ECOLOGY 



under bark, in thatch buildings, etc., Sacharov mentions that it 

 has been found coated with ice and yet subsequently revived and 

 became active. It will be observed that its total water content 

 is very low and 71 per cent, of this water was non-frozen at — 11-1°, 

 while the ratio of fat to live weight appears to be high. 



The work of the three observers mentioned has put us on the 

 track of understanding the way of solving the problem of cold 

 hardiness in insects and that it is linked up with the biophysics of 

 metabolic water in the living creatures. All that we know about 

 this water is that in cold hardy species a high proportion of it is 

 held in a peculiar phase that does not freeze until relatively very 

 low temperatures are attained. Whether this water is actually 

 bound or adsorbed in the strict physical sense will not be discussed 

 here. 



In a state of nature we may picture to ourselves what appears 

 to happen to an insect when it enters into hibernation. It ceases 

 to feed, and wanders until it discovers an appropriate resting 

 place. At this period it has a liberal store of fat in its tissues that 

 has accumulated during the preceding period of activity. Its 

 water content is reduced and, as the prevailing temperature of its 

 environment gradually falls away during autumn and early 

 winter, the ratio of non-freezable to total water in its body increases 

 in relation to the fall in its freezing and supercooling points. 

 Cold dry weather, it would seem, aids the acquisition of the insect's 

 powers of resistance, whereas cold damp weather is much less 

 favourable. It is probable that vast numbers of insects perish 

 when the acquisition of cold hardening is interfered with, since 

 the fatal supercooling point is higher and more quickly reached. 

 The farmer, in cultivating his land during winter, disturbs the 

 winter quarters of immense numbers of insects. These are often 

 brought in the process into contact with much lower temperatures 

 than their hardening capacity enables them to resist, and large 

 numbers are killed. 



For the most recent contribution to the subject of freezing 

 temperatures in relation to insect life, the reader is referred to a 

 paper by Salt (1936). In addition to giving a critical review of 

 the literature up to date, this author describes improved apparatus 



