H'^BERNATTON OF INSECTS. 451 



coons of silk or other materials. Yet a very great pro- 

 portion of insects in all their states are necessarily sub-* 

 jected to an extreme degree of cold. Many eggs and 

 pupsB are exposed to the air without any covering ; 

 and many, both larva? and perfect insects, are sheltered 

 too slightly to be secure from the frost. This they are 

 either able to resist, remainino- unfrozen though ex- 

 posed to tlie severest cold, or, which is still more sur^ 

 prising, are uninjured by its intensest action, recover- 

 ing their vitality even after having been frozen into 

 lumps of ice. 



The eggs of insects are filled with a fluid matter, in- 

 cluded in a skin infinitely thinner than that of hens' 

 eggs, which John Hunter found to freeze at about 1 5° 

 of Fahrenheit. Yet on exposing several of the former, 

 including those of the silkworm, for five hours to a 

 freezing mixture which made Falirenheit's thermo- 

 meter fall to 38" below zero, Spallanzani found that 

 Ihey were not frozen, nor their fertility in the slightest 

 degree impaired. Others were exposed even to 56" 

 below zero, without being injured^. — A less degree of 

 cold suffices to freeze many pupae and larvae, in both 

 which states the consistency of the animal is almost as 

 fluid as in tJiat of the egg. Their vitality enables them 

 to resist it to a certain extent, and it must be consi- 

 derably below the freezing point to affect them. The 

 winter of 1813-14 was one of the severest we have had 

 for many years, Fahrenheit's thermometer having been 

 more than once as low as 8" when the ground was 

 wholly free from snow ; yet almost the first objects 

 which I observed in my garden, in the commencement 



* Tracts, '2'2. 

 2 G 2 



