364 



HYBERNATION OF INSECTS. 



The degree of cold which most insects in their different 

 states, while torpid, are able to endure with impunity is very- 

 various; and the habits of the different species, as to the 

 situation which they select to pass the winter, are regulated 

 by their greater or less sensibility in this respect. Many 

 insects, though able to sustain a degree of cold sufficient to 

 induce torpidity, would be destroyed by the freezing tem- 

 perature, to avoid which they penetrate into the earth or hide 

 themselves under non-conducting substances ; and there can 

 be little doubt that it is with this view that so many species 

 while pupae are thus secured from cold by cocoons of silk or 

 other materials. Yet a very great proportion of insects, in all 

 their states 3 are necessarily subjected to an extreme degree of 

 cold. Many eggs and pupae are exposed to the air without 

 any covering ; and many, both larvae and perfect insects, are 

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

 are able to resist, remaining unfrozen though exposed to the 

 severest cold, or, which is still more surprising, are uninjured 

 by its intensest action, recovering their vitality even after 

 having been frozen into lumps of ice. 



The eggs of insects are filled with a fluid matter, included 

 in a skin infinitely thinner than that of hens' eggs, which 

 John Hunter found to freeze at about 15° of Fahrenheit. 

 Yet on exposing several of the former, including those of the 

 silk-worm, for five hours to a freezing mixture which made 

 Fahrenheit's thermometer fall to 38° below zero, Spallanzani 

 found that they were not frozen, nor their fertility in the 

 slightest degree impaired. Others were exposed even to 56° 

 below zero, without being injured. 1 



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 that of the egg. Their vitality enables 

 them to resist it to a certain extent, and it must be con- 



that bees preserve their activity, and even leave the hive and collect pollen, when 

 the external temperature is 40° -38, and that of the hive only 47°'28 (Table XVT. 

 Nov. 6), and they may, consequently, feed their brood, and attend to the usual 

 interior occupations of the hive, at a temperature not lower than this, to which 

 lower temperature it does not appear likely, from Mr. Newport's observations, 

 the interior of their hives often descends in our winters. 

 1 Tracts, 22. 



