HYBERNATION OF INSECTS 451 
are thus secured from cold by cocoons of silk or other 
materials, Yet a very great proportion of insects in all 
their states are necessarily subjected to an extreme de- 
gree of cold. Many eggs and pupe are exposed to the 
air without any covering; and many, both larvee and per- 
fect insects, are sheltered too slightly to be secure from 
the frost. This they are either able to resist, remaining 
unfrozen though exposed to the severest cold, or, which 
is still more surprising, are uninjured by its intensest ac- 
tion, recovering their vitality even after having been fro- 
zen 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 15° of 
Fahrenheit. Yet on exposing several of the former, in- 
cluding those of the silk-worm, for five hours to a freez- 
ing 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?. 
A less degree of cold suffices to freeze many pupa and 
larvze, 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 considerably 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 
a Tracts, 22. 
DCG 
