HYBERNATION OF INSECTS. 455 
of insects have usually thinner skins than pupe, and yet 
they are unaffected by a degree of cold much superior. 
In the present state, then, of our knowledge of animal 
physiology, we must confess our ignorance of the cause 
of these phenomena, which seem never to have been suf- 
ficiently adverted to by general speculators on the na- 
“ ture of animal heat. We may conjecture, indeed, either 
that they are owing to some peculiar and varying attrac- 
tion for caloric inherent in the fluids which compose the 
animal, and which in the egg state, like spirit of wine, 
resist our utmost producible artificial cold; or that, as 
John Hunter seems to infer with respect to a similar fa- 
culty in a minor degree in the hen’s egg, the whole are 
to be referred to some unknown power of vitality. The 
Jatter seems the most probable supposition; for Spallan- 
zani found that the blood of marmots, which remains 
fluid when they are exposed to a cold several degrees 
below zero of Fahrenheit, freezes at a much higher tem- 
perature when drawn from the animal*; and it is rea- 
sonable to conjecture that the same result would follow 
if the fluids filling the eggs of insects were collected se- 
parately, and then exposed to severe cold. 
Spring is, of course, the period when insects shake off 
the four or five months’ sleep which has sweetly banish- 
ed winter from their calendar, quit their dormitories, 
and again enter the active scenes of life. It is impossi- 
ble to deny that the increased temperature of this season 
is the immediate cause of their reappearance ; for they 
leave their retreats much earlier in forward than in back~ 
@ Rapports de P Air, §e. i. 215. 
