528 HYBERNATION OF INSECTS. 
imago state, after being four times in succession exposed to a cold of 40° 
below zero, and four times revivified by being brought into the warm at. 
mosphere of the cabin, Indeed, the circumstance that animals of a much 
more complex organisation than insects, namely serpents and fishes, have 
been known to revive after being frozen, is sufficient to dispel any doubts 
on this head. John Hunter, though himself unsuccessful in his attempts 
to reanimate carp and other animals that had been frozen, confesses 
that the fact itself is so well authenticated as to admit of no question." 
On what principle a faculty so extraordinary and so contrary to our com- 
mon conceptions of the nature of animal life depends, I'shall not attempt 
to explain. Nor can anything very satisfactory be advanced with regard 
to the source of the power which many insects in some states, and almost 
all in the egg state, have of resisting intense degrees of cold without be- 
coming frozen. It is clear that the usual explanation of the same faculty 
to a less degree in the warm-blooded animals —the constant production 
of animal heat from the caloric set free in the decomposition of the re- 
spired air — will not avail us here. For many large larvee, as Reaumur 
has observed, are destroyed by a less degree of cold than smaller species 
whose respiratory organisation is necessarily on a much less extensive 
scale; and the eggs of insects, in which, though they probably are in some 
degree acted upon by the oxygen of the atmosphere, nothing like respira- 
tion takes place, can endure a much greater intensity of cold than either 
the larvae or pup produced from them. 
Nor can we refer the effect in question to the thinness or thickness — 
the greater or less non-conducting power—of the skin of the animal. 
Reaumur found that the subterranean pupz of many moths perished with 
a cold of 7° or 8° R. below zero (14° F.), while ‘the exposed pupx of 
Pontia Brassice and other species endured 15° or 16° without injury ?; 
(a proof, by the way, that the different economy of these insects, as to 
their choice of a situation in their state of pup, is regulated by their 
power of resisting cold); but no difference in the substance of the exterior 
skin is perceptible. And the eggs of insects have usually thinner skins 
than pup, and yet they are unaffected by a degree of cold much su- 
perior. 
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 sufficiently adverted to by general speculators on the 
nature of animal heat, We may conjecture, indeed, either that they are 
owing to some peculiar and varying attraction 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 faculty in a minor degree in the 
hen’s egg, the whole are to be referred to some unknown power of vitality. 
The latter seems the most probable supposition ; for Spallanzani found 
that the blood of marmots, which remains fluid when they are exposed to 
acold several degrees below zero of Fahrenheit, freezes at » much higher 
temperature when drawn from the animal’; and it is reasonable to cone 
1 Observations on the Animal Economy, 99. 
2 Reaum. ii. 146—, 
5 Rapports del Air, &e, ii, 216, 
