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. 1 



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 larvae, 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 larva? or pupae 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 pupa? of many moths perished with 

 a cold of 7 or 8 R. below zero (14 F.), while the exposed pupae of 

 Pontia BrassiccB and other species endured 15 or 16 without injury 2 ; 

 (a proof, by the way, that the different economy of these insects, as to 

 their choice of a situation in their state of pupae, 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 pupa?, 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 

 a cold several degrees below zero of Fahrenheit, freezes at a much higher 

 temperature when drawn from the animal s j and it is reasonable to con- 



1 Observations on the Animal Economy, 99. 



* Reaum. ii. 146. 



5 Rapports de I' Air, &c. ii. 215. 



