562 HYBERNATION OF INSECTS. 



the imago state, after being four limes in succession exposed to a cold of 

 40° below zero, and four times revivified by being brought into the warm 

 atmosphere of the cabin. Indeed, the circumstance that animals of a 

 much more complex organization 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 

 common conceptions of the nature of animal life depends, I shall not 

 attempt to explain. Nor can any thing 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 with- 

 out becoming frozen. It is clear that the usual explanation of the same 

 faculty to a less degree in the warm-blooded animals — the constant pro- 

 duction of animal heat from the caloric set free in the decomposition of 

 the respired 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 organization is necessarily on a much less exten- 

 sive 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 

 respiration takes place, can endure a much greater intensity of cold than 

 either the larvae 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 pupae 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"; 

 (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 

 pupae, 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 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 inhe- 

 rent 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"'; and it is rea- 

 sonable to conjecture that the same result would follow if the fluids filling 

 the eggs of insects were collected, separately, and then exposed to severe cold. 



* Observations on the Animal Economy, 99. 



' Keaum. ii. 146 — . ^ jtapports de I'Air, fee. ii. 215. 



