HYBERNATION OF INSECTS. 



367 



Reaumur has observed, are destroyed by a less degree of cold 

 than smaller species whose respiratory organisation is neces- 

 sarily 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 respir- 

 ation 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 Bras- 

 sices and other species endured 15° or 16° without injury 1 ; 

 (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 in- 

 herent in the fluids which compose the animal, and which in 

 the egg state, like spirit of wine, resist our utmost pro- 

 ducible artificial cold ; or that, as J ohn Hunter seems to infer, 

 with respect to a similar faculty in a minor degree in the 

 lien's egg, the whole are to be referred to some unknown 

 power of vitality. The latter seems the most probable sup- 

 position; 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 2 ; and it is reason- 

 able to conjecture that the same result would follow if the 



1 Reaum. ii. 146 — . 



2 Rapports de VAir, &c. ii. 215. 



