HYBERNATION OF INSECTS. 455 



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 ani- 

 mal 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, in- 

 deed, either that they are owing to sonie 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 arti- 

 ficial 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 un- 

 known 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 Fahren- 

 heit, freezes at a much higher temperature when drawn 

 from the animal^; and It is reasonable 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. 



Spring is, of course, the period when insects shake 

 oft* the four or five months' deep sleep which has sweetly 

 bfinished winter from their calendar, quit their dormi- 

 tories, and again enter the active scenes of life. It is 

 impossible to deny that the increased temperature of 

 this season is the immediate cause of their revival ; for 



^ Rapports dc l' Air, S)C. ii. 215. 



