HYBEllNATIOX OF INSECTS. 457 



sooner thun those of Bomhi/x chri/sorrhca'^. The rea- 

 son is obvious ; but cannot be referred to mere sensa- 

 tion. The former live on grass, and on the leaves of 

 plantain, which they can meet with at the beginning of 

 March — the period of their appearance : the latter 

 eat only the leaves of trees which expand a month 

 later. It might, indeed, be still contended, that this 

 fact is susceptible of explanation by supposing that the 

 organization of these two species of larva, though ai> 

 parently similar, is yet in fact ditfereut, that of the 

 one being constituted so as to be acted upon by a Je^s 

 degree of heat than that of tise other. But without here 

 entering upon the reasons which seem to me to make 

 this explanation unsatisfactory, I shall pass on, in con- 

 cluding this letter, to advert to the causes which have 

 been assigned for the hybernation and torpidity of ani- 

 mals, and to state my own ideas on the subject, which 

 will equally apply to the termination of this condition 

 in spring. 



The authors who have treated on these phenomena 

 have generally'' referred them to the operation of cold 

 npon the animals in which tliey are witnessed, but act- 

 ing in a diiferent manner. Some conceive that cold, 

 combined with a degree of fatness arising from abund- 

 ance of food in autumn, produces in them an agreeable 

 sensation of drowsiness, such as we know, from the ex- 

 perience of Sir Joseph Banks and Dr. Solander in Terra 



« Rcaum. ii. 170, 



* Here must be excepted my lamented friend the late Dr. Reeve of 

 Norwich, who, in Ills ingeuious Essay on the Tvrpidili/ of ylnimnls, lias 

 come to nearly the same conclusion as is adopted in this letter ; but, by 

 omitting to make a distinction between torpidity and hybernation, he 

 ha Hot donr ju.tite to his own ideas. 



