HYBERNATION OP INSECTS. 



369 



obvious ; but cannot be referred to mere sensation. 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 appear- 

 ance ; 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 

 organisation of these two species of larva, though apparently 

 similar, is yet in fact different, that of the one being consti- 

 tuted so as to be acted upon by a less degree of heat than 

 that of the other; and this solution would be satisfactory if 

 the torpidity of these larvae were uninterrupted up to the 

 very period at which they quit their nest. But facts do not 

 warrant any such supposition. You have seen that the 

 temperature of a mild day, even in winter, awakens many 

 insects from their torpidity, though without inducing them 

 to leave their hybernacula ; and it is therefore highly impro- 

 bable that the larvse of P. chrysorrhea should not often have 

 their torpid state relaxed during the month of March, when 

 we have almost constantly occasional bright days elevating 

 the thermometer to above 50°. Yet as they still do not, like 

 the larva? of M. Cinxia, leave their nest, it seems obvious 

 that something more than the sensation of heat is the regu- 

 lator of the movements of each. Not, however, to detain 

 you here unnecessarily, I shall not enlarge on this point, but 

 shall pass on, in concluding this letter, to advert to the causes 

 which have been assigned for the hybernation and torpidity 

 of animals, 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 1 referred them to the operation of cold upon the 

 animals in which they are witnessed, but acting in a different 

 manner. Some conceive that cold, combined with a degree 

 of fatness arising from abundance of food in autumn, pro- 

 duces in them an agreeable sensation of drowsiness, such as 



1 Here must be excepted my lamented friend the late Dr. Reeve of Norwich, 

 who, in his ingenious Essay on the Torpidity of Animals, has come to nearly the 

 same conclusion as is adopted in this letter ; but, by omitting to make a dis- 

 tinction between torpidity and hybernation, he has nA done justice to his own 

 ideas. 



VOL. II. 



B B 



