HYBERNATION OF INSECTS. 529 



jecture 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 off the four or five 

 months' sleep which has sweetly banished winter from their calendar, quit 

 their dormitories, and again enter the active scenes of life. It is im- 

 possible to deny that the increased temperature of this season is the im- 

 mediate cause of their reappearance ; for they leave their retreats much 

 earlier in forward than in backward springs. Thus in the early spring of 

 1805 (to me a memorable one, since in it I began my entomological career, 

 and had anxiously watched its first approaches in order to study practi- 

 cally the science of which I had gained some theoretical knowledge in the 

 winter) insects were generally out by the middle of March ; and before 

 the 30th, I find, on referring to my entomological journal, that I had taken 

 and investigated (I scarcely need add, not always with a correct result) 

 fifty-eight coleopterous species; while in the untoward spring of 1816 

 I did not observe even a bee abroad until the 20th of April ; and the first 

 butterfly that I saw did not appear until the 26th. 



There are, however, circumstances connected with this reappearance, 

 which seem to prove that something more than the mere sensation of 

 warmth is concerned in causing it. I shall not insist upon the remarkable 

 fact which Spallanzani has noticed, that insects reappear in spring at a 

 temperature considerably lower than that at which they retired in autumn ; 

 because it may be plausibly enough explained by reference to their in- 

 creased irritability in spring, the result of so long an abstinence from food, 

 and their consequent augmented sensibility to the stimulus of heat. But 

 if the mere perception of warmth were the sole cause of insects ceasing to 

 hybernate, then we might fairly infer, that species of apparently similar 

 organisation, and placed in similar circumstances, would leave their winter 

 quarters at the same time. This, however, is far from being the case. 

 Reaumur observed that the larvae of Melitcea Cinxia quitted their nest a full 

 month sooner than those of Porthesia Chrysorrhea. 1 The reason is 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 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 organisa- 

 tion of these two species of larva, though apparently similar, is yet in fact 

 different, that of the one being constituted so as to be acted upon by a less 

 degree of heat than that of the other ; and this solution would be satis- 

 factory 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 in- 

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

 bable that the larvae of P. Chrysoirhea 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 larvae of M. Cinxia^ leave their nest, it seems 



' Keaum. ii. 170. 

 M M 



