HYBERNATION OF INSECTS. 563 



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 

 impossible to deny that the increased temperature of this season is the 

 immediate 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 

 practically 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 30tli, 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 '20ih of April ; and 

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



Tliere 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 

 increased 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 fiiirly infer, that species of apparently 

 similar organization, 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 Melitaa Cinxia quitted 

 their nest a full month sooner than those of Porthesia chrysorrheM.^ 

 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 organization of these two species of larva, though appa- 

 rently 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 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 improbable 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 larvae of 

 M. Cinxia, leave their nest, it seems obvious that something more than 

 the sensation of heat is the regulator of the movements of each. Not, 

 however, to detain you here unnecessarily, I shall not enlarge on this 



' Reaum. ii. 170. 



