HYBERNATION OF INSECTS. 451 



add, not always with a correct result,) fifty-eight coleo- 

 pterous species: while in the last untoward spring (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 plau- 

 sibly 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 v/ere the sole cause of insects ceasing to hyber- 

 nate, then we might fairly infer, that species of apparently 

 similar organization, and placed in similar circumstances, 

 would leave their winter quarters at the same time. This, 

 however, is for from being the case. Reaumur observed 

 that the larvae of Melitcea Cinxia quitted their nest a 

 full month sooner than those o^ Arctia clirysorrhea^. The 

 reason is obvious ; but cannot be referred to mere sen- 

 sation. 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 sus- 

 ceptible of explanation by supposing that tl^e organization 

 of these two species of larva, though apparently similar, 



"^ Reauni. ii. 170. 

 2 G 2 



