368 



HYBERNATION OF INSECTS. 



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 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 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 ob- 

 serve even a bee abroad until the 20th of April ; and the first 

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



There are, however, circumstances connected with this re- 

 appearance, 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 percep- 

 tion of warmth were the sole cause of insects ceasing to hy- 

 bernate, 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 



1 Reaum. ii. 170. 



