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 entomo.ogical journal, that I had taken 
and investigated (1 scarcely need add, not always with a correct result) 
filty-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 Melitea Cinwia quitted their nest a full 
month sooner than those of Porthesia Chrysorrhea.’ The reason is obyious, 
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. 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 J. Cinwia, leave their nest, it seems 
1 Reaum. ii, 170. 
MT Mf 
