HYBERNATION OF INSECTS. 457 
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 the organization 
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 satisfactory if 
the torpidity of these larvee were uninterrupted up to the 
‘very period at which they quit their nest. But facts 
do not warrant any such supposition. You have seen? 
than the temperature of a mild day even in winter awaken 
many insects from their torpidity, though without indu- 
cing them to leave their hybernacula; and it is therefore 
highly improbable that the larvee of B. chrysorrhea should 
not often have their torpid state relaxed during the 
month of March, when we have almost constantly occa- 
sional bright days elevating the thermometer to above 
50°. Yetas they still do not, like the larve of P. Cinaxia, 
leave their nest, it seems obvious that something more 
than the sensation of heat is the regulator of the move- 
ments of each. Not, however, to detain you here unne- 
cessarily, I shall not enlarge at present on this point, but 
shall pass on, in concluding this letter, to advert to the 
causes which have been assigned for the hybernation and 
torpidity of animals, and to state my own ideas on the 
subject, which will equally apply to the termination of 
this condition in spring. 
@ See above, 443-6. 
