In the present state, then, of our knowledge of animal physiology, we must confess our ignorance of the cause of these phenomena, which seem never to have been sufficiently adverted to by general speculators on the nature of animal heat. We may conjecture, indeed, either that they are owing to some peculiar and varying attraction for caloric inherent in the fluids which compose the animal, and which in the egg state, like spirit of wine, resist our utmost producible artificial cold; or that, as John Hunter seems to infer with respect to a similar faculty in a minor degree in the hen's egg, the whole are to be referred to some unknown power of vitality. The latter seems the most probable supposition; for Spallanzani found that the blood of marmots, which remains fluid when they are exposed to a cold several degrees below zero of Fahrenheit, freezes at a much higher temperature when drawn from the animal[753]; and it is reasonable to conjecture 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 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 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 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 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 far from being the case. Reaumur observed that the larvæ of Melitæa Cinxia quitted their nest a full month sooner than those of Arctia chrysorrhea[754]. 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 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 larvæ were uninterrupted up to the very period at which they quit their nest. But facts do not warrant any such supposition. You have seen[755] 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 larvæ of A. 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 larvæ 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 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.
The authors who have treated on these phenomena have generally[756] referred them to the operation of cold upon the animals in which they are witnessed, but acting in a different manner. Some conceive that cold combined with a degree of fatness arising from abundance of food in autumn, produces in them an agreeable sensation of drowsiness, such as we know, from the experience of Sir Joseph Banks and Dr. Solander in Terra del Fuego, as well as from other facts, is felt by man when exposed to a very low temperature; yielding to which, torpidity ensues. Others, admitting that cold is the cause of torpidity, maintain that the sensations which precede it are of a painful nature; and that the retreats in which hybernating animals pass the winter are selected in consequence of their endeavours to escape from the disagreeable influence of cold.
I have before had occasion to remark[757] the inconclusiveness of many of the physiological speculations of very eminent philosophers, arising from their ignorance of Entomology, which observation forcibly applies in the present instance. The reasoners upon torpidity have almost all confined their view to the hybernating quadrupeds, as the marmot, dormouse, &c. and have thus lost sight of the far more extensive series of facts supplied by hybernating insects, which would often at once have set aside their most confidently-asserted hypotheses. If those who adopt the former of the opinions above alluded to, had been aware that numerous insects retire to their hybernacula (as has been before observed) on some of the finest days at the close of autumn, they could never have contended that this movement, in which insects display extraordinary activity, is caused by the agreeable drowsiness consequent on severe cold; and the very same fact is equally conclusive against the theory, that it is to escape the pain arising from a low temperature that insects bury themselves in their winter quarters.
In fact, the great source of the confused and unsatisfactory reasoning which has obtained on this subject, is, that no author, as far as my knowledge extends, has kept steadily in view, or indeed has distinctly perceived, the difference between torpidity and hybernation; or, in other words, between the state in which animals pass the winter, and their selection of a situation in which they may become subject to that state.
That the torpidity of insects, as well as of other hybernating animals, is caused by cold, is unquestionable. However early the period at which a beetle, for example, takes up its winter quarters, it does not suffer that cessation of the powers of active life which we understand by torpidity, until a certain degree of cold has been experienced; the degree of its torpidity varies with the variations of temperature; and there can be no doubt that, if it were kept during winter from the influence of cold, it would not become torpid at all—at least this has proved the fact with marmots and dormice thus treated; and the Aphis of the rose (A. Rosæ), which becomes torpid in winter in the open air[758], retains its activity and gives birth to a numerous progeny upon rose trees preserved in greenhouses and warm apartments.
But, can we, in the same way, regard mere cold as the cause of the hybernation of insects? Is it wholly owing to this agent, as most writers seem to think—to feelings either of a pleasurable or painful nature produced by it—that previously to becoming torpid they select or fabricate commodious retreats precisely adapted to the constitution and wants of different species, in which they quietly wait the accession of torpidity and pass the winter? In my opinion, certainly not.
In the first place, if sensations proceeding from cold lead insects to select retreats for hybernating, how comes it that, as above shown, a large proportion of them enter these retreats before any severe cold has been felt, and on days considerably warmer than many that preceded them? If this supposition have any meaning, it must imply that insects are so constituted that, when a certain degree of cold has been felt by them, the sensations which this feeling excites impel them to seek out hybernacula. Now the thermometer in the shade on the 14th of October 1816, when I observed vast numbers thus employed, was at 58°:—this then, on the theory in question, is a temperature sufficiently low to induce the requisite sensations. But it so happens, as I learn from my meteorological journal (which registers the greatest and least daily temperature as indicated by a Six's thermometer), that on the 31st of August 1816 the greatest heat was not more than 52°, or six degrees lower than on the 14th of October: yet it was six weeks later that insects retired for the winter!