But it may be objected, that it is perhaps not so much the precise degree of cold prevailing on the day when insects select their hybernacula, that regulates their movements, as the lower degree which may have obtained for a few nights previously, and which may act upon their delicate organization so as to influence their future proceedings. Facts, however, are again in direct opposition to the explanation; for I find that, for a week previously to the 14th of October 1816, the thermometer was never lower at night than 48°, while in the first week of August it was twice as low as 46°, and never higher than 50°.[759]
As a last resource, the advocates of the doctrine I am opposing, may urge, that possibly insects may even have their sensations affected by the cold some days before it comes on, in the same way as we know that spiders and some other animals are influenced by changes of weather previously to their actual occurrence. But once more I refer to my meteorological journal; and I find that the average lowest height of the thermometer, in the week comprising the latter end of October and beginning of November 1816, was 431⁄7°; while in the week comprising the same days of the month of the end of August and beginning of September it was only 445⁄7°—a difference surely too inconsiderable to build a theory upon.
I have entered into this tedious detail, because it is of importance to the spirit of true philosophizing to show what little agreement there often is between facts and many of the hypotheses, which authors of the present day are, from their determination to explain every thing, led to promulgate. But in truth there was no absolute need for imposing this fatigue upon your attention; for the single notorious consideration that in this climate, as well as in more southern ones, we not unfrequently have sharp night-frosts in summer, and colder weather at that season than in the latter end of autumn and beginning of winter, and yet that insects do hybernate at the latter period, but do not at the former, is an ample refutation of the notion that mere cold is the cause of the phenomenon. If, indeed, the hybernacula of insects were simply the underside of any dead leaf, clod, or stone, that chanced to be in the neighbourhood of their abode, it might still be contended, that such situations were always resorted to by them on the occurrence of a certain degree of cold, but that they remained in them only when its continuance had induced torpidity: and it seems to have been in this view that most reasoners on this subject have regarded the hybernation of the larger animals, to which they have exclusively directed their attention. But had they been acquainted (as surely the investigators of such a question ought to have been) with the economy of the class of insects, in which not merely a few species, as among quadrupeds, but ninety-nine hundredths of the whole, in our climates, hybernate, they would have known that their hybernacula are in general totally distinct from their ordinary retreats in casual cold weather; and that many of them even fabricate habitations requiring considerable time and labour, expressly for the purpose of their winter residence—which last fact in particular, on their theory, admits of no satisfactory explanation. We may say, and truly, that the sensation of fatigue causes man to lie down and sleep; but we should laugh at any one who contended that this sensation forced him first to make a four-post bedstead to repose upon.
In the second place, if we grant for a moment that it is cold which drives insects to their hybernacula, there are other phenomena attending the state of hybernation which on this supposition are inexplicable. If cold led insects to enter their winter quarters, then they ought to be led by the cessation of cold to quit them. But, as has been before observed, we have often days in winter milder than at the period of hybernating, and in which insects are so roused from their torpidity as to run about nimbly when molested in their retreats; yet though their irritability must have been increased by a two or three months inactivity and abstinence, they do not leave them, but quietly remain until a fresh accession of cold again induces insensibility.
In short, to refer the hybernation of insects to the mere direct influence of cold, is to suppose one of the most important acts of their existence given up to the blind guidance of feelings which in the variable climates of Europe would be leading them into perpetual and fatal errors—which in spring would be inducing them to quit their ordinary occupations, and prepare retreats and habitations for winter to be quitted again as soon as a few fine days had dispelled the frosty feel of a May week; and in a mild winter's day, when the thermometer, as is often the case, rises to 50° or 55°, would lure them to an exposure that must destroy them. It is not, we may rest assured, to such a deceptious guide that the Creator has intrusted the safety of so important a part of his creatures: their destinies are regulated by feelings far less liable to err.
What, you will ask, is this regulator? I answer Instinct—that faculty to which so many other of the equally surprising actions of insects are to be referred; and which alone can adequately account for the phenomena to be explained. Why, indeed, should we think it necessary to go further? We are content to refer to instinct, the retirement of insects into the earth previously to becoming pupæ, and the cocoons which they then fabricate; and why should we not attribute to the same energy, their retreat into appropriate hybernacula, and the construction by many species of habitations expressly destined for their winter residence! The cases are exactly analogous; and the insect knows no more that its hybernaculum is to protect it from too severe a degree of cold during winter, than does the full-fed caterpillar when it enters the earth that it shall emerge a glorious butterfly.
I am, &c.