Towards the close of autumn the whole insect world, particularly the tribe of beetles, is in motion. A general migration takes place: the various species quit their usual haunts, and betake themselves in search of secure hybernacula. Different species, however, do not select precisely the same time for making this change of abode. Thus many lady-birds, field-bugs, and flies, are found out of their winter quarters even after the commencement of frost; while others, as Schmid has remarked, make good their retreat long before any severe cold has been felt: in fact, I am led to believe, from my own observations, that this is the case with the majority of coleopterous insects; and that the days which they select for retiring to their hybernacula, are some of the warmest days of autumn, when they may be seen in great numbers alighting on walls, rails, path-ways, &c. and running into crevices and cracks, evidently in search of some object very different from those which ordinarily guide their movements. I have noticed this assemblage in different years, but more particularly in the last autumn (1816). Walking on the banks of the Humber on the 14th of October about noon,—the day bright, calm, and deliciously mild, Fahrenheit's thermometer 58° in the shade,—my attention was first attracted by the path-ways swarming with numerous species of rove-beetles (Staphylinus, Oxytelus, Aleochara, &c.), which kept incessantly alighting, and hurrying about in every direction. On further examination I found a similar assemblage, with the addition of multitudes of other beetles, Halticæ, Nitidulæ, Rhyncophora, Cryptophagi, &c. on every post and rail in my walk, as well as on a wall in the neighbourhood; and on removing the decaying mortar and bark, I found that some had already taken up their abode in holes, from their situation with their antennæ folded, evidently meant for winter quarters. I am not aware that any author has noticed this remarkable congregation of coleopterous insects previously to hybernating, which it is so difficult to explain on any of the received theories of torpidity, except the pious Lesser, who so expressly alludes to it, and without quoting any other authority, that he would seem to have derived the fact from his own observation[723].
The site chosen by different perfect insects for their hybernacula is very various. Some are content with insinuating themselves under any large stone, a collection of dead leaves, or the moss of the sheltered side of an old wall or bank. Others prefer for a retreat the lichen or ivy-covered interstices of the bark of old trees, the decayed bark itself, especially that near the roots, or bury themselves deep in the rotten trunk; and a very great number penetrate into the earth to the depth of several inches. The aquatic tribes, such as Dytisci, Hydrophili, &c. burrow into the mud of their pools; but some of these are occasionally met with under stones, bark, &c. In every instance the selected dormitory is admirably adapted to the constitution, mode of life, and wants of the occupant. Those insects which can bear considerable cold without injury, are careless of providing other than a slight covering; while the more tender species either enter the earth beyond the reach of frost, or prepare for themselves artificial cavities in substances such as moss and rotten wood, which conduct heat with difficulty, and defend them from an injuriously low temperature. It does not appear that any perfect insect has the faculty of fabricating for itself a winter abode similar to those formed of silk, &c. by some larvæ. Schmid, indeed, has mentioned finding Rhagium mordax and Inquisitor in such abodes, constructed, as he thought, of the inner bark of trees; but these, as Illiger has suggested, were more probably the deserted dwellings of lepidopterous larvæ, of which the beetles in question had taken possession[724].—Most insects place themselves in their hybernacula in the attitude which they ordinarily assume when at rest; but others choose a position peculiar to their winter abode. So most of the ground-beetles (Eutrechina) adhere by their claws to the under side of the stone, which serves for their retreat, their backs being next to the ground; in which posture, probably, they are most effectually protected from wet. Gyrohypnus sanguinolentus, and other rove-beetles of the same genus, coils itself up like a snake, with the head in the centre.
The majority of insects pass the winter in perfect solitude. Occasionally, however, several individuals of one species, not merely of such insects as Anchomenus prasinus, a beetle, Pyrrhocoris apterus, a bug, &c., which usually in summer also live in a sort of society, but of others which are never seen thus to associate, as Haltica oleracea, Carabus intricatus, and several Coccinellæ, &c. are found crowded together. This is perhaps often more through accident than design, as individuals of the same species are frequently met with singly; yet that it is not wholly accidental, seems proved by the fact that such assemblages are generally of the same genus and even species. Sometimes, however, insects of dissimilar genera and even orders are met with together. Schmid once in February found the rare Lomechusa strumosa torpid in an ant-hill in the midst of a conglomerated lump of ants, with which it was closely intertwined[725].
By far the greater proportion of insects pass the winter only in one or other of the several states of egg, pupa, larva, or imago, but are never found to hybernate in more than one. Some species, however, depart from this rule. Thus Aphis Rosæ, Cardui, and probably many others of the genus, hybernate both in the egg and perfect state[726]; Cinthia Cardui, Gonepteryx Rhamni, and some other species, usually in the pupa, but often in the perfect state also; and Vanessa Io, according to the accurate Brahm, in the three states of egg, pupa, and imago[727]. It is probable that in these instances the perfect insects are females, which, not having been impregnated, have their term of life prolonged beyond the ordinary period.
The first cold weather, after insects have entered their winter quarters, produces effects upon them similar to those which occur in the dormouse, hedgehog, and others of the larger animals subject to torpor. At first a partial benumbment takes place; but the insect if touched is still capable of moving its organs. But as the cold increases all the animal functions cease. The insect breathes no longer, and has no need of a supply of air[728]; its nutritive secretions cease, and no more food is required; the muscles lose their irritability[729]; and it has all the external symptoms of death. In this state it continues during the existence of great cold, but the degree of its torpidity varies with the temperature of the atmosphere. The recurrence of a mild day, such as we sometimes have in winter, infuses a partial animation into the stiffened animal: if disturbed, its limbs and antennæ resume their power of extension, and even the faculty of spirting out their defensive fluid is re-acquired by many beetles[730]. But however mild the atmosphere in winter, the great bulk of hybernating insects, as if conscious of the deceptious nature of their pleasurable feelings, and that no food could then be procured, never quit their quarters, but quietly wait for a renewal of their insensibility by a fresh accession of cold.
On this head I have had an opportunity of making some observations which, in the paucity of recorded facts on the hybernation of insects, you may not be sorry to have laid before you. The 2nd of December 1816 was even finer than many of the preceding days of the season, which so happily falsified the predictions that the unprecedented dismal summer would be followed by a severe winter. The thermometer was 46° in the shade; not a breath of air was stirring; and a bright sun imparted animation to troops of the winter gnat (Trichocera hiemalis), which frisked under every bush; to numerous Psychodæ; and even to the flesh-fly, of which two or three individuals buzzed past me while digging in my garden. Yet though these insects, which I shall shortly advert to as exceptions to the general rule, were thus active, the heat was not sufficient to induce their hybernating brethren to quit their retreats. Removing some of the dead bark of an old apple-tree, I soon discovered several insects in their winter quarters. Of the little beetle Lebia quadrinotata, Duftschmid Faun. Austr. (Carabus punctomaculatus, Ent. Brit.), I found six or eight individuals, and all so lively, that though remaining perfectly quiet in their abode until disturbed, they ran about with their ordinary activity as soon as the covering of bark was displaced. The same was the case with a colony of earwigs. Two or three individuals of Lebia quadrimaculata showed more torpidity. When first uncovered, their antennæ were laid back; and it was only after the sun had shone some seconds upon them that they exhibited symptoms of animation, and after stretching out these organs began to walk. Close by them lay a single weevil (Anthonomus Pomorum), but in so deep a sleep that at first I thought it dead. It gave no sign of life when placed on my hand, quite hot with the exercise of digging; and it was only after being kept there some seconds, and breathed upon several times, that it first slowly unfolded its rostrum, and then its limbs. It deserves remark, that all these insects, thus differently affected, were on the same side of the tree, under a similar covering of bark, and apparently equally exposed to the sun, which shone full upon the covering of their retreat[731].
All insects, however, do not undergo this degree of torpidity. In fact, there are some, though but few, which cannot, at least in our climate, strictly be said to hybernate, understanding by that term passing the winter in one selected situation in a greater or less degree of torpor, without food. Not to mention Cheimatobia brumata, and some other moths, which are disclosed from the pupæ in the middle of winter, and can therefore be scarcely regarded as exceptions to the rule, some insects are torpid only in very severe weather, and on fine mild days in winter come out to eat. This is the case with the larva of Euprepia fuliginosa[732]; and Lyonet asserts that there are many other caterpillars which eat and grow even in the midst of slight frost[733]. Amongst perfect insects, troops of Trichocera hiemalis, the gnat whose choral dances have been before described[734], may be constantly seen gamboling in the air in the depth of winter when it is mild and calm, accompanied by the little Psychoda, so common in windows, several Muscidæ, spiders, and occasionally some Aphodii and Staphylinidæ: and the societies of ants, as well as their attendant Aphides, are in motion and take more or less food during the whole of that season when the cold is not intense. The younger Huber informs us that ants become torpid only at 2° Reaum. below freezing (27° Fahrenheit), and apparently endeavour to preserve themselves from the cold, when its approach is gradual, by clustering together. When the temperature is above this point they follow their ordinary habits (he has seen them even walk upon the snow), and can then obtain the little food which they require in winter from their cows the Aphides, which, by an admirable provision, become lethargic at precisely the same degree of cold as the ants, and awake at the same period with them[735].
Lastly, there are some few insects which do not seem ever to be torpid, as Podura nivalis, L., and the singular apterous insect recently described by Dalman, Chionea araneoïdes[736], both of which run with agility on the snow itself; and the common hive-bee; though with regard to the precise state in which this last passes the winter, this part of its economy has not been made the subject of such accurate investigation as is desirable.
Many authors have conceived that it is the most natural state of bees in winter to be perfectly torpid at a certain degree of cold, and that their partial reviviscency, and consequent need of food in our climate, are owing to its variableness and often comparative mildness in winter; whence they have advised placing bees during this season in an ice-house, or on the north side of a wall, where the degree of cold being more uniform, and thus their torpidity undisturbed, they imagine no food would be required. So far, however, do these suppositions and conclusions seem from being warranted, that Huber expressly affirms that, instead of being torpid in winter, the heat in a well-peopled hive continues +24° or 25° of Reaumur (86° Fahrenheit), when it is several degrees below zero in the open air; that they then cluster together and keep themselves in motion in order to preserve their heat[737]; and that in the depth of winter they do not cease to ventilate the hive by the singular process of agitating their wings before described[738]. He asserts also that, like Reaumur, he has in winter found in the combs brood of all ages; which, too, the observant Bonnet says he has witnessed[739]; and which is confirmed by Swammerdam, who expressly states that bees tend and feed their young even in the midst of winter[740]. To all these weighty authorities may be added that of John Hunter, who, as before noticed, found a hive to grow lighter in a cold than in a warm week of winter; and that a hive from November 10th to February 9th lost more than four pounds in weight[741]; whence the conclusion seems inevitable, that bees do eat in winter.