LETTER XXVI.

_ON THE HYBERNATION AND TORPIDITY OF INSECTS._

If insects can boast of enjoying a greater variety of food than many other tribes of animals, this advantage seems at first sight more than counterbalanced in our climates, by the temporary nature of their supply. The graminivorous quadrupeds, with few exceptions, however scanty their bill of fare, and their carnivorous brethren, as well as the whole race of birds and fishes, can at all seasons satisfy, in greater or less abundance, their demand for food. But to the great majority of insects, the earth for nearly one half of the year is a barren desert, affording no appropriate nutriment. As soon as winter has stripped the vegetable world of its foliage, the vast hosts of insects that feed on the leaves of plants must necessarily fast until the return of spring: and even the carnivorous tribes, such as the predaceous beetles, parasitic _Hymenoptera_, _Sphecina_, &c. would at that period of the year in vain look for their accustomed prey.

How is this difficulty provided for? In what mode has the Universal Parent secured an uninterrupted succession of generations in a class of animals for the most part doomed to a six months' deprivation of the food which they ordinarily devour with such voracity? By a beautiful series of provisions founded on the faculty, common also to some of the larger animals, of passing the winter in a state of torpor--by ordaining that the insect shall live through that period, either in an incomplete state of its existence when its organs of nutrition are undeveloped, or, if the active epoch of its life has commenced, that it shall seek out appropriate _hybernacula_, or winter quarters, and in them fall into a profound sleep, during which a supply of food is equally unnecessary.

In two of the four states of existence common to insects, in which different tribes pass the winter, namely, the egg and the pupa state, the organs for taking food (except in some cases in the latter) are not developed, and consequently the animal is incapable of eating. The existence of insects in these states during the winter, differs from their existence in the same form in summer only in the greater length of its term. In both seasons food is alike unnecessary, so that their hybernation in these circumstances has little or nothing analogous to that of larger animals. With this, however, strictly accords their hybernation in the larva and imago states, in which their abstinence from food is solely owing to the torpor that pervades them, and the consequent non-expenditure of the vital powers.--I shall attend to the peculiarities of their hybernation in each of these states in the order just laid down; premising that we have yet much to learn on this subject, no observations having been instituted respecting the state in which multitudes of insects pass the winter.

It is probable that some insects of almost every order hybernate in the _egg_ state: though that these must be comparatively few in number, seems proved from two considerations: first, That the majority of insects assume the imago, and deposit their eggs in the summer and early part of autumn, when the heat suffices to hatch them in a short period: and secondly, That the eggs of a very large proportion of insects require for their due exclusion and the nutriment of the larvæ springing from them, conditions only to be fulfilled in summer, as all those which are laid in young fruits and seeds; in the interior and galls of leaves; in insects that exist only in summer, &c. &c. The insects which pass the winter in the egg state are chiefly such as have several broods in the course of the year, the females of the last of which lay eggs that, requiring more heat for their development than then exists, necessarily remain dormant until the return of spring.

The situation in which the female insect places her eggs in order to their remaining there through the winter, is always admirably adapted to the degree of cold which they are capable of sustaining; and to the ensuring a due supply of food for the nascent larvæ. Thus, with the former view, _Acrida verrucivora_ and many other insects whose eggs are of a tender consistence, deposit them deep in the earth out of the reach of frost; and with the latter, _Trichoda Neustria_, _Lasiocampa castrensis_, _Hypogymna dispar_, and some other moths, departing from the ordinary instinct of their congeners, which teaches them to place their eggs upon the _leaves_ of plants, fix theirs to the stem and branches only. That this variation of procedure has reference to the hybernation of the eggs of these particular species, is abundantly obvious. Insects whose eggs are to be hatched in summer, usually fix them slightly to the leaves upon which the larvæ are to feed. But it is evident that, were this plan to be adopted by those whose eggs remain through the winter, their progeny might be blown away along with the leaf to which they are attached, far from their destined food. These, therefore, choose a more stable support, and carefully fasten them, as has just been observed, either to the trunk or branches of the tree, whose young leaves in spring are to be the food of the excluded larvæ. The latter plan is followed by the female of _Trichoda Neustria_, which curiously gums her eggs in bracelets round the twigs of the hawthorn, &c. But another provision is demanded. Were these eggs of the usual delicate consistence, and to be attached with the ordinary slight gluten, they would have a poor chance of surviving the storms of rain and snow and hail to which for six or eight months they are exposed. They are therefore covered with a shell much more hard and thick than common; packed as closely as possible to each other; and the interstices are filled up with a tenacious gum, which soon hardens the whole into a solid mass almost capable of resisting a penknife. Thus secured, they defy the elements and brave the blasts of winter uninjured.--The female of _Hypogymna dispar_, whose eggs have a more tender shell, glues them in an oval mass to the stem of a tree (whence the German gardeners call the larvæ _Stamm-raupe_), and then covers them with a warm non-conducting coat of hairs plucked from her own body, equally impervious to cold and wet.

Another of those beautiful relations between objects at first sight apparently unconnected, which at every step reward the votaries of Entomology, is afforded by the coincidence between the period of the hatching in spring of eggs deposited before winter, and of the leafing of the trees upon which they have been fixed, and on whose foliage the larvæ are to feed: which two events, requiring exactly the same temperature, are always simultaneous. Of this fact I have had a striking exemplification the last spring (1816). On the 20th of February, observing the twigs of the birches in the Hull Botanic Garden to be thickly set, especially about the buds, with minute oval black eggs of some insect with which I was unacquainted, I brought home a small branch and set it in a jar of water in my study, in which is a fire daily, to watch their exclusion. On the 28th of March I observed that a numerous brood of Aphides (not _A. Betulæ_, as the wings were without the dark bands of that species) had been hatched from them, and that two or three of the lower buds had expanded into leaves, upon the sap of which they were greedily feasting. This was full a month before either a leaf of the birch appeared, or the egg of an Aphis was disclosed in the open air.--To view the relation of which I am speaking with due admiration, you must bear in mind the extremely different periods at which many trees acquire their leaves, and the consequent difference demanded in the constitution of the eggs which hybernate upon dissimilar species, to ensure their exclusion, though acted upon by the _same_ temperature, earlier or later, according to the early or late foliation of these species. There is no visible difference between the conformation of the eggs of the Aphis of the birch and those of the Aphis of the ash; yet in the _same_ exposure those of the former shall be hatched, simultaneously with the expansion of the leaves, nearly a month earlier than those of the latter: thus demonstrably proving that the hybernation of these eggs is not accidental, but has been specially ordained by the Author of nature, who has conferred on those of each species a peculiar and appropriate organization.

A much greater number of insects pass the winter in the _pupa_ than in the egg state; probably nine-tenths of the extensive order _Lepidoptera_, many in _Hymenoptera_, and several in other orders. In placing these pupæ in security from the too great cold of winter and the attacks of enemies, the larvæ from which they are to be metamorphosed exhibit an anxiety and ingenuity evidently imparted to them for this express design. A few are suspended without any covering, though usually in a sheltered situation. But by far the larger number are concealed under leaves, in the crevices or in the trunk of trees, &c., or inclosed in cocoons of silk or other materials which will be described to you in a subsequent letter, and often buried deep under ground out of the reach of frost.--One reason why so many lepidopterous insects pass the winter as pupæ, has been plausibly assigned by Rösel, in remarking that this is the case with all the numerous species which feed on annual plants. As these have no local habitation, dying one year and springing up from seed in another quarter the next, it is obvious that eggs deposited upon them in autumn would have no chance of escaping destruction; and that even if the larvæ were to be hatched before winter, and to hybernate in that state, they would have no certainty of being in the neighbourhood of their appropriate food the next spring. By wintering in the pupa state, these accidents are effectually provided against. The perfect insect is not ready to break forth until the food of the young, which are to proceed from its eggs, is sprung up.

To the insects which hybernate in the _larva_ state, of course belong, in the first place, all those which exist under that form more than one year; as many _Melolonthæ_, _Elateres_, _Cerambyces_, _Buprestes_, and several species of _Libellula_, _Ephemera_, &c. There are also many larvæ which, though their term of life is not a year, being hatched from the egg in autumn, necessarily pass the winter in that state, as those of several _Anobia_ and other wood-boring insects; of _Semasia Wœberana_ and others of the same family; of the second broods of several butterflies, &c. Many of these residing in the ground or in the interior of trees need no other hybernacula than the holes which they constantly inhabit; some, as the aquatic larvæ, merely hide themselves in the sides or muddy bottom of their native pools; while others seek for a retreat under moss, dead leaves, stones, and the bark of decaying trees. Most of these can boast of no better winter quarters than a simple unfurnished hole or cavity; but a few, more provident of comfort, prepare themselves an artificial habitation. With this view the larva of _Cossus ligniperda_, as formerly observed in describing the habitations of insects[717], forms a covering of pieces of wood lined with fine silk; those of _Hepiolus Humuli_, _Xylina radicea_, and some other moths, excavate under a stone a cavity exactly the size of their bodies, to which they give all round a coating of silk[718], and the larvæ of _Pieris Cratægi_ inclose themselves in autumn in cases of the same material[719], and thus pass the cold season in small societies of from two to twelve, under a common covering formed of leaves. Bonnet mentions a trait of the cleanliness of these insects which is almost ludicrous. He observed in one of these nests a sort of sack containing nothing but grains of excrement; and a friend assured him that he had seen one of these caterpillars partly protrude itself out of its case, the hind feet first, to eject a similar grain; so that it would seem the society have on their establishment a scavenger, whose business it is to sweep the streets and convey the rejectamenta to one grand repository[720]! This, however singular, is rendered not improbable from the fact that beavers dig in their habitations holes solely destined for a like purpose[721].

A very considerable number of insects hybernate in the _perfect_ state, chiefly of the orders _Coleoptera_, _Hemiptera_, _Hymenoptera_, and _Diptera_, and especially of the first. _Vanessa Urticæ_, _Io_, and a few other lepidopterous species, with a small proportion of the other orders, occasionally survive the winter; but the bulk of these are rarely found to hybernate as perfect insects. Of coleopterous insects, Schmid, to whom we are indebted for some valuable remarks on the present subject[722], says that he never found or heard of any Entomologist finding a hybernating individual of the common cockchafer (_Melolontha vulgaris_) or of the stag-beetle (_Lucanus Cervus_); and suggests that it is only those insects which exist but a short period as larvæ, as most of the tribe of weevils, lady-birds, &c., that survive the winter in the perfect state; while those which live more than one year in the larva state, as the species just mentioned, are deprived of this privilege.

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.

On the other hand, Reaumur adopts (or rather, perhaps, has in great measure given birth to) the more commonly received notion, that bees in a certain degree of cold are torpid and consume no food. These are his words:--"It has been established with a wisdom which we cannot but admire,--with that wisdom with which every thing in nature has been made and ordained,--that during the greater part of the time in which the country furnishes nothing to bees, they have no longer need to eat. The cold which arrests the vegetation of plants, which deprives our fields and meadows of their flowers, throws the bees into a state in which nourishment ceases to be necessary to them: it keeps them in a sort of torpidity (_engourdissement_), in which no transpiration from them takes place; or, at least, during which the quantity of that which transpires is so inconsiderable, that it cannot be restored by aliment without their lives being endangered. In winter, while it freezes, one may observe without fear the interior of hives that are not of glass; for we may lay them on their sides, and even turn them bottom upwards, without putting any bee into motion. We see the bees crowded and closely pressed one against the other: little space then suffices for them[742]." In another place, speaking of the custom in some countries of putting bee-hives during winter into out-houses and cellars, he says that in such situations the air, though more temperate than out of doors during the greater part of winter, "is yet sufficiently cold to keep the bees in that species of torpidity which does away their need of eating[743]." And lastly, he expressly says that the milder the weather, the more risk there is of the bees consuming their honey before the spring, and dying of hunger; and confirms his assertion by an account of a striking experiment, in which a hive that he transferred during winter into his study, where the temperature was usually in the day 10° or 12° R. above freezing (59° F.), though provided with a plentiful supply of honey, that if they had been in a garden would have served them past the end of April, had consumed nearly their whole stock before the end of February[744].

Now, how are we to reconcile this contradiction?--for, if Huber be correct in asserting that in frosty weather bees agitate themselves to keep off the cold, and ventilate their hive;--if, as both he and Swammerdam state, they feed their young brood in the depth of winter--it seems impossible to admit that they ever can be in the torpid condition which Reaumur supposes, in which food, so far from being necessary, is injurious to them. In fact, Reaumur himself in another place informs us, that bees are so infinitely more sensible of cold than the generality of insects, that they perish when in numbers so small as to be unable to generate sufficient animal heat to counteract the external cold, even at 11° R. above freezing[745] (57° F.); which corresponds with what Huber has observed (as quoted above) of the high temperature of well-peopled hives, even in very severe weather. We are forced, then, to conclude that this usually most accurate of observers has in the present instance been led into error, chiefly, it is probable, from the clustering of bees in the hives in cold weather; but which, instead of being, as he conceived, an indication of torpidity, would seem to be intended, as Huber asserts, as a preservative against the benumbing effects of cold.

Bees, then, do not appear to pass the winter in a state of torpidity in our climates, and probably not in any others. Populous swarms inhabiting hives formed of the hollow trunks of trees, used in many northern regions, or of other materials that are bad conductors of heat, seem able to generate and keep up a temperature sufficient to counteract the intensest cold to which they are ordinarily exposed. At the same time, however, I think we may infer, that though bees are not strictly torpid at that lowest degree of heat which they can sustain, yet that when exposed to _that_ degree they consume considerably less food than at a higher temperature; and consequently that the plan of placing hives in a north aspect in sunny and mild winters may be adopted by the apiarist with advantage. John Hunter's experiment, indeed, cited above, in which he found that a hive grew lighter in a cold than in a warm week, seems opposed to this conclusion; but an insulated observation of this kind, which we do not know to have been instituted with a due regard to all the circumstances that required attention, must not be allowed to set aside the striking facts of a contrary description recorded by Reaumur and corroborated by the almost universal sentiment of writers on bees.--After all, however, on this point, as well as on many others connected with the winter economy of these endlessly-wonderful insects, there is evidently much yet to be observed, and many doubts which can be satisfactorily dispelled only by new experiments.

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The degree of cold which most insects in their different states, while torpid, are able to endure with impunity, is very various; and the habits of the different species, as to the situation which they select to pass the winter, are regulated by their greater or less sensibility in this respect. Many insects, though able to sustain a degree of cold sufficient to induce torpidity, would be destroyed by the freezing temperature, to avoid which they penetrate into the earth or hide themselves under non-conducting substances; and there can be little doubt that it is with this view that so many species while pupæ are thus secured from cold by cocoons of silk or other materials. Yet a very great proportion of insects in all their states are necessarily subjected to an extreme degree of cold. Many eggs and pupæ are exposed to the air without any covering; and many, both larvæ and perfect insects, are sheltered too slightly to be secure from the frost. This they are either able to resist, remaining unfrozen though exposed to the severest cold, or, which is still more surprising, are uninjured by its intensest action, recovering their vitality even after having been frozen into lumps of ice.

The eggs of insects are filled with a fluid matter, included in a skin infinitely thinner than that of hens' eggs, which John Hunter found to freeze at about 15° of Fahrenheit. Yet on exposing several of the former, including those of the silk-worm, for five hours to a freezing mixture which made Fahrenheit's thermometer fall to 38° below zero, Spallanzani found that they were not frozen, nor their fertility in the slightest degree impaired. Others were exposed even to 56° below zero, without being injured[746].

A less degree of cold suffices to freeze many pupæ and larvæ, in both which states the consistency of the animal is almost as fluid as in that of the egg. Their vitality enables them to resist it to a certain extent, and it must be considerably below the freezing point to affect them. The winter of 1813-14 was one of the severest we have had for many years, Fahrenheit's thermometer having been more than once as low as 8° when the ground was wholly free from snow; yet almost the first objects which I observed in my garden, in the commencement of spring, were numbers of the caterpillars of the gooseberry-moth (_Abraxas grossulariata_), which, though they had passed the winter with no other shelter than the slightly projecting rim of some large garden-pots, were alive and quite uninjured; and these and many other larvæ never in my recollection were so numerous and destructive as in that spring: whence, as well as from the corresponding fact recorded with surprise by Boerhaave, that insects abounded as much after the intense winter of 1709, during which Fahrenheit's thermometer fell to 0, as after the mildest season, we may see the fallacy of the popular notion, that hard winters are destructive to insects[747].

But though many larvæ and pupæ are able to resist a great degree of cold, when it increases to a certain extent they yield to its intensity and become solid masses of ice. In this state we should think it impossible that they should ever revive. That an animal whose juices, muscles, and whole body have been subjected to a process which splits bombshells, and converted into an icy mass that may be snapped asunder like a piece of glass, should ever recover its vital powers, seems at first view little less than a miracle; and, if the reviviscency of the wheel animal (_Vorticella rotatoria_), and of snails, &c. after years of desiccation, had not made us familiar with similar prodigies, might have been pronounced impossible; and it is probable that many insects when thus frozen never do revive. Of the fact, however, as to several species, there is no doubt. It was first noticed by Lister, who relates that he had found caterpillars so frozen, that when dropped into a glass they chinked like stones, which nevertheless revived[748]. Reaumur, indeed, repeated this experiment without success; and found that when the larvæ of _Lasiocampa Pityocampa_ were frozen into ice by a cold of 15° R. below zero (2° F. below zero), they could not be made to revive[749]. But other trials have fully confirmed Lister's observations. My friend Mr. Stickney, before mentioned as the author of a valuable _Essay on the Grub_ (larva of _Tipula oleracea_)--to ascertain the effect of cold in destroying this insect, exposed some of them to a severe frost, which congealed them into perfect masses of ice. When broken, their whole interior was found to be frozen. Yet several of these resumed their active powers. Bonnet had precisely the same result with the pupæ of _Pontia Brassicæ_, which, by exposing to a frost of 14° R. below zero (0° F.), became lumps of ice, and yet produced butterflies[750]. Indeed, the circumstance that animals of a much more complex organization than insects, namely, serpents and fishes, have been known to revive after being frozen, is sufficient to dispel any doubts on this head. John Hunter, though himself unsuccessful in his attempts to reanimate carp and other animals that had been frozen, confesses that the fact itself is so well authenticated as to admit of no question[751].

On what principle a faculty so extraordinary and so contrary to our common conceptions of the nature of animal life depends, I shall not attempt to explain. Nor can any thing very satisfactory be advanced with regard to the source of the power which many insects in some states, and almost all in the egg state, have of resisting intense degrees of cold without becoming frozen. It is clear that the usual explanation of the same faculty to a less degree in the warm-blooded animals--the constant production of animal heat from the caloric set free in the decomposition of the respired air--will not avail us here. For, first, the hive-bee, which has the capacity of evolving animal heat in a much greater degree than any other insect, is killed by a cold considerably less than that of freezing. Secondly, many large larvæ, as Reaumur has observed, are destroyed by a less degree of cold than smaller species whose respiratory organization is necessarily on a much less extensive scale. And thirdly, the eggs of insects--in which, though they probably are in some degree acted upon by the oxygen of the atmosphere, nothing like respiration takes place--can endure a much greater intensity of cold than either the larvæ or pupæ produced from them.

Nor can we refer the effect in question to the thinness or thickness--the greater or less non-conducting power--of the skin of the animal. Reaumur found that the subterranean pupæ of many moths perished with a cold of 7° or 8° R. below zero (14° F.), while the exposed pupæ of _Pontia Brassicæ_ and other species endured 15° or 16° without injury[752]; (a proof, by the way, that the different economy of these insects, as to their choice of a situation in their state of pupæ, is regulated by their power of resisting cold;) but no difference in the substance of the exterior skin is perceptible. And the eggs of insects have usually thinner skins than pupæ, and yet they are unaffected by a degree of cold much superior.

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!

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 43-1/7°; while in the week comprising the same days of the month of the end of August and beginning of September it was only 44-5/7°--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.

FOOTNOTES:

[717] VOL. I. 452.

[718] Brahm, _Ins. Kal._ ii. 59. 118.

[719] I have reason to think that the larvæ of some species of _Hemerobius_ thus protect themselves by a net-like case of silken threads; at least I found one to-day (December 3d, 1816) inclosed in a case of this description concealed under the bark of a tree: and it is not very likely that it could be a cocoon, both because the inhabitant was not a pupa, which state, according to Reaumur, is assumed soon after the cocoon is fabricated (iii. 385); and because the same author describes the cocoons of these insects as perfectly spherical and of a very close texture (384); while this was oblong, and the net-work with rather wide meshes.

[720] _Œuv._ ii. 72.

[721] _Ibid._ ix. 167.

[722] Illig. _Mag._ i. 209-228.

[723] Lesser, _L._ .256.--Lyonet inserts a note to explain that Lesser's remark is to be understood only of such insects as live in societies; and adds, that solitary species do not assemble to pass the winter together. Lesser, however, says nothing about these insects passing the winter _together_, as his translator erroneously understands him; but merely that they assemble as if _preparing_ to retire for the winter, which my own observations, as above, confirm. His expression in the original German is, "gleichsam als wenn sie sich zu ihrer winter-ruhe fertig machen wolten." Edit. Frankfurt und Leipsig 1738, p. 152.

[724] Illig. _Mag._ i. 216.

[725] Illig. _Mag._ i. 491.

[726] Kyber in German _Magazin der Entomologie_, ii. 2.

[727] _Ins. Kal._ ii. 188.

[728] Spallanzani, _Rapports de l'Air, &c._ i. 30.

[729] Carlisle in _Phil. Trans._ 1805, p. 25.

[730] Schmid in Illig. _Mag._ i. 222.

[731] Since writing the above, I have had another opportunity of confirming the observations here made. The last week of January 1817, in the neighbourhood of Hull, was most delicious weather--calm, sunny, dry, and genial--the wind south-west, the thermometer from 47° to 52° every day, and at night rarely below 40°; in fact, a week much finer than we can often boast of in May: the 27th of the month was the most delightful day of the whole: the air swarmed with _Trichocera hiemalis_, _Psychodæ_, and numerous other _Diptera_, and the bushes were hung with the lines of the gossamer-spider as in autumn. Yet, with the exception of _Aphodius contaminatus_, I did not observe a single coleopterous insect on the wing, nor even an individual tempted to crawl on the trunks of the trees, under the dead bark of which I found many in a very lively state. Five or six individuals of _Haltica Nemorum_ were still very lethargic; and two of _Geotrupes stercorarius_, which I accidentally dug up from their hybernacula in the earth at the depth of six or eight inches, though the _Acari_ upon them were quite alert, exhibited every symptom of complete torpor.

[732] Brahm, _Ins. Kal._ ii. 31.

[733] Lesser, _L._ i. 255.

[734] See above, p. 4. 375.

[735] _Recherches_, 202.--In digging in my garden on the 26th of January 1817, I turned up in three or four places colonies of _Myrmica rubra_, Latr. in their winter retreats, each of which comprised apparently one or two hundred ants, with several larvæ as big as a grain of mustard, closely clustered together, occupying a cavity the size of a hen's egg, in tenacious clay, at the depth of six inches from the surface. They were very lively; but though Fahrenheit's thermometer stood at 47° in the shade, I did not then, nor at any other time during the very mild winter, see a single ant out of its hybernaculum.

[736] Kongl. _Vet. Acad. Handling._ 1816. 104.

[737] Huber i. 134.

[738] Ibid. ii. 344. 358. See above, p. 192--.

[739] Bonnet _On Bees_, 104.

[740] Huber, i. 354.

[741] _Phil. Trans._ 1790. 161.

[742] Reaum. v. 667.

[743] Ibid. 682.

[744] Ibid. 668.

[745] Reaum. 678. Compare also 673.

[746] _Tracts_, 22.

[747] Vid. Spence in _Transactions of the Horticult. Soc. of London_, ii. 148. Compare Reaum. ii. 141.

[748] Lister, Goedart. _de Insectis_, 76.

[749] Reaum. ii. 142.

[750] _Œuvres_, vi. 12.

[751] _Observations on the Animal Economy_, 99.

[752] Reaum. ii. 146-.

[753] _Rapports de l'Air, &c._ ii. 215.

[754] Reaum. ii. 170.

[755] See above, 438--.

[756] Here must be excepted my lamented friend the late Dr. Reeve of Norwich, who, in his ingenious _Essay on the Torpidity of Animals_, has come to nearly the same conclusion as is adopted in this letter; but, by omitting to make a distinction between torpidity and hybernation, he has not done justice to his own ideas.

[757] VOL. I. 32.

[758] Kyber in Germar's _Mag. der Ent._ ii. 3.

[759] Since the publication of the first edition of this volume, I have had an opportunity of making some observations which strongly corroborate the above reasoning. The month of October in the present year (1817) set in extremely cold. From the 1st to the 6th, piercing north and north-west winds blew; the thermometer at Hull, though the sun shone brightly, in the day-time was never higher than from 52° to 56°, nor at night than 38°; in fact, on the 1st and 3rd it sunk as low as 34°, and on the 2nd to 31°: and on those days, at eight in the morning, the grass was covered with a white hoar frost; in short, to every one's feelings the weather indicated December rather than October. Here then was every condition fulfilled that the theory I am opposing can require; consequently, according to that theory, such a state of the atmosphere should have driven every hybernating insect to its winter quarters. But so far was this from being the case, that on the 5th, when I made an excursion purposely to ascertain the fact, I found all the insects still abroad which I had met with six weeks before in similar situations.