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.
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.