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.
* * * * *
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.