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.
LETTER XXVII.
_ON THE INSTINCT OF INSECTS._
The greater part of those surprising facts connected with the
manners and economy of insects, of which the relation has occupied
the preceding letters, is to be referred, I have told you, to their
instinct. But _what_, you will ask, is this instinct?--of what nature
is this faculty which produces effects so extraordinary?
To this query I do not pretend to give any satisfactory answer. As I
am quite of Bonnet's opinion, that philosophers will in vain torment
themselves to define instinct, until they have spent some time in the
head of an animal without actually _being_ that animal--a species
of metempsychosis through which I have never passed--I shall not
attempt to explain what this mysterious energy _is_. It will not,
however, I imagine, be very difficult to show what it is _not_; and
some observations with this view, followed by an enumeration of
peculiarities which distinguish the instincts of insects from those of
other tribes of animals, and a short inquiry whether their actions are
guided solely by instinct, will form the substance of this letter.
I. It is quite superfluous at this day to controvert the explanations
of instinct advanced by some of the philosophers of the old school,
such as that of Cudworth, who referred this faculty to a certain
_plastic nature_; or that of Des Cartes, who contended that animals
are mere _machines_. Nor, I fancy, would you thank me for entering
into an elaborate refutation of the doctrine of Mylius, that many of
the actions deemed instinctive are the effect of painful corporeal
feelings; the cocoon of a caterpillar, for instance, being the result
of a fit of the colic, produced by a superabundance of the gum which
fills its silk-bags, and which exuding, is twisted round it, by its
uneasy contortions, into a regular ball. Still less need I advert to
the notable discovery of some pupils of Professor Winckler, that the
brain, alias the soul, of a bee or spider, is impressed at the birth
of the insect with certain geometrical figures, according to which
models its works are constructed,--a position which these gentlemen
demonstrate very satisfactorily by a memorable experiment in which
they themselves were able to _hear triangles_.
It is as unnecessary to waste any words in refutation of the nonsense
(for it deserves no better name) of Buffon, who refers the instinct
of societies of insects to the circumstance of a great number of
individuals being brought into existence at the same time, all acting
with equal force, and obliged by the similarity of their internal and
external structure, and the conformity of their movements, to perform
each the same actions, in the same place, in the most convenient mode
for themselves, and least inconvenient for their companions; whence
results a regular, well-proportioned, and symmetrical structure:
and he gravely tells us that the boasted hexagonal cells of bees are
produced by the reciprocal pressure of the cylindrical bodies of
these insects against each other[760]!!
Nor is it requisite to advert at length to the explanations of
instinctive actions more recently given by Steffens, a German
author (one of the transcendentalists, I conclude, from the
incomprehensibility of his book to my ordinary intellect), who
says that the products of the vaunted instinct of insects are
nothing but "shootings out of inorganic animal masses" (_anorgische
anschüsse_)[761]; and by Lamarck[762], who attributes them to
certain inherent inclinations arising from habits impressed upon the
organs of the animals concerned in producing them, by the constant
efflux towards these organs of the nervous fluid, which during a
series of ages has been displaced in their endeavours to perform
certain actions which their necessities have given birth to. The
mere statement of an hypothesis of which the enunciation is nearly
unintelligible, and built upon the assumption of the presence of
an unseen fluid, and of the existence of the animal some millions
of years, is quite sufficient, and would even be unnecessary if it
were not of such late origin. Neither shall I detain you with any
formal consideration of the hypothesis advanced by Addison and some
other authors, that instinct is an immediate and constant impulse of
the Deity; which, to omit other obvious objections, is sufficiently
refuted by the fact, that animals in their instincts are sometimes at
fault, and commit mistakes, which on the above supposition could not
in any case happen.
The only doctrine on the subject of instinct requiring any thing
like a formal refutation, is that which, contending for the identity
of this faculty with reason in man, maintains that all the actions
of animals, however complicated, are, like those of the human race,
the result of observation, invention, and experience. This theory,
maintained by the sceptics, Pythagoras, Plato, and some other ancient
philosophers, and in modern times by Helvetius, Condillac, and
Smellie, has been by none more ingeniously supported than by Dr.
Darwin, who in the chapter treating on instinct, in the first volume
of his _Zoonomia_, has brought forward a collection of facts which
give it a great air of plausibility. This plausibility, however, is
merely superficial; and the result of a rigorous examination by any
competent judge is, that the greater part of Dr. Darwin's facts bear
more strongly in favour of the dissimilarity of instinct and reason
than of their identity: and that those few which seem to support the
latter position, are built upon the relations of persons ignorant
of natural history, who have confused together distinct species
of animals. Thus, because some anonymous informant told him that
hive-bees when transported to Barbadoes, where there is no winter,
ceased to lay up a store of honey, Dr. Darwin infers that all the
operations of these insects are guided by reason and the adaptation
of means to an end--a very just inference, if the statement from
which it is drawn were accurate; but that it is not so, is known to
every naturalist acquainted with the fact that many different species
of bees store up honey in the hottest climates; and that there is no
authentic instance on record of the hive-bees' altering in any age
or climate their peculiar operations, which are now in the coldest
and in the hottest regions precisely what they were in Greece in the
time of Aristotle, and in Italy in the days of Virgil. Indeed the
single fact, depending on the assertions of such accurate observers
as Reaumur and Swammerdam, that a bee as soon after it is disclosed
from the pupa as its body is dried and its wings expanded, and before
it is possible that it should have received any instruction, betakes
itself to the collecting of honey or the fabrication of a cell, which
operations it performs as adroitly as the most hoary inhabitant
of the hive, is alone sufficient to set aside all the hear-say
statements of Dr. Darwin, and should have led him, as it must every
logical reasoner, to the conclusion, that these and similar actions
of animals cannot be referred to any reasoning process, nor be deemed
the result of observation and experience.--It is true, it does not
follow that animals, besides instinct, have not, in a degree, the
faculty of reason also; and as I shall in the sequel endeavour to
show, many of the actions of insects can be adequately explained on
no other supposition. But to deny, as Dr. Darwin does, that the art
with which the caterpillar weaves its cocoon, or the unerring care
with which the moth places her eggs upon food that she herself can
never use, are the effects of instinct, is as unphilosophical and
contrary to fact, as to insist that the eagerness with which, though
it has never tasted milk, the infant seeks for its mother's breast,
is the effect of reason.
Instinct, then, is _not_ the result of a plastic nature; of a system
of machinery; of diseased bodily action; of models impressed on the
brain; nor of organic shootings-out:--it is not the effect of the
habitual determination for ages of the nervous fluid to certain
organs; nor is it either the impulse of the Deity, or reason. Without
pretending to give a logical definition of it, which while we are
ignorant of the essence of reason is impossible, we may call the
instincts of animals those unknown faculties implanted in their
constitution by the Creator, by which, independent of instruction,
observation, or experience, and without a knowledge of the end
in view, they are impelled to the performance of certain actions
tending to the well-being of the individual and the preservation
of the species: and with this description, which is in fact merely
a confession of ignorance, we must, in the present state of
metaphysical science, content ourselves.