the glow-worm are worthy of being recorded.--One of the receptacles
being extracted with a penknife, continued luminous; but on being
immersed in camphorated spirit of wine, became immediately extinct.
The animal, with one of its receptacles uninjured, being plunged
into the same spirit, became apparently lifeless in less than a
minute; but the receptacle continued luminous for five minutes, the
light gradually disappearing.--Having extracted the luminous matter
from the receptacles, in two days they were healed, and filled with
luminous matter as before. He found this matter to lose its luminous
property, and become dry and glossy like gum, in about two minutes;
but it recovered it again on being moistened with saliva, and again
lost it when dried. When the matter was extracted from two or three
glow-worms, and covered with liquid gum-arabic, it continued luminous
for upwards of a quarter of an hour.
[711] _Phil. Trans._ 1810, p. 287.
[712] _Ibid._ 1801, p. 483.
[713] See above, p. 225.
[714] Müller in _Illig. Mag._ iv. 178.
[715] iv. 49.
[716] _Phil. Trans._ 1799. 157.
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].