_rete mucosum_; but in _insects_, as I formerly stated[469], since the
time of Swammerdam it has generally been believed by entomologists,
that the larva includes a series of cases or envelopes, one within
the other, containing in the centre the germe of the future perfect
insect, whose development and final exclusion take place only when
these cases have been successively cast off. This hypothesis, as was
explained to you on a former occasion[470], has been controverted by a
late writer, Dr. Herold; who affirms that the skins of caterpillars are
also successively produced out of the _rete mucosum_. I have however,
I hope, satisfied you that the old system is most consonant to nature
and probability: but as we are now to enter at large upon the _Moults_
of insects, it will not be without use if I add a few additional
reasons which seem to me still further to prove the correctness of
Swammerdam's system, as far as it relates to that subject. With regard
to the mere formation of the skin from the _rete mucosum_, were this
the whole question few would hesitate to adopt the sentiments of M.
Herold; but when we come to consider further--that the number of moults
of individuals of the _same_ species is always the same, and that it
varies in _different_ species, and takes place at certain periods,--we
begin to suspect that something more than the mere formation of a new
skin upon an old one being cast is to be accounted for; and that the
law which prescribes its own definite number of skins to each species,
must begin to act in the primordial formation of the larva. Again,
the _hairs_ observable in the higher animals do not take their origin
from the epidermis solely, but are planted below it in the _rete
mucosum_, or deeper[471]; so that the change of skin does not affect
them; but in the larvæ of insects they are a continuation of that
integument, since, when the moult takes place, they always remain on
the rejected skin[472]: this is the case, also, even with spines. If
you shave a caterpillar ready to change its skin, either partially or
generally, you will find that the parts in the new skin that correspond
with those that are denuded, are equally hairy with those that were
not[473]; and if you pay attention to the new-clad animal, you will
find further, that the hairs never grow after a moult. From hence it
follows, that the _hairs_ have their place and take their whole growth
between the new skin and the old[474]. Whether the _spines_, simple or
compound, lately described to you, that arm some larvæ are similarly
circumstanced, seems not as yet to have been ascertained; but as the
spinous ones of certain _Tenthredines_ L. and _Lepidoptera_ at their
last moult have no spines, the presumption is, that, whether incased
or not, they are mere appendages of the skin on which they appear.
A new set of hairs, therefore, and probably of spines in spinous
larvæ, accompanying each skin, and these varying very much in size,
composition, &c. though a new membrane may be admitted to be formed
from an action in the _rete mucosum_ without a pre-existent germe of
it, it seems not easy to conceive how these hairs and spines can spring
up and grow there, each according to a certain law, without existing
previously as a kind of _corculum_ or _punctum saliens_; and that the
germes of the tubercles, in which the hairs are so generally planted,
according to a certain arrangement and in a given number, should also
pre-exist, seems most consonant to reason. These and the several skins
may all co-exist in their primordial germes, and remain beyond the
discovery of our highest powers of assisted vision, till a certain
period when they may first enter the range of the microscope-aided eye.
It does not therefore follow, because these _primordia semina rerum_
are not discoverable, that therefore they may not exist. Our faculties
and organs are too limited and of too little power to enable us to see
the essences of being.
Upon the supposition that the hypothesis of Swammerdam is the true
one, we may imagine that the envelope that lies within all the rest is
that which covers the insect in its pupa state. Above this are placed
several others, which successively become external integuments. These
changes or casting of the skin in larvæ, analogous, as before observed,
to that of serpents, are familiar to every breeder of silk-worms, in
which _four_ such changes occur: the first at the end of about _twelve_
days from its birth, and the three next each at the end of _half_ that
time from the moulting which preceded it. With some exceptions[475],
similar changes of the skin take place in all larvæ, not however in
the same number and at the same periods. Most indeed undergo this
operation only three or four times; but there are some that moult
oftener, from five up to eight (_Arctia villica_), nine (_Callimorpha
Dominula_), or even ten times; for so often, M. Cuvier informs us,
the caterpillar of the tiger-moth (_Callimorpha Caja_) casts its
exuviæ. It has been observed that the caterpillars of the day-flying
_Lepidoptera_ (_Papilio_ L.) usually change only _three_ times, while
those of the night-flying ones (_Phalæna_ L.) change _four_[476]. The
periods that intervene between each change depend upon the length of
the insect's existence in the larva state. In those which live only a
few weeks or months, they are from eight to twenty days; while in those
that live more than a year, as the cockchafer, &c. they are probably
proportionably longer: though we know very little with regard to the
moult of any insects besides the _Lepidoptera_.
A day or two previously to each change of its skin, the larva ceases
eating altogether; it becomes languid and feeble, its beautiful
colours fade, and it seeks for a retreat in which it can undergo
this important and sometimes dangerous and even fatal operation in
security. Here, either fixing itself by its legs to the surface on
which it rests, or, as is the case with many caterpillars, by its
prolegs, to a slight web spun for this purpose, it turns and twists
its body in various directions, and alternately swells and contracts
its different segments. The object of these motions and contortions
seems to be, to separate the exterior skin, now become dry and rigid,
from the new one just below it. After continuing these operations for
some hours, resting at intervals without motion, as if exhausted by
their violence, the critical moment arrives: the skin splits in the
back, in consequence of the still more violent swelling of the second
or third segment: the opening thus made is speedily increased by a
succession of swellings and contractions of the remaining segments:
even the head itself often divides into three triangular pieces, and
the inclosed larva by degrees withdraws itself wholly from its old
skin. All larvæ, however, do not force their way through this skin
in precisely the same place. Thus, those of the hawthorn butterfly
(_Pieris Cratægi_), according to Bonnet[477] make their way out by
forcing off what may be called their skull, or the horny part of
their head, without splitting the skin, which remains entire; others
have been observed to make their way out at the side and the belly.
Reaumur noticed the larva of _Zygæna Filipendulæ_, previously to its
last moult, actually biting off and detaching several portions of its
old skin; and before this, drops of a fluid resembling water were
seen to exude from it[478].
The skin when cast is often so entire, that it might be mistaken
for the larva itself; comprising not only the covering of the main
trunk with the hairs which clothed it, but of the very skull, eyes,
antennæ, palpi, jaws, and legs; which, if examined from within, are
now found to be hollow, and to have incased, like so many sheaths,
similar parts in the new skin. That the feet of the newly-coated
larva were actually sheathed, as fingers in a glove, in the same
parts of the exuviæ, may be proved by a very simple experiment: if
a leg of one just ready to cast its skin be cut off, the same limb
will be found mutilated when that change has ensued. The anal horns,
also, of the larvæ of the hawk-moth (_Sphinx_ L.) and other similar
protuberances, are incased in each other in like manner; but hairs
are laid flat between the two skins, and contribute considerably
towards their more easy separation. Thus, if you saved the skins cast
by the larva of _Callimorpha Caja_, for instance, you would appear to
have ten different specimens of caterpillars, furnished with every
external necessary part, and differing only in size, and the colour
perhaps of the hairs, and all representing the same individual.
But further changes than this take place. Swammerdam says, speaking
of the moult of the grub of _Oryctes nasicornis_, a beetle common
in Holland, but not satisfactorily ascertained to inhabit Britain,
"Nothing in all nature is in my opinion a more wonderful sight than
the change of skin in these and other the like worms. This matter,
therefore, deserves the greatest consideration, and is worthy to be
called a specimen of nature's miracles; for it is not the external
skin only that these worms cast, like serpents, but the throat and a
part of the stomach, and even the inward surface of the great gut,
change their skin at the same time. But this is not the whole of
these wonders; for at the same time some hundreds of pulmonary pipes
within the body of the worm cast also each its delicate and tender
skin. These several skins are afterwards collected into eighteen
thicker, and, as it were, compounded ropes, nine on each side of the
body, which, when the skin is cast, slip gently and by degrees from
within the body through the eighteen apertures or orifices of the
pulmonary tubes before described, having their tops or ends directed
upwards towards the head. Two other branches of the pulmonary
pipes that are smaller, and have no points of respiration, cast a
skin likewise." ... "If any one separates the cast little ropes or
congeries of the pulmonary pipes with a fine needle, he will very
distinctly see the branches and ramifications of these several pipes,
and also their annular composition[479]."--Bonnet makes a similar
observation with regard to _caterpillars_; but he appears to have
observed it more particularly, at least the change of the intestines,
previously to the metamorphosis of the insect, when he says with the
excrements it casts the inner skin of the stomach and viscera[480].
Both these great men appear to have recorded the result of their
own actual observations with regard to the proceedings of two very
different kinds of insects; the one the grub of a beetle, and the
other the caterpillars of _Lepidoptera_. The account of the former is
given quite in detail, as that of a person who is describing what he
has actually _seen_: yet by a later and very able physiologist, Dr.
Herold, it is affirmed that the inner skin of the intestinal canal
is never cast, that canal constantly retaining its two skins. He
further affirms, that they are only the _large_ trunks of the Tracheæ
that cast their skins, none being detached from their _smaller_
ramifications[481]. When men so eminent for their anatomical skill
and nicety, and for their depth and acumen, disagree, the question
must be regarded as undecided till further observations throw
sufficient weight into one scale or the other.
The larva which has undergone this painful process is at first
extremely weak: all its parts are soft and tender; even the corneous
ones, as the head and the legs, are then scarcely more than membranous,
and are all bathed with a fluid, which, before the moult, intervenes
between the two skins, and facilitates their separation[482]: and
it is only after some _hours_, or in some cases even _days_, during
which it lies without motion, that this humidity evaporates, all its
parts become consolidated, and it recovers its strength sufficiently
to betake itself to its wonted food. Its colour, too, is usually at
first much paler than before, and its markings indistinct, until their
tints have been enlivened by exposure to the air, when they become
more fresh, vivid, and beautiful to appearance than ever. When a few
meals have invigorated its languid powers, the renovated animal makes
up for its long abstinence by eating with double voracity. . A similar
preparatory fast, and succeeding state of debility, accompany every
change of the larva's skin. Each time except the last, the old skin is
succeeded by a new one, with few exceptions, similar to the one it has
discarded. Previously to the final change, which discloses the pupa, it
quits the plant or tree on which it had lived, and appears to be quite
unsettled, wandering about and crossing the paths and roads, as if in
quest of some new dwelling. It now abstains from food for a longer time
than before a common moult, empties itself copiously, and as I have
just said, if Swammerdam and Bonnet are to be depended upon, casts the
skin that lines the stomach and intestines, as well as that of the
Tracheæ.
I have observed above, that all larvæ, with few exceptions, change
their skins in the manner that I have described. These exceptions
are principally found in the order _Diptera_, of which those of
the Linnean genera _Musca_, _Œstrus_, and probably all that, like
the maggot of the common flesh-fly, have membranous contractile
heads, never change their skin at all, not even preparatory to their
becoming pupæ. The skin of the pupa, though often differing greatly
in shape from that of the larva, is the same which has covered this
last from its birth, only modified in figure by the internal changes
that have taken place, and to which its membranous texture readily
accommodates itself. The larvæ of the Dipterous genera _Tipula_,
_Culex_, and those which have corneous heads, like other larvæ
change their skins several times previously to becoming pupæ[483].
The grubs, also, of bees, wasps, ants? and probably many other
_Hymenoptera_, do not change their skin till they assume the pupa,
nor the larva of the female _Coccus_[484].
If you feel disposed to investigate the reasons of that law of the
Creator which has ordained that the skins of the higher animals shall
be daily, and imperceptibly, and as it were piece by piece renewed,
while those of insects are cast periodically and simultaneously,--the
proximate cause must be sought for in the nature of the two kinds
of skin, the one being more pliable and admitting a greater degree
of tension than the other, and being so constructed as to scale off
more readily. If, ascending higher, you wish to know why the skins
of insects are so differently circumstanced from our own, the most
apparent reason is, to accommodate the skin to the very rapid growth
of these animals, which a gradual and slower change would have
impeded too much, or the skin have suffered constant dilapidation
and injury; therefore their Beneficent Creator has furnished them
with one which will stretch to a certain point, and during a certain
period, and then yield to the efforts of the inclosed animal, and be
thrown aside as a garment that no longer fits the wearer.
viii. And this leads me to a subject to which I am desirous now to
bespeak your attention,--the _Growth_, I mean, and size of Insects in
this state. As to _size_, larvæ differ as much as insects in their
perfect state: these last, however, never grow after their exclusion
from the pupa, while larvæ increase in bulk in a proportion, and
often with a rapidity, almost without a parallel in the other tribes
of animals. Thus Lyonnet found, that the caterpillar of the great
goat-moth (_Cossus ligniperda_ F.) after having attained its full
growth is at least 72,000 times heavier than when it was first
excluded from the egg[485]; and of course had increased in size in
the same proportion. Connected with the size of larvæ, is the mode
in which their accretion takes place. This, with respect to the more
solid parts, as the head, legs, &c., is not, as in other animals,
by gradual and imperceptible degrees, but suddenly and at stated
intervals. Between the assumption of a new skin and the deposition of
an old one, no increase of size takes place in these parts, while the
rest of the body grows and extends itself, till, becoming too big for
these solid parts, nature restores the equilibrium between them by a
fresh moult[486], in which the augmentation of bulk, especially in
these parts, is so great, that we can scarcely credit the possibility
of its being cased in so small an envelope. Malpighi declares, that
the head of a silk-worm that has recently cast its skin is four times
larger than before the change[487]. It is very probable, also, that
when the outer skin becomes rigid, it confines the body of the larva
within a smaller compass than it would expand to if unconfined, so
that, when this compression is removed, the soft and elastic new
integument immediately swells out, and the animal appears all at
once much larger than it was before the moult. In fact, the proximate
cause of the rupture and rejection of the old skin is the expansion
of the included body, which at length becomes so distended as to
split its envelope, aided, indeed, as before described, by the
contortions of the creature itself.
The larvæ most notorious for the _rapidity_ of their growth are those
of _Musca carnaria_ and other flesh-flies: some of which Redi found
to become from 140 to more than 200 times heavier in twenty-four
hours[488]: an increase of weight and size in so short a time truly
prodigious, but essential for the end of their creation--the rapid
removal of dead and putrescent animal matter. As the skins of these
larvæ are never changed, we may conclude, if the cause of the change
of skin in other larvæ above surmised be accurate, that their skins
are more contractile and capable of a greater degree of tension than
those of larvæ that are subject to moulting. And two peculiarities
observable in them confirm this idea: in the first place, their head
is not hard and corneous, as that of the others, but capable of being
shortened or lengthened; and in the next, their breathing-pores are
not in the sides, but at the extremities of the body, while in the
_moulting_ larvæ there are two in almost every segment, which must form
so many callous points that impede the stretching of the skin to the
utmost. The hairs, spines, and tubercles, that are so often found on
caterpillars, must also form so many points of resistance that prevent
that full extension of the integument which it might otherwise admit.
There is not always that proportion between the size of larvæ and of
the insects that proceed from them that might have been supposed,
some small larvæ often producing perfect insects larger than some of
those proceeding from such as are of greater size.
ix. As insects often live longest in the state we are treating of,
I shall say something next upon the _age_ of larvæ, or the period
intervening between their exclusion from the egg and their becoming
pupæ. This is exceedingly various, but in every case nicely adapted
to their several functions and modes of life. The grubs of the
flesh-fly have attained their full growth, and are ready to become
pupæ, in _six_ or _seven_ days; the caterpillar of _Argynnis Paphia_,
a butterfly, in _fourteen_ days; the larvæ of bees in _twenty_ days;
while those of the great goat-moth (_Cossus ligniperda_) and of the
cockchafer (_Melolontha vulgaris_) live _three_ years, or at least
survive three winters, before the same change. That of another
lamellicorn beetle (_Oryctes nasicornis_ F.) is said to be extended
to _four_ or _five_; that of the wire-worm (_Elater segetum_) to
_five_. That of the stag-beetle (_Lucanus Cervus_) is affirmed by
Rösel to be extended to _six_ years; but the most remarkable instance
of insect longevity is recorded by Mr. Marsham in the _Linnean
Transactions_[489]. A specimen of _Buprestis splendida_, a beautiful
beetle never before found in this country, made its way out of a deal
desk in an office in London in the beginning of the year 1810, which
had been fixed there in the year 1788 or 1789; so that according to
every appearance it had existed in this desk more than twenty years.
Ample allowance being made for its life as a pupa, we may conclude
that it had existed as a larva at least half the above period. The
grubs of the species of the genus _Cynips_ L. attain their full size
in a short time; but they afterwards remain five or six months in the
gall before they become pupæ[490].
With few exceptions it may be laid down, that those larvæ which live
on dead animals, in fungi, in dung, and in similar substances, are of
the shortest duration in this state; and that those which live under
the earth, on the roots of grass, &c. and in wood, the longest: the
former becoming pupæ in a few days or weeks, the latter requiring
several months, or even years, to bring them to maturity. The larvæ
which live on the leaves of plants seem to attain a middle term
between the one and the other,--seldom shorter than a few weeks, and
rarely longer than seven or eight months. Aquatic larvæ appear to be
subject to no general rule: some, as the larvæ of _Gnats_, becoming
pupæ in two or three weeks; and others, as those of the _Ephemeræ_,
which are thus compensated for their short life as flies, in as
many years[491]. The cause of all these differences is obviously
dependent on the nature of the food, and the purposes in the economy
of creation to which the larvæ are destined.
x. The last part of the history of larvæ relates to their _Preparations
for assuming the pupa state_. When they have acquired their full size,
after having ceased to take food, by a copious evacuation they empty
the intestinal canal, even rejecting the membrane that lines it and
the stomach[492]; their colours either change totally, or fade; and
they make themselves ready for entering upon a new stage of their
existence. Some merely rest in a state of inactivity in the midst of
the substances in which they feed, as if conscious of their inability
to select any safer abode. Of this description are most Coleopterous,
Hymenopterous, and Dipterous larvæ, that feed under ground, or in the
interior of trees, fruits, and seeds.
But a still larger tribe, those which feed on leaves, animals, &c.
act as if more sensible of the insecurity of this to them important
epoch. They are about to exchange their state of vigour and activity
for a long period of deathlike sleep. The vigilant caution which
was wont to guard them from the attack of their enemies will be
henceforward of no avail. Destitute of all the means of active
defence, their only chance of safety during their often protracted
night of torpor must arise from the privacy of their place of repose.
About this, therefore, they exhibit the greatest anxiety. Many, after
wandering about as if bewildered, retire to any small hole on the
surface of the earth, covering themselves with dead leaves, moss, or
the like, or to the chinks of trees, or niches in walls and other
buildings, or similar hiding-places. Many penetrate to the depth of
several inches under ground, and there form an appropriate cavern
by pushing away the surrounding earth; to which they often give
consistence by wetting it with a viscid fluid poured from the mouth.
The larvæ of other insects undertake long and arduous journeys in
search of appropriate places of shelter. Those of flesh-flies, now
satiated with the mass of putridity in which they have wallowed,
leave it, and conceal themselves in any adjoining heap of dust.
The grubs of the gad-fly (_Œstrus_) creep some of them out of the
backs of cattle, in tumours of which they have resided, and suffer
themselves to fall to the earth; while others, which have fed in the
stomach of horses, quit their hold, and by a still more extraordinary
and perilous route are carried through the intestines the whole
length of their numerous circumvolutions, and are discharged at the
anus. And without enumerating other instances, various aquatic larvæ,
as that of a common fly (_Elophilus pendulus_), &c. leave the water,
now no longer their proper element, and betake themselves to the
shore, there to undergo their metamorphosis.