MOULTING OF THE LARVA.
We are now to enter upon a very interesting part of the history of a
larva: this is called _moulting_, and consists in the larva casting
off its old skin, and appearing clothed in a new garment, often more
brilliant than before. The change is well and clearly described in
the following passage from the Introduction to Entomology, of Messrs.
Kirby and Spence:--"A day or two previously to each change of skin,
the larva ceases eating altogether; it becomes languid and feeble;
its beautiful colours fade; and it seeks 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
pro-legs, 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
and third segments. 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 enclosed larva by degrees withdraws itself wholly from its old
skin." The engraving is intended to show the larva just escaping out of
its old skin.
[Illustration]
Sometimes the larva comes out of its skin at the side; sometimes it
has been seen to bite off portions of the skin. The most common way is
that above described. The skin, when cast in this manner, resembles a
sort of universal coat, which has fitted into every crack and joint
of the insect's body; and just as a lady's glove, if we could suppose
it tinted of a flesh-colour, and marked for the various markings on
the hand, might be mistaken, if cast down after inflating it with air,
for a hand cut off, so, only far more closely, does the cast skin
represent, in the minutest particular, the larva which has emerged
from it. It is a perfect mould of all its parts, even to the very
antennæ, eyes, jaws, &c. "Thus," say the authors last quoted, "if you
saved the skins cast by the larva of the insect called _Callimorpha_,
or _Arctia Caja_, you would appear to have ten different specimens
of caterpillars, furnished with every external necessary part, and
differing only in size and in the colour, perhaps, of the hairs, and
all representing the same individual."
In order to show how completely this is the case, some singular
experiments have been made by various observers, in the following
manner:--Just before the larva was about to cast off its skin, they
have, by means of a sharp instrument, cut off one or two of its feet.
The larva was then allowed to moult, and was carefully examined, and it
was invariably found that the feet cut off when in its old skin were
also wanting when it appeared in its new robes; thus plainly proving
that the feet were really sheathed in the old skin completely. It was
just as if we had gloves on our hands and were to put one finger or
more between a pair of sharp and powerful shears, we should, of course,
find that when we took off the glove we should be short of one or two
fingers!
But the hairs are not thus sheathed. The old skin, if the larva was a
hairy one, such as many of those with which we are familiar under the
more popular title "caterpillars," is cast off with the hairs attached
to it--a circumstance which makes the cast skin look still more like
the real larva. How, then, it may be asked, does the larva acquire
new hairs to take the place of the old ones? Were we to take a larva
just before its moulting, and by a sharp and delicate instrument, to
slit open and raise its old skin, we should soon perceive how this has
been contrived. We should there perceive sundry little tufts of very
delicate hairs, lying down smoothly on the surface of the larva's body,
and arranged in certain directions, with great regularity. When the
old skin is cast, in the course of a little time these hairs stand
upright, and assume precisely the same appearance as those on the
former skin.
The keeper of silk-worms can tell us well, and perhaps with many a
sigh, that the period of moulting is one of great peril to his tender
charge. The larvæ are sick and feeble for some time after each period
of moulting; and large numbers die at this time, apparently unable
to bear the exhaustion attending the loss of their old skin. Larvæ
generally are much debilitated by each moult that takes place; for a
variable period, sometimes for some hours, sometimes for a whole day,
they will refuse food, and lie without motion. All their parts are very
soft and tender, and require hardening by exposure to the air. After
a certain period, however, these effects pass off; the larva recovers
its original strength, its body becomes firm, its colour brightens, and
appears more brilliant than ever; and, above all, like convalescents
among ourselves, its appetite is twice as keen as before, which, when
we call to mind what has been already said about the voracity of larvæ,
it will be confessed, appears scarcely possible.
These changes of skin take place a definite number of times in the
larvæ of each species. The breeders of silk-worms tell us that that
precious, if not priceless, little larva, sheds its skin four times;
other larvæ moult oftener than these: some five times, some seven
times, some eight times, and some nine, or even ten times; others,
again, only moult thrice. The ordinary number of moults appears to be
about three or four. Some, however, never moult at all in the larva
state. The larva of the common bee, and that of the flesh-fly, and some
others, are among this number.
Whatever may be the general opinion upon the want of beauty and
comeliness in insects, and particularly during the larva stage of their
history, it is very certain that it is quite erroneous. We may possibly
fail, unaided by the pencil and colours of the artist, in conveying
even an approach to an adequate conception of the rich and glowing
raiment with which it has pleased God to clothe these minute and
humble beings. But a very little actual observation of a few even of
the commoner larvæ or caterpillars of the butterfly and moth species,
will convince the reader that few other created things have been so
exquisitely adorned with magnificence of colour and apparel as the
larvæ of these insects. Here will be found hues so rich and deep as to--
"Make the rose's blush of beauty pale,
And dim the rich geranium's scarlet blaze."
Here is a larva of a moth before us whose head is a delicate orange
colour, whose belly and feet a pale pure green, whose sides are of
primrose tint, variegated with slashes of green, and along the back
there runs a beautiful band of white, the intensity of which is most
agreeably relieved by a vein of lovely pink, which divides it in
the middle. There is another clothed in purple, green, and white;
another, the larva of the butterfly called the "Camberwell Beauty,"
is of a brilliant black, dotted with spots glowing like carbuncles
along its back; another has a coat which seems as if set with precious
stones, blazing with an intensity of colour which cannot be conveyed
by description. In a word, larvæ, on the whole, can boast as varied a
display of beautiful hues as can almost the whole array of flowers in
our gardens. In consequence of the, unfortunately, too general feeling
of loathing, to which allusion has already been made, but few persons
are in reality acquainted with their beauties. Their diminutive size
also renders them less attractive than they might otherwise prove,
since we are generally most pleased with the beauty of an object when
its size makes it so manifest to our eyes, as to render it impossible
it should escape our attention.
It is a very remarkable fact, and corresponds, to a certain extent,
with the influence of the same agent upon plants[G]--that light seems
to be in some measure necessary in order to develop the colours of the
larva. Thus it has been noticed, that those larvæ which are, by their
habits, much in darkness, dwelling, perhaps, in caves of the earth, or
immured in the heart of a piece of timber, or inhabiting a cell scooped
out of the solid rock, are, most frequently, of a uniform whitish
colour. Some experiments have shown that when these whitish larvæ have
been brought out of darkness and exposed to the sunlight, their colour
has turned to brown. Very probably this effect is due to precisely the
same kind of change as takes place when a fair-skinned European travels
into a southern clime, his face and hands becoming so brown, tanned,
and dark, as to form a ludicrous contrast to the whiteness of his
chest or arms, which are not exposed to the influence of the solar ray.
[Illustration: _Larvæ resembling Twigs._]
There is still something more to be said about the coats of the
larva. To look at some of them, it would be difficult for a person
unacquainted with entomology, to guess at the real nature of the object
before him. Some look like dry twigs. Some look far more like little
_Cactuses_ than insects. Some are clothed with _hairs_, arranged in
the most curious and eccentric manner. Some have hairs so long as
to give them all the appearance of very minute shock-dogs. In some,
the hairs are arranged like stars; in others, like the quills of the
"fretful porcupine." The backs of other larva look just as if they
were studded over with little camel-hair pencils. Madame Merian
has described the larva of an insect found in Surinam as having the
various divisions of its body ornamented with three blue tubercles,
like turquoise beads, from each of which proceeds a long, delicate,
feathery plume of a black colour. Another, described by the same lady,
is splendidly adorned on each side with fifty red tubercles, shining
like coral, from each of which proceed five or six long hairs. Some,
again, are covered as thickly as possible with sharp processes, like
thorns, sufficiently strong and sharp to pierce the skin of any one who
incautiously ventures to take them up. The body of a small caterpillar
brought from Brazil, is described by Kirby and Spence as possessing a
body so thickly planted with spines, like the branches of a tree, as
absolutely to wear the appearance of a forest or thicket in miniature!
It has been stated, indeed, by some travellers, that the larva of some
insects has the power of darting out, when alarmed by the approach of
anything, such as the human hand, from as many knobs or protuberances
in its back, eight bunches of little stings, with which it inflicts a
very painful and venomous wound.
[Illustration: _Singularly coated Larva._]
The larvæ of some insects again seem as if they were clothed from head
to foot in the most delicate and snowy white cotton wool. These may
sometimes be seen on our apple-trees; but one of the most singular is
figured in the cut, from an engraving given in Baron de Geer's work.
The larva is covered completely with a coat of cottony flakes of the
most dazzling white, and these are arranged, as will be perceived,
with great regularity. The larva has, in this white and warm raiment,
somewhat of the aspect of a coachman buried in the old-fashioned great
coats with the many capes.
Having dwelt so long upon the food and clothing of the insect in the
larva form, it is right we should now devote a little space to consider
how they _breathe_. Some one perhaps will say, _Breathe_?--do larvæ
breathe? Most certainly; and respiration, or the function of breathing,
is not more necessary to us than it is to these creatures. For this
purpose, however, we must begin a fresh chapter.