When the winter has stripped the leaves off the trees, the Lepidoptera
are seen no more; but as soon as the leaves begin to show themselves
on the trees and shrubs, this tribe of the insect race again makes its
appearance. Caterpillars of all kinds are gnawing at the leaves, even
before they are fully developed. Many of them have just emerged from
the eggs which the perfect insects had laid at an earlier period;
others have passed the winter in this state.
When they come out of the egg the young caterpillars are in shape more
or less elongated and cylindrical. Their body is composed of twelve
segments, or rings. In front is the head; then come three segments, on
which are the front legs, and which constitute the thorax; the other
segments constitute the abdomen. The head is formed of two scaly
parts. It is often very deeply hollowed out on its upper side, and
divided into two lobes, which contain in the angle formed by their
separation the different parts of the mouth. The head is uniform,
rarely having, so far as our caterpillars are concerned, any
protuberance; but in the tropical species it is often armed with
prickles, spikes, and extraordinary appendages. They are provided with
six small simple eyes, isolated from each other. The mouth is armed
laterally with a pair of very solid horny mandibles, articulated by
means of vigorous muscles, and moving horizontally. It is the function
of the mandibles, as with the jaws, to divide the creature's food. On
the middle of a broad under-lip one may perceive a little elongated
tubular organ, pierced with a microscopic orifice. This organ is the
spinning apparatus, which the animal uses in fabricating the threads
which it will one day require. It is a tube composed of longitudinal
fibres. It presents only one orifice, cut obliquely, and capable of
applying itself exactly to the body on which the larva is placed. From
the contractile nature of this organ and the form of its orifice,
combined with the faculty the insect possesses of moving it in all
directions, result the great differences we observe in the diameter
and form of the threads.
[Illustration: Fig. 94.--Scaly legs of the Caterpillar of the Gipsy
Moth (_Liparis dispar_).]
The external organs of the trunk and abdomen are the legs, the
spiracles, and various occasional appendages. The legs are of two
different kinds. The one, to the number of six, attached by pairs to
the trunk, are covered with a shiny cartilage, and armed with hooks.
These are the true legs. Fig. 94 represents, after Réaumur's "Mémoire
sur les Différentes Parties des Chenilles,"[36] the scaly legs of the
caterpillar of the Gipsy Moth. The others are membranous, fleshy,
generally conical or cylindrical, contractile, and taking, according
to the will of the animal, very different forms. Fig. 95 represents,
after the same Memoir of Réaumur's, the different forms of the
membranous legs of the silkworm caterpillar. This plate gives a
sufficiently good idea of the shape of these organs, and of the
hooks, circular or semi-circular, with which they are furnished.
[36] Tome i., p. 164; Plate III., Figs. 1, 2.
[Illustration: Fig. 95.--Membranous legs of the Silkworm (_Bombyx mori_).]
In Fig. 96 are represented, after the same author, two membranous legs
of a large caterpillar, of which the hooks of the feet are fastened
into a branch of a shrub.
[Illustration: Fig. 96.--Membranous legs of a large Caterpillar
embracing a twig.]
Caterpillars have from two to ten false legs, the scaly legs being
always six in number. The pro-legs, as the fleshy ones are called, are
divided into _hinder_ and _intermediate_. The former are two in
number; the intermediate are rarely more than eight in number.
In the caterpillars which have the full number of legs--that is to
say, sixteen--there are two empty spaces, where the body has no
support: the one between the legs and the pro-legs, formed by the
fourth and fifth segment; the other, between the intermediate pro-legs
and the anal legs, formed by the tenth and eleventh ring.
The variations which caterpillars present, so far as the number and
situations of their pro-legs are concerned, are the following:--
The greatest number among them have ten pro-legs; others have only
eight; others only six--these may be called semi-loopers; others only
four, one pair being situated on the last ring, and the other on the
ninth, as in the case of looper caterpillars. And, lastly, there are
others which have only two pro-legs.
The various forms, numbers, and positions of these organs, produce
great differences in the mode of locomotion of caterpillars. Those
provided with ten or eight membranous legs have in walking only a very
slight undulating motion. Their bodies are parallel to the plane which
supports them. They can walk very quickly; but their steps are short
and quickly repeated. Others, on the contrary, in proportion as the
number of their false legs diminish, and the spaces between the legs
increase, walk in a more irregular and quaint manner.
[Illustration: Fig. 97.--Looper Caterpillar.]
[Illustration: Fig. 98.--Caterpillar curved into an arch.]
[Illustration: Fig. 99.--Caterpillar at full length.]
If the reader will glance at Fig. 97, taken from Réaumur's "Mémoire
sur les Chenilles en général,"[37] which represents a looper
caterpillar, with four membranous legs, he will see that there is a
considerable space between the posterior legs and the first pair of
pro-legs, along which the body has no points of support. If one of
these caterpillars, lying quiet and at full length, determines to
walk, in order to take its first step (Fig. 98) it begins by humping
its back, curving into an arch that part which has no legs, and
finishes by assuming the position seen in Fig. 99. In the former
position it has its two intermediate legs against the posterior legs,
and, in consequence, it has brought forward the hinder part of its
body, a distance equal to the interval of the five segments which
separate them. There it hooks on by its _intermediate_ and _hind_
legs. Then it has only to raise and straighten the five rings which
had formed the loop, and to advance its head to a distance equal to
the length of five segments. The step is thus made, the caterpillar
making the same movements in taking the second and following steps.
[37] Tome i., p. 49, Plate I., Fig. 6.
This sort of gait has gained for them the name of Geometers, because
they seem to measure the road over which they travel. When they make a
step, they apply the part of their body which they have just curved up
to the ground, in exactly the same way as a land surveyor applies his
chain to it.
[Illustration: Fig. 100.--Caterpillar of the Canary-shouldered Thorn
(_Eugonia alniaria_).]
These looper caterpillars cannot shorten nor lengthen their segments
at will, as other caterpillars, but only bend their bodies. There are
many species whose bodies are cylindrical, stiff, and of the same
colour as bark. Their attitudes deceive even the close observer. They
embrace the stem of a leaf or twig with their hinder and intermediate
legs, whilst the rest of their body, vertically elevated, remains
stiff and immovable for hours together. Fig. 100 shows the caterpillar
of the Canary-shouldered Thorn (_Eugonia alniaria_) in this strange
position. Now, this is a feat of strength which the most skilful of
our acrobats, ordinary and extraordinary, which all the Leotards of
the present day, and those who are to succeed them, can never
accomplish. With such a persistency, this caterpillar can sustain its
body in the air for a considerable time, in all the positions
imaginable, between the vertical and the horizontal, and downwards
again in any incline from the horizontal to the vertical. "If one
considers," says Réaumur, "how far we are from having in the muscles
of our arms a force capable of supporting us in such attitudes as
these, we must own that the power of the muscles in these insects is
prodigious."
We will not dwell now on the variableness of the length of the body of
caterpillars; on the fleshy appendages which are to be observed on
them; on the hairs which either beautify or render them hideous,
according to the fancy of the observer; nor on the various colours
with which they are decorated. We will notice these various
characteristics when giving the history of some species of remarkable
Lepidoptera.
Many caterpillars are solitary; others live in companies more or less
numerous, either when young, or during the whole of their existence.
With the exception of a great number of moths, which live at the
expense of our furs, or woollen stuffs, and leather or fatty matters,
all caterpillars feed on plants. From the root to the seeds, no part
of the vegetable is safe from their attacks. The greatest number of
the species, however, prefer the leaves. Those of the most acrid and
poisonous are no more spared than those of the most harmless plants.
There are caterpillars which eat the leaves of the Euphorbia, or
spurge, for instance.
"I wished to try," says Réaumur, "the milk of this plant on my tongue.
It produced hardly any effect upon it at first; but after a quarter of
an hour I found my mouth on fire, and it was a heat which reiterated
garglings with water during many hours in succession could not quench.
This continued till the next day. The heat passed successively from
one part of my mouth to another. I, however, saw many of my
caterpillars drinking greedily the great drops of milk which were at
the end of the broken stem I had presented to them."
Is it not extraordinary that there are caterpillars which live on the
nettle?--that they eat the leaves of this plant, armed as it is with
stinging bristles, which cause such smarting and itching to the skin,
and produce blisters upon it.
It has often been said that each plant has its own peculiar species of
caterpillar. All we can say is, that a certain number of vegetables
only suit certain caterpillars. The species which eat roots are few;
those which live in the interior of stalks or stems which they feed
on are numerous, and those which nourish themselves on the pulp of
fruits are rare. In general, after the leaves, the caterpillars prefer
the flowers: in this they certainly do not show bad taste. Their
growth is more or less rapid, according to the species, according to
the nourishment they take, and according to the season of the year.
Those whose food is succulent grow more rapidly than those which have
for their food dry gramineous plants and coriaceous lichens. Most of
them eat at night, and remain during the day motionless, and as it
were in a state of torpor; others are so voracious that they are
constantly eating. This voracity is indeed sometimes surprising.
Malpighi has observed that a silkworm often eats in a day mulberry
leaves equal to its own weight. How could we provide our horses and
oxen with provender, if they required each day their own weight of hay
and grass? There are even some caterpillars which are still more
voracious than that. Réaumur weighed several caterpillars of a species
which lives on the cabbage, and gave them bits of cabbage-leaves which
weighed twice as much as their bodies. In less than twenty-four hours
they had entirely consumed them. In this space of time their weight
increased one-tenth. Fancy a man whose weight is 180 lbs. eating in
one day 360 lbs. of meat, and gaining 18 lbs. in weight! Caterpillars
eat by the aid of two jaws or mandibles so broad and solid that,
considering the smallness of the insect, they are equivalent to all
the teeth with which large animals are furnished. It is by the
alternate movement of these mandibles that the caterpillars devour the
leaves with so much greediness and ease.
[Illustration: Fig. 101.--Looper Caterpillar eating the leaves of the
Apricot (after Réaumur).]
"A caterpillar, when it wants to gnaw the edge of a leaf," says
Réaumur, "twists its body in such a way that at least one portion of
the edge of this leaf is passed between its legs. These legs hold fast
that portion of the leaf which is to be cut by the insect's jaws (Fig.
101). To give the first bite the caterpillar elongates its body, and
carries its head as far forward as possible. The portion of the leaf
which is between the open jaws is cut through the instant the teeth
meet each other; the bites succeed each other quickly; there is not
one, or scarcely one of them, that does not detach a bit, and each bit
is swallowed almost as soon as cut off. At each fresh bite the head
approaches the legs in such a way that during the succession of bites
it describes an arc; it hollows out the portion of the leaf in a
segment of a circle, and it is always in this order that it gnaws it."
But there is a phenomenon in the life of caterpillars which we ought
to point out, and which has attracted the attention of the most
illustrious observers. All caterpillars change their skins many times
during their life. It is not, indeed, enough to say that they change
their skins; the skins or cases they cast are so complete that they
might be taken for entire caterpillars. The hairs, the case of the
legs, the claws with which the legs are provided, the hard and solid
parts which cover the head, the jaws--all these are found in the skin
which the insect abandons. What an operation for the poor little
animal! This labour is so enormous, so troublesome, that one cannot
form a just idea of it. One or two days before this grand crisis, the
caterpillar leaves off eating, loses its usual activity, and becomes
motionless and languid. Their colour fades, their skin dries little by
little, they bow their backs, swell out their segments. At last this
dried-up skin splits below the back, on the second or third ring, and
lets us have a glimpse of a small portion of the new skin, easily to
be recognised by the freshness and brightness of its colours.
"When once the split has been begun," says Réaumur, "it is easy for
the insect to extend it; it continues to swell out that part of its
body which is opposite the slit. Very soon this part raises itself
above the sides of the split; it does the work of a wedge, which
elongates it; thus the split soon extends from the end or the
commencement of the first ring as far as the other side of the end of
the fourth. The upper portion of the body which corresponds to these
four rings is then laid bare, and the caterpillar has an opening
sufficiently large to serve it as an egress through which it can
entirely leave its old skin. It curves its fore part, and draws it
backwards; by this movement it disengages its head from under its old
envelope, and brings it up to the beginning of the slit; at once it
raises it, and puts it out through this slit. The moment afterwards it
stretches out its fore part and lowers its head. There now remains for
the caterpillar nothing but to draw its hinder part from the old
case."
This excessively laborious operation is finished in less than a
minute. The new livery which the caterpillar has just put on is fresh
and bright in colour. But the animal is exhausted by its fast, and the
efforts which it has made. It requires a few hours in which to regain
its equilibrium, and at the same time its former activity and
voracity.