Having attained its full development, the caterpillar ceases to eat,
as at the approach of a moult, it empties its intestinal canal by
copious ejections; it loses its colours, and becomes dull and livid,
and thus prepares itself to enter a new phase of its existence.
Some, when about to transform themselves into chrysalides, suspend
themselves to foreign bodies. Others spin a cocoon, composed of silk
and other substances, which secures them against the attacks of their
enemies and the action of the atmosphere. Those which suspend
themselves can be divided under two heads, according to the mode of
their suspension:--1. Those which suspend themselves perpendicularly
by the tail. 2. Those which, after having fixed themselves by the same
part, suspend themselves horizontally, by means of a silk thread
passed round the body.
To understand the difficulty which the first of these operations
presents, we must consider the problem which the caterpillar has to
solve. In this problem there are two unknown quantities to be
discovered. Firstly, the caterpillar must suspend itself firmly; and
secondly, the pupa, having no communication with the object which
supports it, must be suspended in the same manner. This problem is
difficult, apparently impossible, to solve. It is only by watching
these insects at work that one can discover the wonderful mysteries of
their lives. Swammerdam, Valisnieri, and other observers who have
studied insects, had not, however, observed the manoeuvres of
caterpillars in this curious phase of their existence. It is to
Réaumur again that science is indebted for the most charming and
valuable observations on this point. He got together a great number of
caterpillars of the small Tortoise-shell Butterfly (_Vanessa urticæ_),
black prickly caterpillars which are common on the stinging-nettle,
where they live in companies, and suspend themselves by the tail. When
the time approaches at which the caterpillars of this species ought to
undergo their transformations, they usually leave the plant which had
up to that time served them as food. After having wandered about a
little, they select some convenient spot, where they hang themselves
up head downwards (Figs. 102, 103).
In order to hang itself in this way, the caterpillar begins by
covering, with threads drawn in different directions, a pretty large
extent of the surface of the body against which it wishes to fix
itself. After having covered it thus with a kind of thin cobweb, it
adds different layers of threads on a small portion of this surface,
in such a manner that the upper one is always smaller than that upon
which it is laid. In this manner a small hillock of silk is formed,
the tissue of which is not at all compact. It resembles an assemblage
of loose or badly interwoven threads. The membranous feet of the
caterpillar are armed with hooks of different lengths, with the aid of
which it suspends itself. By alternately contracting and elongating
its body, it pushes its hindermost legs against the hillock of silk,
presses against it the hooks of its feet, so as to get them better
entangled, and lets its body fall in a vertical position.
[Illustration: Figs. 102, 103.--Caterpillars of the small
Tortoise-shell Butterfly (_Vanessa urticæ_) undergoing their
metamorphoses.]
[Illustration: Figs. 104, 105.--Chrysalides of the small
Tortoise-shell Butterfly freeing themselves from the Caterpillar skin.]
It remains hanging thus, often for twenty-four hours, during which
time it is occupied in a difficult task, that of splitting its skin.
In order to effect this, it incessantly curves and recurves its body
(Fig. 102), until at last a split appears on the skin of the back, and
through this split emerges a part of the body of the chrysalis. This
acts as a wedge, and little by little the split widens from the head
to the last of the true legs, and beyond them. Then the opening is
sufficient to allow of the chrysalis drawing out its anterior portion
from the envelope, which it immediately does. To set itself entirely
free, the chrysalis lengthens and shortens itself alternately (Fig.
105). Each time that it shortens itself, and when it consequently
distends the part of its body which is outside the old skin, that part
acts against the edges of the slit, and gradually pushes the old skin
upwards. Thus the caterpillar skin ascends, its plaits are pushed
nearer and nearer together, and it is soon reduced to a packet so
small that it covers only the end of the tail of the chrysalis (Fig.
106).
But here comes the culminating point, the most difficult part of the
operation. The chrysalis, which is shorter than the caterpillar, is at
some distance from the silky network to which it must fix itself; it
is only supported by that extremity of the caterpillar's skin which
had not been split open. It has neither legs nor arms, and yet it must
free itself from this remaining part of the skin, and reach the
threads to which it is to suspend itself.
[Illustration: Fig. 106.--Chrysalis of the small Tortoise-shell
Butterfly completing the operation of casting its larval skin.]
[Illustration: Fig. 107.--Chrysalis divested of the larva skin.]
The supple and contractile segments of the chrysalis serve for the
limbs which are wanting to it. Between two of these segments, as with
a pair of pincers, the insect seizes a portion of the folded skin, and
with such a firm hold that it is able to support the whole of its body
on it. It now curves its hinder parts slightly, and draws its tail
entirely out of the sheath in which it was enclosed. It then reposes
for an instant only, for it has not yet finished the laborious
operation of its deliverance. It must free itself entirely from the
dry skin which surrounds the extremity of its body.
The insect curves the part which is below its tail in such a manner
that that part can embrace and seize the packet to which it holds on.
It then gives to its body a violent shock, which makes it spin round
many times on its tail, and that with great rapidity. During all these
pirouettes the chrysalis acts against the skin; the hooks of its legs
fray the threads, and break them or disentangle themselves from them.
Sometimes the threads do not break at once. Then the animal
recommences its revolutions in an opposite direction, and this time it
is almost certain to succeed. Réaumur, however, saw a chrysalis which,
after having tired itself in vain in its endeavours to get entirely
free of its old skin, despairing of ever being able to manage it,
abandoned it where it was so solidly fixed. We represent (Fig. 107),
rather magnified, the chrysalis arrived at its final state, and
suspended to a branch of a tree by a network of silk.[38]
[38] It has been remarked that only those whose continuance in
the pupal state is short, undergo their metamorphosis in this
apparently inconvenient position.--ED.
We come now to the mode of suspension employed by those caterpillars,
which, after having fixed themselves by the tail, strengthen the
support by means of a small silk cord passed round their body.
It is again to Réaumur, that indefatigable observer of the habits of
insects, that we go for the details of this manner of suspension.
According to Réaumur, these caterpillars make and put on this belt in
three different ways. But of these three ways the simplest, and the
least liable to meet with accident, is that employed by the larva of
the Cabbage Butterfly (_Pieris brassicæ_). When the time for its
metamorphosis is only a few days distant, one may observe this
caterpillar engaged in stretching threads on different parts of the
case in which it is confined. It then chooses a spot, which it covers
entirely with threads, some more compact than the others, and disposed
in layers, which cross each other in different directions. These
threads form a thin white cloth, against which the belly of the
caterpillar and later that of the chrysalis are applied. Very soon we
see a small hillock of silk rising. The caterpillar hooks itself on to
this by the claws of its hinder feet, and sets to work to secure
itself.
To understand this process, it suffices to know that after having
lengthened its body to a certain point, this caterpillar can turn back
its head on to its back, and reach to the fifth ring, having its three
pairs of true legs in the air. But without putting the caterpillar
into such an unnatural position, let us take it in a position in which
it is simply bent sideways in such a manner that its head, with the
thread-spinning apparatus, which is below, can be applied opposite and
pretty near to one of the legs belonging to the first pair of
membranous legs. Our caterpillar begins by fixing on this point a
thread, which is the first of those that are intended to tie it up
securely.
"This thread," says the illustrious author of the "Mémoires pour
servir à l'Histoire des Insectes," "must pass over the caterpillar's
body, and be attached by its other end near the leg corresponding to
that near which the first end was fastened. To spin the thread the
proper length, and at the same time to fix it in its proper place, the
caterpillar has only to bring round its head to the fifth segment. The
thread will be drawn from the spinning apparatus as the head advances
over half the circumference of the circle which it has to describe;
and when it has described this, there will only remain for it to
secure the second end of the thread against the support. Thus the
head, which was at first placed against one of the legs, advances
little by little on the outline of the fifth ring as far as to its
middle (Fig. 108). It is the facility the caterpillar has of reversing
its body that enables it to make its head perform this journey, in
proportion as it moves it over the circumference of the ring, it
twists its body. And at last, when it has brought it over the top of
the segment, its body is exactly folded in two; it draws it little by
little from this situation by bending towards the other side, and by
causing its head to pass gently over the last quarter of the circle.
At last the caterpillar finds itself bound on the second side; the
head rests on the thread-covered plane, and the insect fixes the
second end of the thread."
[Illustration: Fig. 108.--Caterpillars of the Cabbage Butterfly
(_Pieris brassicæ_).]
It has only to repeat the same manoeuvre as many times as there are
threads wanted to make a strong band. But each thread embraces the
head, or rather the lower part of the head, for it knows how to make
each thread it spins glide into the bend or crease of its neck by a
little movement of its head. It must disengage its head from under the
band, not a difficult operation. It causes it to slide along the
threads near one of the places where they are fixed, and it is then in
the position indicated by the foregoing engraving (Fig. 109).
[Illustration: Fig. 109.--Caterpillar of the Pieris brassicæ.]
About thirty hours after the caterpillars have succeeded in making
themselves fast, they have completed their transformation into
chrysalides (Fig. 110), where the chrysalis of the above-mentioned
caterpillar is seen in two different positions, and held by the same
band which first supported the caterpillar.
[Illustration: Fig. 110.--Pupæ of Pieris brassicæ.]
Those caterpillars which construct cocoons make them of silk and other
substances. These cocoons are, for the most part, oval or elliptical,
sometimes boat-shaped, and ordinarily white, yellow, or brown in
colour. The threads may very slightly adhere together, or be closely
united by a gummy substance with which the caterpillar lines the
interior of the cocoon, and which it expels from the anus. Some
cocoons are composed of a double envelope, others are of a uniform
texture. Some are of a tissue so close that they entirely hide the
chrysalis contained within; others form a very light covering, through
which the chrysalis can be easily perceived (Fig. 111).
Among caterpillars that make a very slight cocoon, some, as the
_Catocalas_, gather together two or three leaves into a ball, to
protect them. Others strengthen their cocoons, and render them opaque
by adding earth or other substances, often obtained from their own
bodies. Some, after having spun their cocoon, cast forth through the
anus three or four masses of a matter resembling paste, which they
apply with their head to the inside of the cocoon, and which, drying
quickly, becomes pulverulent. Others employ for the same purpose the
hairs with which their bodies are covered.
[Illustration: Fig. 111.--Cocoon, after Réaumur.]
[Illustration: Fig. 112.--Larvæ of Catocala fraxini.]
[Illustration: Fig. 113.--Larva of Acronycta aceris.]
The larva of _Acronycta aceris_ (Fig. 113) is covered with tufts of
yellow hair. Réaumur made these caterpillars work under his own eye in
glass vases. They make the layer which is to form the exterior surface
of their shell, or cocoon, of pure silk, and when it is thick enough,
tear out their hair, now from one place, now from another. But we will
leave the illustrious observer to relate this operation himself, which
must without doubt be painful to the poor animal:--
"Its two jaws are the pincers the caterpillar uses in seizing a
portion of one or other of the tufts of hair; and when it has seized
it, it tears it out without much difficulty. It at once places this
against the tissue it has already commenced, in which it entangles it
at first simply by pressure; it fixes it then more securely by
spinning over it. It does not leave off tearing out its hairs till it
has entirely stripped them off. When the caterpillar has taken between
its jaws and torn out a whole tuft of hair, the head carries it and
deposits it on some part of the lower surface of the cocoon; but it
does not leave the hairs of such a large parcel together. The next
moment one sees its head moving about very quickly; then taking a
portion of the hairs from the little heap, it distributes them about
on the neighbouring parts of the cocoon. If one opens one of these
shells before the caterpillar has become a chrysalis, the larva, which
is quite naked, and which was only known by its hair, can be no longer
recognised."
[Illustration: Fig. 114.--Larva of Acronycta aceris.]
[Illustration: Fig. 115.--Larva of Acronycta aceris taken out of its
cocoon.]
[Illustration: Fig. 116.--Larva of Chelonia caja.]
The caterpillar of the Tiger Moth, or Woolly Bear, called by Réaumur
_Marte_ or _Herisson_ (_Chelonia caja_, Fig. 116), is covered with
long inclined hairs. This caterpillar also makes use of its hairs for
strengthening the tissue of its cocoon; but whether it feels the pain
more acutely than the former, or whether it would suffer more, it does
not tear out its hairs. It adopts another system; it cuts them. The
caterpillar is then enveloped on all sides in its hair, which is to
serve in the construction of its cocoon.
[Illustration: Fig. 117.--Larva of Chelonia caja forming its cocoon.]
[Illustration: Fig. 118.--Small Caterpillar of the Pimpernel.]
[Illustration: Fig. 119.--Cocoon of the same.]
Another species uses its hairs in the composition of its cocoon; but
it adopts an entirely peculiar way of tearing them out, when the
tissue of its cocoon has become a species of network of pretty closely
packed rings. Réaumur one day saw one part of the cocoon bristling
with hairs. These were the hairs of a part of the back of the
caterpillar, which it had pushed through the rings of its cocoon. The
caterpillar then moved about as if rubbing this part of its back
successively in opposite directions against the interior surface of
the cocoon. In this way the hairs were very soon torn out and kept
retained in the rings of the cocoon. This cocoon is then bristly
inside, and does not at all suit the future chrysalis, which does not
like to be touched by any but smooth surfaces. The caterpillar then
works with its head, to lay the hairs along the interior surface, and
to keep them down by threads, which it draws over them. At another
time Réaumur saw a small hairy caterpillar, which appeared to live on
lichens, using its hair in another way. It tore them out to make its
cocoon, but it was not to lay them down and work them into a tissue.
It set them straight up like the stakes of palisades, on the
circumference of an oval space, in which it was placed. Shut up within
this palisade, it spun a light white web. This web supports the hairs,
causing the greater part of them to curve at their upper extremity,
in such a manner as to form a sort of cradle.
It remains for us now to speak of the caterpillars that make their
cocoons of silk, together with other materials. Réaumur saw the
Pimpernel caterpillar arranging and sticking together the leaves of
that plant, and spinning underneath them a thin cocoon of white silk
(Fig. 119).
Some caterpillars make their cocoons on the surface of the earth, and
even with earth. These cocoons are spherical or oblong. Their exterior
is more or less well shaped, but their interior is always smooth,
polished, shining like moistened earth, worked up together into a kind
of paste, and carefully smoothed out. This cocoon is besides lined
with a covering of silk of variable thickness. The shell is not made
of earth alone; threads of silk may be seen in it, crossing each
other, and binding together the particles of earth.
[Illustration: Fig. 120.--Larva of Cucullia verbasci.]
[Illustration: Fig. 121.--Cocoon of the Cucullia verbasci.]
These subterranean workers do not allow their proceedings to be easily
observed. Réaumur was fortunate enough to be able to notice their
skill in the construction of their shells or cocoons. The _Cucullia
verbasci_ (Fig. 120) makes itself a thick and very compact cocoon of
the form of an egg (Fig. 121). Réaumur took one of these out of the
ground before it is fortified. He tore it partially open, and placed
it in a glass vase containing sand, but the poor insect was not long
in repairing the disorder caused by the rough hand of our naturalist.
It only took four hours to restore its cocoon to its former state.
"It began," says Réaumur, "by coming almost entirely out, and left
only its hinder part within. It moved its head forwards as far as was
necessary to enable it to seize a particle of earth. As soon as it had
got its load, it re-entered the interior of the cocoon. It deposited
the grain of earth, and came out again immediately, as it did at
first, to pick up another grain, which it carried likewise into the
interior of the cocoon. This operation it continued for more than an
hour.... The provision of materials being got together, the
caterpillar now devoted his whole attention to working them up. It
began by spinning over one part of the edges of the opening. After
having put over this a small band of very loose web, the caterpillar's
head left the opening, the insect went right back again into its
cocoon, and the head returned to the opening loaded with a little
grain of earth, which it entangled in the silky threads. It then
entangled in them two or three, or a greater number of grains,
according to the quantity of threads it had spun. It bound them into
these with other threads, after which it drew threads over the edges
of another part. By thus going round the whole rim of the opening, and
by carrying and fixing the grains of earth in the threads which were
the last stretched over the opening, it rendered its diameter smaller
and smaller."
It was by working with its head that our mason gave to the new wall of
its cocoon the necessary curvature. It was interesting to know how, as
it could no longer put out its head, it could stop up the orifice.
"It knew how to change its manoeuvres. When the opening was reduced
to a circle of only a few lines in diameter, it drew threads from a
point on the circumference to another on the other side.... Thus the
opening was covered in with a rather open network.... As soon as this
web was finished, it got a grain of earth (which it had laid by until
it was wanted), brought it up, placed it against the web, and by
pushing and pressing it, made it pass through the web until it reached
the exterior, and so in succession the whole of the web was covered
with grains of earth.... It was not satisfied with rendering the
exterior of this place exactly like the rest of the shell; it
fortified it thoroughly; it added to it, one after another, layers of
grains of earth, till it was as solid and as thick as the rest."
The larva of _Pyralis corticalis_, which is found on oak trees in the
month of May, shows to what a point these little insects carry their
industry in the construction of their cocoons, in the choice of their
materials, in their manner of working them up, and in the forms they
cause them to assume. Réaumur one day saw this caterpillar on a small
branch, between two triangular appendages (Figs. 122, 123). This was
the beginning of a cocoon. Each triangular blade was composed of a
great number of small, thin, rectangular plates, taken from the bark
of the twig. The caterpillar detached with its jaws a small band of
bark, and fitted it on, and adjusted it with admirable precision
against the edge already formed. It then fixed it securely with silk
threads. Réaumur saw this caterpillar work and erect in this way a
large blade during an hour and a half.
[Illustration: Figs. 122, 123.--Cocoon of Pyralis corticalis
(magnified, proper size 2/3 inch).]
"When one sees," he says,[39] "an insect which, to construct a cocoon,
begins by collecting together an infinite number of small plates of
bark in order to compose of them two flat triangular blades; which, to
gain its end, takes means that seem so roundabout, although they are
the most suitable and the quickest it could adopt, one is very much
tempted to consider such an insect, when one sees it thus acting,
possessed of reason."
[39] Mem. 12, vol. i., p. 487.
These two blades are at last transformed into a regular cocoon. The
little animal, which is at the same time architect, cabinet-maker, and
weaver, arranges it in such a way as to form a hollow cone, which it
only remains for it to shut. Réaumur calls this sort of cocoon or
shell, _la coque en bateau_, the boat-shaped cocoon. Some caterpillars
weave cocoons of the same form with pure silk.
To bring this subject to an end, we will mention the industry of the
Puss Moth (_Dicranura vinula_), and that of a small _Tinea_, which
eats the barley stored away in our granaries.
The larva of the puss moth employs in the construction of its shell
the wood of the tree on which it has lived. It bites it up, and mixing
it with a glutinous fluid which it secretes from its mouth, reduces it
to a sort of paste, which it then uses in the formation of an
envelope, of such hardness that a knife can hardly cut into it.
The _Tinea_ lines the interior of a grain, of which it has previously
devoured the contents, with a coating of silk, and divides it thus
into two different chambers. In one of these it is to change into a
pupa; in the other it places its excrement. And so the little careful
architect constructs its house in such a manner as to find in it
tranquillity, cleanliness, and comfort.
When caterpillars have not within their reach the materials they are
in the habit of employing, like good workmen, they content themselves
with what they can get. Réaumur reared a caterpillar which formed its
cocoon of pieces of the paper of which the box was made in which it
was imprisoned.
What an extraordinary condition! what a strange phase of vitality does
the chrysalis present to us--a being occupying the middle state
between the caterpillar and the perfect insect! How little does it
resemble that which it previously was, and what it will become! In
appearance it is scarcely a living being; it takes no nourishment, and
has no digestive organs; can neither walk nor drag itself along, and
hardly bends the joints of its body. The outside skin of the
chrysalis appears to be cartilaginous; it is generally smooth,
although some species have hairs scattered over their bodies.
We distinguish in chrysalides two opposite sides. The one is the
insect's back, the other its under side. On the upper part of the
latter (Fig. 124) we perceive various raised portions, formed and
arranged like the bands round the heads of mummies; the back is plain
and rounded in a great number of pupæ; but a great many others have on
the upper part, along the edges which separate the two sides, little
humps, eminences broader than they are thick, ending in a sharp point
(Fig. 125).
[Illustration: Fig. 124.--A conical pupa.]
[Illustration: Fig. 125.--Pupa having angular projections.]
[Illustration: Fig. 126.--Angular pupa of a Butterfly.]
The head of the angular pupæ terminates sometimes in two angular
parts, which diverge from each other like two horns (Fig. 126). In
some other cases they are curved into the form of a crescent. These
appendages sometimes give to the pupa the appearance of a mask,
especially as an eminence placed on the middle of the back is rather
like a nose, and the small cavities may represent the eyes (Fig. 125).
The colours of angular pupæ attract our attention. Some are superbly
tinted; they appear to be wrapped in silk and gold. Others have only
spots of gold and silver on their belly or their back. All, however,
have not this remarkable splendour, not these metallic spots. Some are
green, yellow, and spotted with gold. Generally they are brown.
Réaumur has shown that this golden colour is not due, as was thought
for a long while, to colouring matter, but to a little whitish
membrane, placed under the skin, which reflects the light through the
thin outer pellicle, in such a manner as to produce the optical
illusion which imparts to the robe of the chrysalis the golden hues of
a princess in grand costume. _All is not gold that glitters_, Réaumur
proves literally, in the case of chrysalis.[40]
[40] The word is derived from [Greek: chrysos], golden; for
that reason pupa is a better word than chrysalis, as this only
strictly applies to a very small number; for the same reason
aurelia is a bad word.--ED.
Let us add that the chrysalis remains thus superbly dressed as long as
it is tenanted, but loses its colour as soon as the butterfly has
quitted it.
The cone-shaped pupæ belong to the twilight and night-flying
Lepidoptera, and to those butterflies whose caterpillars are
onisciform, or in shape resembling a wood-louse. They are generally
oval, rounded at the head, and more or less conical at the lower end.
Their colour is generally of a uniform chestnut brown.
What a mystery is that which is accomplished in the transition from
the chrysalis to the perfect state! Those great changes from the larva
state to that of the pupa, and from the pupa to that of the imago, are
accomplished with such rapidity, that the phenomena were looked on as
sudden metamorphoses, like those related in mythology. It has been
thought also that there was in these changes from one state to another
a sort of resurrection. There is here neither sudden metamorphosis,
nor, as we will show, resurrection. In fact, the chrysalis is a living
being; it indeed shows its vitality by exterior movements. Under the
old skin of a caterpillar about to moult, under the envelope which is
soon to be cast off, the new integuments are being prepared.
Some days before the moult, split the caterpillar's skin, and you will
find the skin which is to take its place already beneath. If, some
days before the transformation of the caterpillar into a chrysalis, it
is dissected, the rudiments of wings and antennæ may be discovered. If
a chrysalis is examined on the outside, all the parts of the future
insect can be distinguished under the skin: the wings, the legs, the
antennæ, the proboscis, &c.; only, these parts are folded and packed
away in such a manner that the chrysalis can make no use of them. It
could not, moreover, make use of them on account of their incomplete
development. Fig. 127 shows, after Réaumur,[41] a chrysalis magnified
and seen from its lower side, on which we observe:--_a_, the wings; _b
b_, the antennæ; _t_, the trunk or proboscis.
[41] Tome i., p. 382, planche 26, Fig. 6.
There is a moment when these parts, pressed one against each other,
and as it were swathed up like a mummy, are very easily seen, for they
are, as we may say, laid bare. This moment is that in which the pupa
has just quitted the caterpillar's skin. It is then still soft and
tender. Its body is moistened with a liquid, which, drying rapidly,
becomes opaque, coloured, and of a membranous consistency. The result
is that the parts which did not cohere in the least when the chrysalis
made its first appearance, are fastened together, so that although
they could at first be seen, through a layer of transparent fluid,
they are now hidden under a sort of veil or cloak. It is necessary to
seize then the moment of the birth of the chrysalis, to observe it
accurately.
On examining the pupa before the liquid which pervades these parts has
had time to dry, it resembles the perfect insect. All the exterior
parts which belong to the imago can be distinguished. One recognises
the head, which is then resting on the thorax; the two eyes and the
antennæ (Fig. 128), which are brought forward like two ribbons; the
wings also brought over the thorax, but these are separated
artificially in the drawing we have given after Réaumur;[42] and
lastly in the space left between the wings, the six legs, and the body
of the insect.
[42] Tome i., p. 382, planche 26, Fig. 7.
[Illustration: Fig. 127.--Chrysalis of the large Tortoise-shell
Butterfly (_Vanessa polychloros_), magnified, seen from the lower side.]
To sum up: the pupa, when it approaches the period for being hatched
is only a swaddled butterfly. Directly it has strength enough to rid
itself of its wrapping, the insect frees itself from its fetters; it
flies away, brilliant and free, and its many-coloured wings glitter in
the sun.
The duration of the pupa state is variable, according to the species
and the temperature. Réaumur placed in a hot-house, in the month of
January, some pupæ which, in the ordinary course of things, would not
have been hatched till the month of May, and a fortnight afterwards
the imagos had appeared. On the other hand, he shut up some pupæ in an
ice-house during the whole of a summer, and thus retarded their being
hatched by a whole year. The influence of the temperature on the
period of emerging, and, consequently, the influence of the seasons on
the length of this period, are completely brought to light by these
experiments.[43]
[43] They hardly seem from later experiments to be so fully
explained. It is a well-known fact that many insects remain in
this state a variable time--the Small Eggar (_Bombyx
lanestris_) sometimes as many as seven years.--ED.
We will now see how the insect delivers itself from the last skin. To
quit the pupa case is not so laborious an operation as it was for the
same insect to quit the caterpillar's skin. This is because the pupa
case is drier; it does not adhere to every part of the body, and is
brittle. Those insects which are enclosed in a cocoon free themselves
of the pupa envelope in the cocoon itself. To witness the last
operation, the cocoon may be opened, and the pupa drawn out of it with
care. If it is then placed in a box, the metamorphosis may be
observed. To study this last evolution more at his ease, Réaumur
covered a large extent of the wall of his study with pupæ of the
_Vanessa polychloros_ and other species.
[Illustration: Fig. 128.--Chrysalis of the Large Tortoise-shell
Butterfly (_Vanessa polychloros_) whose different parts have been
opened before they were fastened down. (_a_, wings; _b b_, antennæ;
_t_, trunk, or proboscis.)]
When the parts of the body of the insect have attained a certain
degree of solidity within the envelope, it has no great difficulty in
making the thin and friable membrane which surrounds it split in
different places. If it even distends itself or moves, a small opening
will be made in the dried skin. If the movements persist, the opening
increases in size, and very soon allows the imago to emerge.
It is on the middle of the upper part of the thorax that the envelope
begins to split. The split extends over the middle of the forehead and
back. The pieces of the thorax open, separate themselves from the
other parts to which they were fixed, and the insect can take
advantage of the opening which is made, and escape. Little by little
also it advances its head. The head is the first out of the old skin,
and the insect sets itself entirely free.
This occupies rather a long time; for we must remember that, under the
pupal envelope, its legs, its antennæ, its wings, and many other
parts, are enclosed in special cases. These peculiar circumstances
show that the animal has much trouble and must employ some time in
setting free all the parts.
At last our prisoner has come out of its narrow cell, and is delivered
from its old covering. What poet can describe to us the sensations of
this charming and frail creature which has just risen from the tomb,
and for the first time is enjoying the splendid light of day, the
radiant sky, and the flowers redolent with delicious perfumes, which
are inviting it to kiss and caress them!
The wings strike one most. They are very small at the time of birth.
[Illustration: Fig. 129.--Moth just emerged.]
[Illustration: Fig. 130.--Moth whose wings are folded up.]
Fig. 129 represents, after Réaumur,[44] a moth at the moment in which
it has just emerged from the pupa. But at the end of a short period
the wings become developed; only they are wrinkled, as Fig. 130, given
by Réaumur, represents.
[44] Tome i., p. 654, planche 46, Fig. 1.
Réaumur having taken between his fingers a very short wing of a
butterfly which was just hatched, drew it about gently in all
directions. He succeeded thus in giving it the whole extent it would
have assumed naturally. According to Réaumur the wing of a butterfly
just born, which appears so small, is really already provided with all
its parts, only it is folded and re-folded on itself. He supposes that
what his hands did to lengthen the butterfly's wings, is done
naturally by the liquids which are contained in the insect which has
just emerged, and whose wings are no longer confined in their cases.
At the time of its birth the wings are flat and thick; as they grow,
little by little they spread themselves out and become curled up. When
they are completely developed and flattened the wings become firm and
hard imperceptibly, and this firmness extends at the same time to the
whole of the body.
[Illustration: Fig. 131.--Moth whose wings are developing.]
[Illustration: Fig. 132.--Moth whose wings are developed.]
Figs. 131 and 132, borrowed, like the preceding, from the 14th Memoir
of Réaumur (_sur la transformation des chrysalides en papillons_),
show the states through which the wings of the same moth pass, before
they are thoroughly developed.
Those pupæ enclosed in cocoons free themselves entirely or in part
from their old skin, in the shell itself; but the imago is still a
prisoner. It has broken through a first enclosure; it must open itself
a way through the second. How does it manage to bore through the often
very solid walls of this second prison, so as to regain its liberty?
Réaumur states that in the Lackey Moth (_Bombyx neustria_) the head is
the only instrument of which the insect makes use in opening a
passage, the compound eyes then acting like files. These files cut the
very fine threads of which the cocoon is composed, and as soon as the
end of the cocoon is pierced through, the insect uses its thorax like
a wedge, to enlarge the hole. It very soon manages to get its two
front legs out, fixes itself by them on to the outside, and little by
little emerges from its prison.