caterpillar effects the changes by withdrawing itself from the old skin,
when it finds itself lodged in too narrow a compass. But to produce this
change, to push off the old covering, and bring forward the new, is a
work of labour and time. Those caterpillars who live in society, and
have a kind of nest or habitation, retire there to change their skins,
fixing the hooks of the feet, during the operation, firmly in the web of
their nest. Some of the solitary species spin at this time a slender
web, to which they affix themselves. A day or two before the critical
moment approaches, the insect ceases to eat, and loses its usual
activity; in proportion as the time of change advances, the colour of
the caterpillar becomes more feeble, the skin hardens and withers, and
is soon incapable of receiving those juices by which it was heretofore
nourished and supported. The insect may now be seen, at distant
intervals, to elevate its back, and stretch itself to its utmost extent;
sometimes to lift up the head, move it a little from side to side, and
then let it fall again; near the change, the second and third rings are
seen to swell considerably; by these internal efforts the old parts are
stretched and distended as much as possible, an operation which is
attended with great difficulty, as the new parts are all weak and
tender. However, by repeated exertions, all the vessels which conveyed
the nourishment to the exterior skin are disengaged, and cease to act,
and a slit is made on the back, generally beginning at the second or
third ring; the new skin may now be just perceived, being distinguished
by the freshness and brightness of its colour; the caterpillar then
presses the body like a wedge into this slit, by which means it is soon
opened from the first down to the fourth ring; this renders it large
enough to afford the insect a passage, which it soon effects in a very
curious manner. The caterpillar generally fasts a whole day after each
moulting, for it is necessary that the parts should acquire a certain
degree of consistency, before it can live and act in its usual manner;
many also perish under the operation. The body having grown under the
old skin, till the insect was become too large for it, it always appears
much larger after it has quitted the exuvia: now as the growth was
gradual, and the parts soft, the skin pressed them together, so that
they lay in a small space; but as soon as the skin is cast off, they are
as it were liberated from their bonds, and distend themselves
considerably. Some caterpillars, in changing their skin, from smooth,
become covered with fine hair; while others, that were covered with this
fine hair, have the last skin smooth.[65] The silk-worm, previous to its
chrysalis or pupa state, casts its skin four times; the first is cast on
the tenth, eleventh, or twelfth day, according to the nature of the
season; the second, in five or six days after; the third in five or six
days more, and the fourth and last in six or seven days after the third.
[65] Valmont de Bomare Dictionnaire Universel d’Histoire Naturelle,
vol. ii. 2d edit. 12mo. p. 394.
Before we describe the change of the larva into the pupa state, it will
be necessary to give the reader an account of those names by which
entomologists distinguish the different appearances of the insect in its
pupa state. It is called Coarctata, when it is straitened or confined to
a case of a globular form, without the smallest resemblance to the
structure of the insect it contains, as in the diptera. It is called
Obtecta, disguised or shrouded, when the insect is inveloped in a
crustaceous covering, consisting of two parts, one of which surrounds
the head and thorax, the other the abdomen. It is termed Incompleta,
when the pupa has perceptible wings and feet, but cannot move them, as
in most of the hymenoptera. Semicompleta; these can walk or run, but
have only the rudiments of wings. The difference between the pupa and
the larva of this class is very inconsiderable, as they eat, walk, and
act, just as they did in their primitive state; the only remarkable
difference is a kind of case which contains the wings that are to be
developed in their fly state. Completa; those designed by this name take
their perfect form at their birth, and do not pass, like other insects,
through a variety of states, though they often change their skin.
It is a general rule, that all winged insects pass through the larva and
pupa state, before they assume their perfect form: there are also
insects which have no wings, and yet undergo similar transformations, as
the bed bug, the flea, &c. Other insects, which have no wings, and which
always remain without them, never pass through the pupa state, but are
subject to considerable changes, as well with respect to the number, as
the figure of their parts; thus mites have four pair of feet, and two
smaller ones at the fore part of the body, near the head; yet some of
these are born with only three pair of feet, the fourth is not perceived
till some time after their birth.[66] The figure of the monoculus
quadricornis of Linnæus (Fauna Suecica, edit. Stockholm, 1761, No. 2049)
changes considerably after its birth.[67] The julus is an insect with a
great number of feet, some species having an hundred pair and upwards.
M. De Geer has given a description of one with more than two-hundred
pair,[68] and yet these at their birth have only three pair, the rest
are not perceived till some time after.
[66] De Geer Memoires pour servir a l’Histoire des Insectes, tom. 1.
p. 154.
[67] Ibid.
[68] Memoires des Scavans etrangers, tom. 3, p. 61.
OF THE CHANGE FROM THE LARVA TO THE PUPA STATE.
I shall now return to the caterpillar, and take notice of the care and
provision it makes to pass from the larva state into that of the pupa or
chrysalis; which is, in general, a state of imperfection, inactivity,
and weakness, through which the insect, when it has obtained a proper
size, must pass; and in which it remains often for months, sometimes for
a whole year, exposed, without any means of escaping, to every event;
and in which it receives the necessary preparations for its perfect
state, and is enabled once more to appear upon the transitory scene of
time. During its passage from one state to the other, as well as when it
is in the pupa form, the microscopical observer will find many
opportunities of exercising his instrument.
The transitions of the caterpillar from one state to another, are to it
a subject of the most interesting nature; for in passing through them,
it often runs the risk of losing its life, that precious boon of heaven,
which is ever accompanied with a degree of delight, proportioned to the
state in which the creature exists, and the use it makes of the gift it
has received. If the caterpillar could therefore foresee the efforts and
exertions it must make to put off its present form, and the state of
weakness and impotence under which it must exist while in the pupa
state, it would undoubtedly choose the most convenient place, and the
most advantageous situation, for the performance of this arduous
operation; one where it would be the least exposed to danger, at a time
when it had neither strength to resist, nor swiftness to avoid the
attack of an enemy. All these necessary instructions the caterpillar
receives from the influence of an all-regulating Providence, which
conveys the proper information to it by its own sensations: hence, when
the critical period approaches, it proceeds as if it knew what would be
the result of its operations. Different species prepare themselves for
the change different ways, suited to their nature and the length of time
they are to remain in this state.
When the caterpillar has attained to its full growth, and the parts of
the future butterfly are sufficiently formed beneath its skin, it
prepares for its change into the pupa state; it seeks for a proper place
in which to perform the important business: the different methods
employed by these little animals to secure this state of rest, may be
reduced to four: 1. Some spin webs or cones, in which they inclose
themselves. 2. Others conceal themselves in little cells, which they
form under ground. 3. Some suspend themselves by their posterior
extremity; 4. While others are suspended by a girdle that goes round
their body. I shall describe the variety in these, as well as the
industry used in constructing them, after we have gone through the
manner in which the caterpillar prepares itself for, and passes through
the pupa state.
Preparatory to the change, it ceases to take any food, empties itself of
all the excrementitious matter that is contained in the intestines,
voiding at the same time the membrane which served as a lining to these
and the stomach. The intestinal canal is composed of two principal
tubes, the one inserted into the other; the external tube is compact and
fleshy, the internal one is thin and transparent; it is the inner tube,
which lines the stomach and intestines, that is voided with the
excrement before the change. It generally perseveres in a state of rest
and inactivity for several days, which affords the external and internal
organs that are under the skin an opportunity of gradually unfolding
themselves. In proportion as the change into the pupa form approaches,
the body is observed often to extend and contract itself; the hinder
part is that which is first disengaged from the caterpillar skin; when
this part of the body is free, the animal contracts and draws it up
towards the head; it then liberates itself in the same manner from the
two succeeding rings, consequently the insect is now lodged in the fore
part of its caterpillar covering; the half which is abandoned remains
flaccid and empty, while the fore part is swoln and distended. The
animal, by strong efforts, still forcing itself against the fore part of
the skin, bursts the skull into three pieces, and forms a longitudinal
opening in the three first rings of the body; through this it proceeds,
drawing one part after the other, by alternately lengthening and
shortening, swelling and contracting the body and different rings; or
else, by pushing back the exuvia, gets rid of its odious reptile form.
The caterpillar, thus stripped from its skin, is what we call the pupa,
chrysalis, or aurelia, in which the parts of the future moth are
inclosed in a crustaceous covering, but are so soft, that the slightest
touch will discompose them. The exterior part of the chrysalis is at
first exceedingly tender, soft, and partly transparent, being covered
with a viscous fluid; this soon dries up, thickens, and forms a new
covering for the animal, capable of resisting external injuries; a case,
which is at the same time the sepulchre of the caterpillar, and the
cradle of the moth; where, as under a veil, this wonderful
transformation is carried on.
The pupa has been called a chrysalis, or creature made of gold, from the
resplendent yellow colour with which some kinds are adorned. Reaumur has
shewn us whence they derive this rich colour; that it proceeds from two
skins, the upper one a beautiful brown, which lies upon or covers a
highly polished and smooth white skin: the light reflected from the
last, in passing through, gives it the golden yellow, in the same manner
as this colour is often given to leather; so that the whole appears
gilt, although no gold enters into the tincture. The chrysalis of the
common white butterfly furnishes a most beautiful object for the
lucernal opake microscope.
Those who are desirous to discover distinctly the various members of the
moth in the pupa, should examine it before the fore-mentioned fluid is
dried up, when it will be found to be only the moth with the members
glued together; these, by degrees, acquire sufficient force to break
their covering, and disengage themselves from the bands which confine
them. While in this state, all the parts of the moth may be traced out,
though so folded and laid together, that it cannot make any use of them;
nor is it expedient that it should, as they are too soft and tender to
be used, and pass through this state merely to be hardened and
strengthened.
To examine the moth concealed under the skin of a caterpillar, one of
them should be taken at the last change; when the skin begins to open,
it should be drowned in spirit of wine, or some strong liquor, and be
left therein for some days, that it may take more consistency and harden
itself; the skin of the caterpillar may then be easily removed: the
chrysalis, or feeble moth, will be first discovered, after which the
tender moth may be traced out, and its wings, legs, antennæ, &c. may be
opened and displayed by an accurate observer.
The parts of the moth or butterfly are not disposed exactly in the same
manner in the body of the caterpillar, as when left naked in the
chrysalis. The wings are longer and narrower, being wound up into the
form of a cord, and the antennæ are rolled up on the head; the tongue is
also twisted up and laid upon the head, but in a very different manner
from what it is in the perfect animal, and different from that which it
lies in within the chrysalis; so that it is by a progressive and gradual
change, that the interior parts are prepared for the pupa and moth
state. The eggs, hereafter to be deposited by the moth, are also to be
found, not only in the chrysalis, but in the caterpillar itself,
arranged in their natural and regular order.
While in this state, the creature generally remains immoveable, and
seems to have no other business but patiently to attend the time of its
change, which depends on the parts becoming hard and firm, and the
transpiration of that humidity which keeps them soft; the powers of life
are as it were absorbed in a deep sleep; the organs of sensation seem
obliterated, being imprisoned by coverings more or less strong, the
greater part remains fixed in those situations which the caterpillar had
selected for them till their final metamorphosis; some, however, are
capable of changing place, but their movements are slow and painful.
The time, therefore, which the moth or butterfly remains in the pupa
state is not always the same, varying in different species, and
depending also upon the warmth of the weather, and other adventitious
circumstances; some remain in that situation for a few weeks; others do
not attain their perfect form for eight, nine, or eleven months: this
often depends on the season in which they assume the pupa form, or
rather on the time of their birth. Some irregularities are also
occasioned by the different temperature of the air, by which they are
retarded or accelerated, so as to be brought forward in the season best
suited to their nature and the ends of their existence. I have heard of
an instance, where the pupa, produced from caterpillars of the same
eggs, nourished in the same manner, and which all spun up within a few
days of each other in the autumn, came into the fly state at three
different and distant periods; viz. one-third of them the spring
following their change, one-third more the succeeding spring, and the
remainder the spring after, making three years from their first
hatching; a further and manifest proof of the beauty and wisdom of the
laws of Divine order, which are continually operating for the best
interests of all created beings. As the transformation of insects is
retarded by cold, and accelerated by heat, the ordinary period of these
changes may sometimes be altered, by placing them in different degrees
of heat or cold; by these they may be awakened sooner to a new state of
existence, or kept in one of profound sleep.[69]
[69] Reaumur Memoires sur les Insectes, tom. 2, mem. 1.
There are some caterpillars which remain in their cone eight or nine
months before they acquire the complete pupa state; so that their
duration in that form is much shorter than it naturally appears to be.
OF THE PREPARATION OF THE CATERPILLAR FOR THE METAMORPHOSES.
The industry of the caterpillar, in securing itself for its change into
the chrysalis, must not be passed by; not only because it naturally
leads the reader to consider and admire that divine agency, by which the
insect is informed, but because the different modes it makes use of
cannot be properly investigated, without the assistance of glasses, it
therefore consequently becomes a proper subject for the microscope; we
shall select from a great variety, a few instances, to animate the
reader in these researches.
Some caterpillars, towards the time of their change, suspend themselves
from the branch of a tree, with the head downwards; in this position
they assume the pupa form, and from thence immerge a butterfly or moth.
In order to secure itself in this position, the insect covers with
threads that part of the branch from which it means to suspend itself;
it places these in different directions, and then covers them with other
threads, laying on several successive thicknesses, each new layer being
smaller in size than that which preceded it; forming, when finished, a
little cone or hillock of silk, as will be found when examined by the
microscope. The caterpillar hooks itself by the hinder feet to this
hillock, and when it has found by several trials that it is strongly
fixed thereto, throws itself forward, letting the body fall with the
head downwards. Soon after it is thus suspended, it bends the fore part
of the body, keeping this bent posture for some time, then straitening
the body, again in a little time bending it, and so on, repeating this
operation till it has formed a slit in the skin upon the back; part of
the pupa soon forces itself through this, and extends the slit as far as
the last crustaceous feet; the pupa then forces upwards the skin, as we
would push down a stocking, by means of its little hooks and the motion
of the body, till it has slipped it off to that part from which the
caterpillar had suspended itself. But the pupa has still to disengage
itself from this small packet, to which the exuvia is now reduced: here
the observer will find himself interested for the little animal, anxious
to learn how the pupa will quit this skin, and how it will be enabled to
fix itself to the hillock, as it has neither arms nor legs. A little
attention soon explains the operation, and extricates the observer from
his embarrassment. It seizes the exuvia by the rings of the body, and
thus holds itself as it were by a pair of pincers; then, by bending the
tail, it frees itself from the old skin, and by the same method soon
suspends itself to the silken mount; it lengthens out the hinder part of
the body, and clasps, by means of its rings, the various foldings of the
exuvia, one after another; thus creeping backward on the spoils, till it
can reach the hillock with the tail; which, when examined by the
microscope, will be found to be furnished with hooks to fix itself by.
It is surprizing to see with what exactness and ease these insects
perform an operation so delicate and dangerous, which is only executed
once in their lives; and nought else can account for it, but the
consideration that HE, who designed that the caterpillar should pass
through these changes, had provided means for that end, regularly
connecting the greater steps by intermediate ones, the desire of
extending their species forming and acting upon the organization, till
the purposes of their life are completed. Different kinds of these
insects require variety in the mode of suspension; some fix themselves
in an horizontal position, by a girdle which they tie round their body;
this girdle appears to the naked eye as a single thread; when examined
with the microscope, it will be found to be an assemblage of fine
threads, lying close to each other, so fixed as to support the
caterpillar, and yet leave it in full freedom to effect the changes.
Like the preceding kind, it fixes the girdle to the branch of a tree; in
this situation it remains for some time motionless, and then begins to
bend, move, and agitate its body in a very singular manner, till it has
opened the exterior covering, which it pushes off and removes much in
the same manner as we have described in the preceding article, and yet
with such dexterity, that the pupa remains suspended by the same girdle.
OF THE IMAGO OR FLY STATE OF INSECTS.
As soon as the moth acquires sufficient strength to break the bonds
which surround it, and of which it is informed by its internal
sensations, it makes a powerful effort to escape from its prison, and
view the world with new-formed eyes. The moth frees itself from the pupa
with much greater ease than the pupa from the caterpillar; for the case
of the pupa becomes so dry, when the moth is near the time of throwing
off its covering, that it will break to pieces if it be only gently
pressed between the fingers; and very few of the parts will be found, on
examination, to adhere to the body. Hence, when the insect has acquired
a proper degree of solidity, it does not require any great exertion to
split the membrane which covers it. A small degree of motion, or a
little inflation of the body, is sufficient for this purpose; these
motions reiterated a few times, enlarge the hole, and afford the moth
room to escape from its confinement. The opening through which they pass
is always at the same part of the skin, a little above the trunk,
between the wings, and a small piece which covers the head; the
different fissures are generally made in the same direction. If the
outer case be opened, it is easy to discover the efforts the insect
makes to emancipate itself from its shell; when the operation begins,
there seems to be a violent agitation in the humours contained in the
little animal; the fluids seem to be driven with rapidity through all
the vessels, and it is seen to agitate its legs, &c. as it were
struggling to get free; these efforts soon break its brittle skin. The
loosening the exterior bands of the pupa is not the only difficulty many
moths have to encounter with; it has often also to pierce the cone or
case in which it has been inclosed, and that at a time when its members
are very feeble, when it is no longer furnished with strong jaws to
pierce and cut its way through; but by the regular laws of divine order,
means are furnished to every creature of attaining the end for which it
was produced: thus, in the present case, some of these insects are
provided with a liquor with which they soften and weaken the end of the
cone; some leave one end feeble, and close it only with a few threads,
so that a slight effort of the head enables the moth to burst the prison
doors, and immerge into day.
When the moth first sees the day, it is humid and moist; but this
humidity soon evaporates, the interior parts dry and harden as well as
the exterior; the wings, which are wrinkled, being thick and small, then
extend themselves, strengthen and harden insensibly, and the fibres
which were at first flexible, become hard and stiff; so much so, that
Malpighi considered them as bones: in proportion as these fibres harden,
the fluid which circulates within them, and extends the wings, loses its
force; so that if any extraneous circumstance prevent the motion of this
fluid, at the first instant of the moth’s escape from its former state,
the wings will then become ill-shaped; often expanding with such
rapidity, that the naked eye cannot trace their unfolding. The wings,
which were scarce half the length of the body, acquire in a few minutes
their full size, so as to be nearly five times as large as they were
before: nor is it the wings only which are thus increased; all their
spots and colours, heretofore so minute as to be scarce discernible, are
proportionably extended, so that what before appeared as only so many
unmeaning and confused points, become distinct and beautiful ornaments;
and those that are furnished with a tongue or trunk, curl and coil it
up. When the wings are unfolded, the tongue rolled up, the moth
sufficiently dried, and the different members strengthened, it takes its
flight. Most of them, soon after they have attained their perfect state,
void an excrementitious substance; Reaumur thinks that they eject very
little, if any, during the rest of their lives.
In the progress of these insects, such changes take place, as we could
have formed no conception of, if the great Author of these wonders had
not been pleased to reward the industrious naturalist with the
discovery.
If the moth be opened down the belly, and the unctuous parts which fill
it, be removed, the gross artery, which has been called the heart, will
be visible, and the contractions and dilatations, by which it pushes
forward the liquor it contains, may be easily observed. One of the most
remarkable circumstances is, that the circulation of this fluid in the
moth is directly contrary to that which took place in the caterpillar;
in this, the liquor moved from the tail to the head, whereas in the
moth, it moves from the head to the tail; so that the fluid which
answers the purposes of the blood in the moth, goes from the superior,
towards the inferior parts, but in the voracious sensual caterpillar,
the order is inverted, it proceeds from the inferior towards the
superior parts; all its members, formerly soft, inactive, and folded up
under an envelope, are expanded, strengthened, and exposed to
observation.
The food of the caterpillar is gross and solid, and even this it is
obliged to earn with much labour and danger; but, when freed as it were
from the jaws of death, and arrived at its perfect form, the purest
nectar is its potion, and the air its element. It was supplied with
coarse food, in the first state, by the painful operation of its teeth,
which was afterwards digested by a violent trituration of the stomach.
The intestines are now formed in a more delicate manner, and suited to a
more pure and elegant aliment, which nature has prepared for its use
from the most fragrant and beautiful flowers. Many internal parts of the
caterpillar disappear in the chrysalis, and many that could not be
perceived before, are now rendered visible: the interior changes are not
less surprizing than those of the exterior form, and are, properly
speaking, creative of them; for it is from these the exterior form
originates, and with these it always corresponds. In a word, the
creature that heretofore crept upon the earth, now flies freely through
the air; and far from creating our aversion by its frightful prickles
and foul appearance, it attracts our notice by the most elegant shape
and apparel, and, from being scarce able to move from one shrub to
another, acquires strength and agility to tower far above the tallest
inhabitant of the forest.
OF THE SILK-WORM.