themselves, in order to prepare for their transformation in security, is
more generally known, as it is from one species of these that we derive
so many benefits, namely from the silk-worm, whose works afford an
ornament for greatness, and add magnificence to royalty. All
caterpillars undergo similar changes with it, and many in the butterfly
state greatly exceed it in beauty: but the golden tissue, in which the
silk-worm wraps itself, far surpasses the silky threads of all the other
kinds; they may indeed come forth with a variety of colours, and wings
bedecked with gold and scarlet, yet they are but the beings of a
summer’s day; both their life and beauty quickly vanish, and leave no
remembrance after them; but the silk-worm leaves behind it such
beneficial monuments, as at once record the wisdom of its Creator, and
his bounty to man.[70]
[70] Pullein on the Culture of Silk.
The substance of which the silk is formed, is a fine yellow transparent
gum, contained in two reservoirs that wind about the intestines, and
which, when they are unfolded, are about ten inches long; they terminate
in two exceeding small orifices near the mouth, through which the silk
is drawn, or spun to the degree of fineness which its occasions may
require. This apparatus has been compared to the instrument used by
wire-drawers, and by which gold and silver is drawn to any degree of
minuteness. From each of these reservoirs proceeds a thread, which are
united afterwards; so that if it be examined by the microscope, it will
be found to consist of two cylinders or threads glued together, with a
groove in the middle; a separation may sometimes be perceived.
When the silk-worm has found a convenient situation, it sets to work,
first spinning some random threads, which serve to support the future
superstructure; upon these it forms an oval of a loose texture,
consisting of what is called the floss-silk; within this it forms a firm
and more consistent ball of silk, remaining during the whole business
within the circumference of the spheroid that it is forming, resting on
its hinder parts, and with its mouth and fore legs directing and
fastening the threads. These threads are not directed in a regular
circular form, but are spun in different spots, in an infinite number of
zig-zag lines; so that when it is wound off, it proceeds in a very
irregular manner, sometimes from one side of the cone, then from the
other. This thread, when measured, has been found to be about
three-hundred yards long, and so fine, that eight or ten are generally
rolled off into one by the manufacturers. The silk-worm usually employs
about three days in finishing this cone; the inside is generally smeared
with a kind of gum, that is designed to keep out the rain: in this cone
it assumes the pupa form, and remains therein from fifteen to thirty
days, according to the warmth of the climate. When the moth is formed,
it moistens the end of this cone, and by frequent motions of the head
loosens the texture of the silk, so as to form a hole without breaking
it.
When the silk-worm has acquired its perfect growth, the reservoirs of
silk are full, and it is pressed by its sensations to get rid of this
incumbrance, and accordingly spins a cone, the altitude and size of
which are proportioned to its wants: by traversing backwards and
forwards, it is relieved, and attains by an innate desire the end for
which it was formed; and thus a caterpillar, whose form is rather
disgusting to the human unphilosophic eye, becomes a considerable object
of manufacture and trade, a source of wealth, and, from the extensive
employment it affords, a blessing to thousands. The size of the cone is
not always proportioned to that of the caterpillar; some that are small
construct larger cones than others which exceed them in bulk.
There is a caterpillar which forms its silken cone in the shape of a
boat turned bottom upwards, whence it is called by Reaumur the “coque en
batteau;” the construction is complicated, and seems to require more art
than is usually attributed to this insect. It consists of two principal
parts, shaped like shells, which are united with considerable skill and
propriety; each shell or side is framed by itself, and formed of an
innumerable quantity of minute silk rings; in the fore part there is a
projection, in which a small crevice may be perceived, which serves,
when opened, for the escape of the moth; the sides are connected with so
much art, that they open and shut as if framed with springs; so that the
cone, from which the butterfly has escaped, appears as close as that
which is still inhabited.
Those caterpillars which are not furnished with a silky cone, supply
that want with various materials, which they possess sufficient skill to
form into a proper habitation, to secure them while preparing for the
perfect state; some construct theirs with leaves and branches, tying
them fast together, and then strengthening the connection; others
connect these leaves with great regularity; many strip themselves of
their hairs, and form a mixture of hair and silk; others construct a
cone of sand, or earth, cementing the particles with a kind of glue;
some gnaw the wood into a kind of saw-dust, and glue it together; with
an innumerable variety of modes suited to their present and future
state.
OF THE BEETLE.
To make the reader more fully acquainted with a subject which affords
such abundant matter for the exercise of his microscope, I shall
proceed to describe, in as concise a manner as I am able, the changes of
a few insects of different classes, beginning with the beetle.
The beetle is of the first or coleopterous class, having four wings. The
two upper ones are crustaceous, and form a case to the lower ones; when
they are shut, there is a longitudinal suture down the back: this
formation of the wings is necessary, as the beetle often lives under the
surface of the earth, in holes which it digs by its own industry and
strength. These cases save the real wings from the damage which they
might otherwise sustain, by rubbing or crushing against the sides of its
abode; they serve also to keep the wings clean, and produce a buzzing
noise when the animal rises in the air. The strength of this insect is
astonishing; it has been estimated that, bulk for bulk, their muscles
are a thousand times stronger than those of a man!
The beetle is only an insect disengaged from the pupa form; the pupa is
a transformation in like manner from the worm or larva, and this
proceeds from the egg; so that here, as in the foregoing instances, one
insect is exhibited in four different states of life, after passing
through three of which, and the various inconveniences attendant on
them, it is advanced to a more perfect state. When a larva, it trains a
miserable existence under the earth; in the pupa form it is deprived of
motion, and as it were dead; but the beetle itself lives at pleasure
above and under ground, and also in the air, enjoying a higher degree of
life, which it has attained by slow progression, after passing through
difficulties, affliction, and death.
If we judge of the rank which the beetle holds in the scale of
animation, from the places where they are generally found, from the food
which nourishes them, from the disgusting and odious forms of many,
from their antipathy to light, and their delight in darkness, we shall
not form great ideas of the dignity of their situation. But as all
things are rendered subservient to the laws of divine order, it is
sufficient for us to contemplate the wonders that are displayed in this
and every other organ of life, for the reception of which, from the
FOUNTAIN AND SOURCE OF ALL LIFE, each individual is adapted, and that in
a manner corresponding to the state of existence it is to enjoy, and the
energies it is called forth to represent.
The egg of the rhinoceros beetle[71] is of an oblong round figure, of a
white colour; the shell thin, tender, and flexible; the teeth of the
worm that is within the shell come to perfection before the other parts;
so that as soon as it is hatched, it is capable of devouring, and
nourishing itself with the wood among which it is placed. The larva or
worm is curiously folded in the egg, the tail resting between the teeth,
which are disposed on each side the belly; the worm in proper time
breaks the shell, in the same manner as a chicken, and crawls from
thence to the next substance suitable for its food. The worm, when it is
hatched, is very white, has six legs, and a wrinkled naked body, but the
other parts are all covered with hair; the head is then also bigger than
the whole body, a circumstance which may be observed in larger animals,
and which is founded on wise reasons.[72] If the egg be observed from
time to time while the insect is within it, the beating of the heart may
be perceived.
[71] Scarabæus Acteon, Lin. Syst. Nat. p. 541-3.
[72] Swammerdam’s Book of Nature, pt. 1, p. 33.
The eggs of the earth-worm, the snail, and the beetle, will afford many
subjects for the microscope, and will be found to deserve a very
attentive examination. Swammerdam was accustomed to hatch them in a
dish, covered with white paper, which he always kept in a moist state.
To preserve these and similar eggs, they must be pierced with a fine
needle; the contained liquors must be pressed out, after which they
should be blown up by means of a small glass tube, and then filled with
a little resin dissolved in oil of spike.
The worm of the rhinoceros beetle, like other insects in the larva
state, changes its skin; in order to effect which, it discharges all its
excrement, and forms a convenient hole in the earth, in which it may
perform the wonderful operation; for it does not, like the serpent, cast
off merely an external covering, but the throat, a part of the stomach,
and the inward surface of the great gut, change at the same time their
skin: as if it were to increase the wonder, and to call forth our
attention to these representative changes, some hundreds of pulmonary
pipes cast also each its delicate skin, a transparent membrane is taken
from the eyes, and the skull remains fixed to the exuvia. After the
operation, the head and teeth are white and tender, though at other
times as hard as bone; so that the larva, when provoked, will attempt to
gnaw iron. For an accurate anatomical description of this worm, I must
refer the reader to Swammerdam; he will find it, like the rest of this
author’s works, well worthy of his attentive perusal. To dissect it, he
first killed it in spirit of wine, or suffocated it in rain water rather
more than lukewarm, not taking it out from thence for some hours. This
preparation prevents an improper contraction of the muscular fibres.
When the time approaches for the worm to assume the pupa form, it
generally penetrates deeper into the ground,[73] or those places where
it inhabits, to find a situation that it can more easily suit to its
subsequent process. Having found a proper place, it forms with the
hinder feet a polished cavity, in this it lies for sometime immoveable;
after which, by voiding excrementitious substances, and by the
evaporation of humidity, it becomes thinner and shorter, the skin more
furrowed and wrinkled, so that it soon appears as if it were starved by
degrees. If it be dissected about this period, the head, the belly, and
the thorax may be clearly distinguished. While some external and
internal parts are changing by a slow accretion, others are gently
distended by the force of the blood and impelled humours. The body
contracting itself, while the blood is propelled towards the head,
forces the skull open in three parts, and the skin in the middle of the
back is separated, by means of an undulating motion of the incisions of
the back; at the same time the eyes, the horns, the lips, &c. cast their
exuvia. During this operation, a thin watery humour is diffused between
the old and new skin, which renders the separation easier. The process
going on gradually, the worm is at last disengaged from its skin, and
the limbs and parts are, by a continual unfolding, transformed into the
pupa state; after which, it twists and compresses the exuvia by the
fundament, and throws it towards the hinder part under the belly. The
pupa is at this time very delicate, tender, and flexible; and affords a
most astonishing appearance to an attentive observer. Swammerdam thinks
it is scarce to be equalled among the wonders which are displayed in the
insect part of the creation; in it the future parts of the beetle are
finely exhibited, so disposed and formed, as soon to be able to serve
the creature in a more perfect state of life, and to put on a more
elegant form.
[73] The larvæ of those beetles which live under ground are in general
heavy, idle, and voracious; on the contrary, the larvæ which inhabit
the waters are exceedingly active.
The pupa[74] of this insect weighs, a little after its change, much
heavier than it does in its beetle state; this is also the case with
the pupa of the bee and hornet. The latter has been found to weigh ten
times as much as the hornet itself; this is probably occasioned by a
superabundant degree of moisture, by which these insects are kept in a
state of inactivity, which may be compared to a kind of preternatural
dropsy, till it is in some measure dissipated; in proportion as this
moisture is evaporated, the skin hardens and dries: some days are
required to exhale this superfluous moisture. If the skin be taken off
at this time, many curious circumstances may be noted; but what claims
our attention most is, that the horn, which is so hard in the male
beetle when in a state of maturity, that it will bear to be sharpened
against a grindstone,[75] in the pupa state is quite soft, and more like
a fluid than a solid substance. How long the scene of mutation continues
is not known; some remain during the whole winter, more particularly
those which quit the larva state in autumn, when a sudden cold checks
their further operations, and consequently they remain in a torpid
state, without any food, for several months. Some species of the beetle
tribe go through all the stages of their existence in a season, while
others employ near four years in the process, and live as winged insects
a year.
[74] Swammerdam’s Book of Nature, p. 144.
[75] Mouffet, p. 152.
When the proper time for the final change arrives, all the muscular
parts grow strong, and are thus more able to shake off their last
integuments, which is performed exactly in the same manner as in the
passage of the insect from the larva to the pupa state; so that in this
last skin, which is extremely delicate, the traces of the pulmonary
tubes, that have been pulled off and turned out, again become visible.
All parts of the insect, and more particularly the wings and their
cases, are at this period swelled and extended by the air and fluids
which are driven into them through the arteries and pulmonary tubes; the
wings are now soft as wet paper, and the blood issues from them on the
least wound; but when they have acquired their proper consistency, which
in the elytra is very considerable, they do not exhibit the least sign
of any fluid within them, though cut or torn almost asunder. The pupa
being disengaged from its skin, assumes a different form, in which it is
dignified with the name of a beetle, and acquires a distinction of sex,
being either male or female. The insect now begins to enjoy a life far
preferable to its former state of existence; from living in dirt and
filth, under briars and thorns, it raises itself towards the skies,
plays in the sun-beam, rejoices in its existence, and is nourished with
the oozing liquors of flowers.
OF THE MUSCA CHAMÆLEON.
I shall now proceed to illustrate the nature of the different
transformations in insects, by giving an account of the musca chamæleon.
In the worm or larva condition it lives in the water, breathes by the
tail, and carries its legs within a little snout near its mouth. When
the time arrives for its pupa state, it goes through the change without
casting off the skin of the larva. Lastly, in the imago, or fly state,
it would infallibly perish in the water, that element which had hitherto
supplied it with life and motion, was not the larva by nature instructed
where to choose a suitable situation for its approaching transformation.
This insect is characterized by Linnæus as “Musca chamæleon. Habitat
larva in aquis dulcibus; musca supra aquam obambulare solet.” In a
former edition of the Fauna Suecica he called it oestrus aquæ; but on a
more minute examination, he found it was a musca; besides, the larvæ of
all known oestri are nourished in the bodies of animals. The larva of
this insect appears to consist of twelve annular divisions, see Plate
XI. Fig. 1. by these it is separated into a head, thorax, and abdomen;
but as the stomach and intestines lie equally in the thorax and
abdomen, it is not easy to distinguish their limits until the insect
approaches the pupa state. The parts most worthy of notice are the tail
and snout. The tail is furnished with an elegant crown or circle of hair
b, disposed quite round it in an annular form; by means of this the tail
is supported on the surface of the water, while the worm or larva is
moving therein, the body in the mean while hanging towards the bottom;
it will sometimes remain in this situation for a considerable time,
without the least sensible motion. When it is disposed to sink to the
bottom by means of its tail, it generally bends the hairs of that part
towards each other in the middle, but much closer towards the extremity;
by these means a hollow space is formed, and the bladder of air pent up
in it looks like a pearl, Fig. 2. Plate XI. It is by the assistance of
this bubble, or little balloon, that the insect raises itself again to
the surface of the water. If this bubble escape, it can replace it from
the pulmonary tubes; sometimes large quantities of air may be seen to
arise in bubbles from the tail of the worm to the surface of the water,
and there mix with the incumbent atmosphere. This operation may be
easily seen by placing the worm in a glass full of water, where it will
afford a very entertaining spectacle. The snout is divided into three
parts, of which that in the middle is immoveable; the two other parts
grow from the sides of the former; these are moveable, vibrating in a
very singular manner, like the tongues of lizards and serpents. The
greatest strength of the creature is fixed in these lateral parts of the
snout; it is on these that it walks when it is out of the water,
appearing, as it were to walk on its mouth, using it to assist motion,
as a parrot does its beak to climb, with greater advantage.
We shall now consider the external figure of this worm, as it appears
with the microscope. It is small toward the head, larger about those
parts which may be considered as the thorax; it then again diminishes,
converging at the abdomen, and terminates in a sharp tail, surrounded
with hairs in the form of the rays of a star.
This worm, the head and tail included, has twelve annular divisions, 1,
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, Fig. 3. Plate XI. Its skin resembles
the covering of those animals that are provided with a crustaceous
habit, more than it does that of naked worms or caterpillars; it is
moderately hard, and like the rough skin called shagreen, being thick
set with a number of grains, evenly distributed. The substance of the
skin is firm and hard, and yet very flexible. On each side of the body
are nine spiracula or holes, for the purpose of respiration; there are
no such holes visible on the tail ring _a_, nor on the third ring
counting from the head; for at the extremity of the tail there is an
opening for the admission and expulsion of air; in the third ring the
spiracula are very small, and appear only under the skin, near the place
where the embryo wings of the future fly are concealed. It is remarkable
that caterpillars, in general, have two rings without these spiracula;
perhaps, because they change into flies with four wings, whereas this
worm produces a fly that has only two. The skin has three different
shades of colour; it is adorned with oblong black furrows, with spots of
a light colour, and orbicular rings, from which there generally springs
a hair, as in the figure before us, only the hair that grows on the
insect’s side is represented; besides this, there are here and there
some other larger hairs, c c. The difference of colour in this worm
arises from the quantity of grains in the same space; for in proportion
as there is a greater or lesser quantity of these, the furrows and rings
are of a deeper or paler colour. The head _d_ is divided into three
parts, and covered with a skin, the grains on which are hardly
discernible. The eyes are rather protuberant, and lie forwards near the
snout. It has also two small horns i i, on the fore part of the head.
The snout is crooked, and ends in a sharp point as at f; but what is
altogether singular and surprizing, though no doubt wisely contrived by
the great and almighty Architect, is, that this insect’s legs are placed
near the snout, between the sinuses, in which the eyes are fixed. Each
of these legs consists of three joints, the outermost of which is
covered with hard and stiff hairs like bristles. From the next joint
there springs a horny bone h h, which the insect uses as a kind of
thumb; the joint is also of a black substance, between bone and horn in
hardness; the third joint is of the same nature. To distinguish these
particulars, the parts that form the upper sides of the mouth and the
eyes must be separated by means of a small fine knife; you may then, by
the assistance of the microscope, perceive that the leg is articulated,
by means of some particular ligaments, with that portion of the insect’s
mouth which answers to the lower jaw in the human frame. We may then
also discern the muscles which serve to move the legs, and draw them up
into a cavity that lies between the snout and those parts of the mouth
which are near the horns i i.
This insect not only walks with these legs at the bottom of the water,
but even moves itself on land by means of them; it likewise makes use of
them to swim, while it keeps its tail on the surface contiguous to the
air, and hangs downward with the rest of the body in the water: in this
situation no motion is perceived in it, but what arises from its legs,
which it moves in a most elegant manner. It is reasonable to conclude
from what has been said, that the principal part of the creature’s
strength lies in these legs; nor will it be difficult for those who are
acquainted with the nature of the ancient hieroglyphics, which are now
opening so clearly, to fix the rank of this insect in animated life, and
point out those orders of being, and the moral state through which it
receives its form and habits of life.
The snout is black and hard, the back part is quite solid, and somewhat
of a globular form, whereas the front f, is sharp and hollow; on the
back part three membranaceous divisions may be observed, by means of
which, and the muscles contained in the snout, the insect can at
pleasure expand or contract it.