animal, great pains have been taken by many naturalists to investigate
the nature of this action in insects; to prove its existence, and
explain in what manner it is carried on. Malpighi, Swammerdam, Reaumur,
and Lyonet have discovered in the caterpillar two air-vessels placed the
whole length of the insect, these they have called the tracheæ; they
have also shewn that an infinite number of ramifications proceed from
these, and are dispersed through the whole body; that the tracheal
vessels communicate with particular openings on the skin of the
caterpillar, termed spiracula; there are nine of these on each side of
the body. These vessels seem calculated for the reception of air; they
contain no fluids, are of a cartilaginous nature, and when cut preserve
their figure, and exhibit a well-terminated opening. Notwithstanding
this discovery, respiration has not been proved to exist in many species
of insects, and the mechanism thereof is very obscure in all; nor is the
absence of it more surprising in the caterpillar or embryo state of
insects, than in that of other animals, where we find that respiration
is by no means necessary to existence previous to their birth, though
indispensably so afterwards.
Reaumur thought that the air entered by the spiracula into the trachea,
but was not expelled by the same orifice, and consequently that the
respiration of insects was carried on in a manner totally different from
that of other animals; that the air was expired through a number of
small holes or pores which are to be found in the skin of the
caterpillar, after having been conducted to them through the extremities
of the finer ramifications of the tracheal vessels; whereas Bonnet, in
consequence of a great variety of experiments, supposed that the
inspiration and expiration of the air was through the spiracula, and
that there was no expiration of air through the pores of the skin. These
experiments were made either by plunging the caterpillars into water, or
anointing them with fat and greasy substances, some all over, others
only partially. The number of small bubbles which are observed to cover
the surface of their bodies, when they are immerged in water, does not
arise from the air which is included within, and then proceeding from
them, but they are formed by the air which is lodged near the surface of
their bodies, in the same manner that it is about all other substances.
To render the experiments more accurate, and prevent the air from
adhering to the skin, before he plunged the caterpillars in water he
always brushed them over with an hair pencil; after this, very few air
bubbles were found on their bodies when immerged in water. Caterpillars
will remain a considerable time under water, without destroying the
principle of life; and they also recover, in general, soon after they
are taken out. To know whether a few only of the spiracula might not be
sufficient for the purposes of respiration, he plunged some partially in
water, so that only two or more spiracula remained in the open air: in
these cases the caterpillar did not become torpid as it did when they
were all immerged in water. One caterpillar, upon which Bonnet made his
experiments, lived eight days suspended in water, with only two of its
anterior spiracula in the air; during this time he observed, that when
the insect moved itself, little streams of bubbles issued from the
anterior spiracula on the left side; from this, and many other
experiments, it appeared to him, that amongst all the eighteen
spiracula, the two anterior and the two posterior are of the greatest
use in respiration.[79] Sometimes when the apertures of these have been
stopped with oil, the caterpillar has fallen into convulsions. If the
posterior part had been oiled, that part became paralytic.
Notwithstanding these experiments, and many more which have been made,
the subject is far from being decided, and many still doubt whether
there is any respiration in insects similar to ours, at least at certain
periods of their life. This opinion seems to be further confirmed by the
experiments of M. Lyonet. He confined several large musk beetles under a
glass for more than half an hour, exposed to the fumes of burning
sulphur; and, though during their continuance there the vapour was so
thick that he could not see them, yet on their being liberated, they did
not seem at all effected thereby.[80]
[79] Philos. Trans. vol. xlv. p. 300.
[80] Lesser Theologie des Insectes, tom. 1, p. 124. Ibid. p. 126.
Supposing respiration to be absolutely necessary to the existence of the
pupæ of different insects, when we reflect on the great solidity of
their cases or cones, it is not easy to conceive how they can live
several months under the earth, in spaces so confined, and almost
impervious to the air: and indeed if they did respire, the same
situation seems to preclude a continuance of the operation, as the air
would soon be corrupted, and unfit for the offices of life. As the
tracheæ are divided and subdivided to a prodigious degree of minuteness,
it has been conjectured by some writers, that they may act as so many
sieves, which, by separations properly contrived, filtrate the air, and
so furnish it to the body of different degrees of purity and subtilty,
agreeable to the purposes and nature of the various parts. The
experiments that have been made with the air-pump are by no means
conclusive; the injury which the insect sustains when the atmospheric
pressure is taken from the body, does not prove that it inspired and
expired the air that we have removed; it only shews that an incumbent
pressure is necessary to their comfortable existence, as it prevents the
fluids from disengaging themselves in an aerial form, and as it
counterbalances and re-acts on the principle of life, and, by keeping
the action thereof in proper tone and order, confines and regulates its
energies.
Though it is difficult to ascertain whether some insects respire, at
least at certain periods of their existence, yet there are others to
whom the inspiration and expiration of air seems absolutely necessary:
there are many aquatic insects which are obliged to keep their tails
suspended on the surface of the water for this purpose. To prove this,
keep the tail under water, and you will perceive the insect to be
agitated and uneasy, and to seek for some opening to expose this part to
the air; if it find none, it soon goes to the bottom and dies. Some
aquatic beetles resist the trial for a considerable time, while their
larvæ can only support it for a few minutes. There is a circumstance
which renders all experiments on this subject with insects doubtful and
difficult, namely, the vast tenaciousness of the life principle in the
lower orders of animated nature, and its dissemination through their
whole frame.
Musschenbroeck inclosed the pupa of a moth in a glass tube, very little
larger than the moth itself, and of the following figure.
[Illustration]
The end A of the tube was drawn out in a capillary form, the other end
was covered with a piece of wet bladder to exclude the air; the
capillary end B was then plunged in water, which rose to D. He placed
the capillary part of the tube before a microscope, on a small
micrometer, in order to observe any motion or change in the situation of
the water; as it is evident the expiration or inspiration of air by the
insect would make it rise or fall alternately. In the first experiment
he observed a small degree of motion at distant intervals, not above two
or three times in an hour; in a second experiment on another subject, he
could observe no motion at all. He then placed some pupæ under the
receiver of an air-pump, in water which he had previously purged of its
air; on exhausting the air from the receiver, he observed one bubble to
arise in a part near the tail, and a few near the wings. The pupæ did
not swell under the operation; on the contrary, on letting in the air,
it was found to be diminished in a small degree, but in less than a
quarter of an hour it recovered its former figure. M. Martinet published
at Leyden, in 1753, a dissertation, in which, it is said, he has clearly
proved by a number of experiments that the pupæ of caterpillars and some
other insects do not respire.
OF RESPIRATION IN THE LARVA OF THE MUSCA PENDULA.
Among the insects in which respiration seems to be most clearly proved,
are the larvæ of the musca pendula, Lin. These, while in the worm state,
live under water in the mud, to which they affix themselves; the
respiration of fresh air in this situation is necessary to their
existence; for this purpose they are furnished with a tail, which often
appears of an excessive length comparatively with the body, as this is
seldom more than three quarters of an inch in length, while the tail is
frequently more than four inches; it is composed of two tubes, which run
one into the other, something similar to the tubes of a refracting
telescope. Besides this, the materials of which the tubes are composed
are capable of a great degree of extension. When the tail is at its full
length, it is exceeding small, not being larger near the extremity than
a horse-hair; there is a little knob at the end, which is furnished with
small hairs, to extend on the water, in some measure resembling those at
the tail of the musca chamæleon.
In the body of the larva are two large tracheal vessels; these
air-vessels extend from the head to the tail, terminate in the respiring
tubes, and receive the air from them. The larva quits the water when the
time of its transformation approaches, and enters into the earth, where
the skin hardens and forms a case in which the pupa is formed; soon
after the change, four tubes or horns are seen projecting from the case:
these Reaumur supposes to be organs for communicating air to the
interior parts of the insect; they are connected with little bladders
which are found filled with air, and by which it is conveyed to the
spiracula of the pupa. The larvæ of gnats, and other small aquatic
insects of the same kind, are furnished with small tubes, that play on
the surface of the water, and convey the air from thence to the insect.
Many other singularities are to be found amongst the aquatic larvæ.
OF THE GENERATION OF INSECTS.
One of the greatest mysteries in nature is generation, or that power by
which the various species of animals, &c. are propagated, enabling one
single individual to give birth to thousands, or even millions of
individuals like itself; all formed agreeable to proportions which are
only known to that ADORABLE WISDOM which has established them. We shall
never be able to form any adequate conception of this power, till we are
acquainted with the principles of life, and can trace their various
gradations in different orders of beings. Many ancient philosophers,
from a misconception and perversion of the sentiments of the more
ancient sages, imagined that insects were produced from corrupt and
putrefied substances; that organized bodies, animated with life, and
framed in a most wonderful manner, owed their origin to mere chance! Not
so the most ancient sages; they taught that every degree of life must
proceed from the fountain and source of all life, and that therefore,
when manifested, it must be replete with infinite wonders; but then they
also shewed, that if in its descent through the higher orders of being
it was perverted, it would be manifested in loathsome forms, and with
filthy propensities; and that according to the degree of reception of
the Divine Goodness and Truth, or the perversion thereof, new forms of
life would be occasionally manifested. The gloom of night still wraps
this subject in obscurity; will the dawn of day ere long gild the
horizon of the scientific world? or is the time of its breaking forth
yet far from us? Be this as it may, insects will be found to conform to
that general law of order which runs through the whole of animated
nature, namely, that the conjunction of the male and female is necessary
for the production of their offspring. Where we cannot ascertain causes,
we must be content with facts.
Though insects are, like larger animals, distinguished into male and
female, yet in some classes there is a kind of mules, partaking of
neither sex, though themselves originating from the conjunction of both:
many other particularities relative to the sexes can only be touched
upon here. In many insects the male and female are with difficulty
distinguished, and in some they differ so widely, that an unskilful
person might easily take the male and female of the same insect for
different species; as for instance, in the phalæna humuli, piniaria,
russula. The dissimilarity is still greater in those insects in which
the male has wings and the female none, as in the coccus lampyris,
phalæna antiqua, &c. In general the male is smaller than the female. The
antennæ of the male are, for the most part, larger than those of the
female. In some moths, and other insects which are furnished with
feathered antennæ, the feathers of the male fly are large and beautiful,
while those of the female are small, and hardly perceptible. Some male
beetles are furnished with a horn, which is wanting in the female.
“Pleraque insectorum genitalia sua intra anum habent abscondita, et
penes solitarios, sed nonnulla penem habent bifidum: cancri autem et
aranei geminos, quemadmodum nonnulla amphibia, et quod mirandum in loco
alieno, ut cancer, sub basi caudæ. Araneus mas palpos habet clavatos,
qui penes sunt, juxta os utrinque unicum, quæ clavæ sexum nec speciem
distinguunt; et fœmina vulvas suas habet in abdomine juxta pectus; heic
vero si unquam vere dixeris: res plena timoris amor, si enim procus
inauspicato accesserit, fœmina ipsum devorat, quod etiam fit, si non
statim se retraxerit. Libellula fœmina genitale suum sub apice gerit
caudæ, et mas sub pectore, adeo ut cum mas collum fœmina forcipe caudæ
arripit, illa caudam suam pectori ejus adplicet, sicque peculiari
ratione connexæ volitent.”
Insects are either oviparous or viviparous; or, in other words, the
species is perpetuated either by their laying of eggs, or bringing forth
their young alive. The former is the more general case; there are but
few instances of the latter. Those insects which pass through the
different transformations already described, cannot propagate till they
arrive at their imago or perfect state; and we believe there is seldom
any conjunction of the sexes in other classes till they have moulted, or
put off their last skin, the cancri and monoculi excepted.
To form a just idea of the ovaries of insects, I could wish the reader
to consult the description that Swammerdam has given of that of the
queen bee, and to take a view of the elegant figure that accompanies it,
a figure that speaks to the eyes, and by them to the imagination.
Malpighi has given a description of the ovaries of the silk-worm moth.
Reaumur mentions six or seven species of two-winged flies that are
viviparous, bringing forth worms, which are afterwards transformed into
flies. The womb of one of these is singularly curious; it is formed of a
band rolled up in a spiral form, and about two inches and an half in
length; so that it is seven or eight times longer than the body of the
fly, and composed of worms placed one on the side of the other with
wonderful art: they are many thousands in number.[81]
[81] Reaumur Mem. des Insectes, tom. 4, p. 415.
OF THE APHIDES OR PUCERONS.
These are a species of insects that have opened new views of the œconomy
of animated beings; they belong to the hemiptera class. The rostrum is
inflected, the antennæ are longer than the thorax; some have four erect
wings, others are entirety without them. Towards the end of the abdomen
there are two tubes ejecting that most delicate juice called honey-dew.
Various names have been applied to them, the proper one is aphis, that
by which they are most generally known, is puceron; they are also
frequently called vine-fretters or plant-lice: many among the genera are
both oviparous and viviparous, bringing forth their young alive in
summer, but in autumn depositing their eggs upon the branches and bark
of trees. The different aphides are very curious objects for the
microscope: they are a very numerous genus, Linnæus has enumerated
thirty-three different species, whose trivial names are taken from the
plant which they inhabit, though it is probable the number is much
larger, as the same plant is often found to support two or three
different sorts of them. Their habits are very singular: an aphis or
puceron, brought up in the most perfect solitude from the very moment of
its birth, in a few days will be found in the midst of a numerous
family; repeat the experiment on one of the individuals of this family,
and you will find this second generation will multiply like its parent;
and this you may pursue through many generations.
M. Bonnet had repeated experiments of this kind, as far as the sixth
generation, which all uniformly presented the observer with fruitful
virgins, when he was engaged in a series of new and tedious experiments,
from a suspicion imparted by M. Trembley in a letter to him, who thus
expresses himself: “I have formed the design of rearing several
generations of solitary pucerons, in order to see if they would all
equally bring forth young. In cases so remote from usual circumstances,
it is allowed to try all sorts of means; and I argued with myself, Who
knows but that one copulation might serve for several generations?” This
“WHO KNOWS” persuaded M. Bonnet that he had not sufficiently pursued his
investigations. He therefore now reared to the tenth generation his
solitary aphides, having the patience to keep an exact account of the
days and hours of the birth of each generation. The result of this
pursuit was, his discovering both males and females among them, whose
amours were not in the least equivocal; the males are produced only in
the tenth generation, and are but few in number; these soon arriving at
their full growth, copulate with the females, and the virtue of this
copulation serves for ten successive generations; all these generations,
except the first from fecundated eggs, are produced viviparous, and all
the individuals are females, except those of the last generation, among
whom some males appear, to lay the foundation of a fresh series.
In order to give a further insight into the nature of these insects, I
shall insert an extract of a description of their different generations,
by Dr. Richardson, as published in the Philosophical Transactions for
the year 1771.
The great variety of species which occur in the insects now under
consideration, may make an inquiry into their particular natures seem
not a little perplexing, but by reducing them under their proper genera,
the difficulty is considerably diminished. We may reasonably suppose all
the insects, comprehended under any distinct genus, to partake of one
general nature; and, by diligently examining any particular species, may
thence gain some insight into the nature of all the rest. With this view
Dr. Richardson chose out of the various sorts of aphides the largest of
those found on the rose-tree, not only as its size makes it the more
conspicuous, but as there are few others of so long a duration. This
sort appears early in the spring, and continues late in the autumn;
while several are limited to a much shorter term, in conformity to the
different trees and plants from whence they draw their nourishment.
If at the beginning of February the weather happen to be so warm, as to
make the buds of the rose-tree swell and appear green, small aphides are
frequently to be found on them, though not larger than the young ones
in summer, when first produced. It will be found, that those aphides
which appear only in spring, proceed from small black oval eggs, which
were deposited on the last year’s shoots; though when it happens that
the insects make too early an appearance, the greater part suffers from
the sharp weather that usually succeeds; by which means the rose-trees
are some years in a manner freed from them. The same kind of animal is
then at one time of the year viviparous, and at another, oviparous.
Those aphides which withstand the severity of the weather seldom come to
their full growth before the month of April, at which time they usually
begin to breed, after twice casting off their exuvia, or outward
covering. It appears that they are all females, which produce each of
them a numerous progeny, and that without having intercourse with any
male insect; they are viviparous, and what is equally singular, the
young ones all come into the world backwards. When they first come from
the parent, they are enveloped by a thin membrane, having in this
situation the appearance of an oval egg; these egg-like appearances
adhere by one extremity to the mother, while the young ones contained in
them extend the other, by that means gradually drawing the ruptured
membrane over the head and body to the hind feet. During this operation,
and for some time after, the fore part of the head adheres, by means of
something glutinous, to the vent of the parent. Being thus suspended in
the air, it soon frees itself from the membrane in which it was
confined; and after its limbs are a little strengthened, is set down on
some tender shoots, and left to provide for itself.
In the spring months there appear on the rose-trees but two generations
of aphides, including those which proceed immediately from the last
year’s eggs; the warmth of the summer adds so much to their fertility,
that no less than five generations succeed one another in the interval.
One is produced in May, which casts off its covering; while the months
of June and July each supply two more, which cast off their coverings
three or four times, according to the different warmth of the season.
This frequent change of their outward coat is the more extraordinary,
because it is repeated more often when the insects come the soonest to
their growth, which sometimes happens in ten days, where warmth and
plenty of nourishment conspired.
Early in the month of June, some of the third generation, which were
produced about the middle of May, after casting off their last covering,
discover four erect wings, much longer than their bodies; and the same
is observable in all the succeeding generations which are produced
during the summer months, but still without any diversity of sex; for
some time before the aphides come to their full growth, it is easy to
distinguish which will have wings, by a remarkable fullness of the
breast, which in the others is hardly to be distinguished from the body.
When the last covering is rejected, the wings, which were before folded
up in a very narrow compass, are gradually extended in a surprizing
manner, till their dimensions are at last very considerable.
The increase of these insects in the summer time is so very great, that
by wounding and exhausting the tender shoots, they would frequently
suppress all vegetation, had they not many enemies to restrain them.
Notwithstanding these insects have a numerous tribe of enemies, they are
not without friends, if those may be considered as such, who are
officious in their attendance for the good things they expect to reap
thereby. The ant and the bee are of this kind, collecting the honey in
which the aphides abound, but with this difference, that the ants are
constant visitors, the bee only when flowers are scarce; the ants will
suck in the honey while the aphides are in the act of discharging it,
the bees only collect it from the leaves on which it has fallen.
In the autumn three more generations of aphides are produced, two of
which generally make their appearance in the month of August, and the
third before the middle of September. The two first differ in no respect
from those which are found in summer; but the third differs greatly from
all the rest. Though all the aphides which have hitherto appeared were
females, in this tenth generation several male insects are found, but
not by any means so numerous as the females.