Part 23

Amongst insects, some are produced in the state in which they will remain during their whole lives; others come forth inclosed in an egg, and are hatched from this into a form that admits of no variation; many come into the world under a form which differs but little from that which they have when arrived at an age of maturity; some again assume various forms, more or less remote from that which constitutes their perfect state; lastly, some go through part of these transformations in the body of the parent, and are born of an equal size with them. By these various changes, a single individual unites within itself two or three different species, and becomes successively the inhabitant of two or three worlds: and how great is the diversity of its operation in these various abodes!

Since it has been shewn that the larva or caterpillar is really the moth, crawling, eating, and spinning, under the form of the worm, and that the pupa is only the moth swathed up, it is clear that they are not three beings, but that the same individual feels, tastes, sees, and acts by different organs, at different periods of its life, having sensations and wants at one time, which it has not at another; these always bearing a relation to the organs which excite them.

OF THE RESPIRATION OF INSECTS.

As respiration is one of the most important actions in the life of every 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.

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.