Part 16
The subjects of that part of the creation we are now going to survey, merit our attention as exceeding the rest of animated nature in their numbers, the singularity of their appearance, and the variety of their forms. Earth, air, and water are filled with hosts of them. Being for the major part very small, and myriads so diminutive, as even to be imperceptible to the unassisted eye, our knowledge of them, and their component parts would be extremely circumscribed and imperfect, were it not for the advantages derived from the use of the microscope; but happily possessed of this valuable instrument, an inexhaustible source of entertainment and instruction is afforded to the curious inquirer into the wonders of nature. The beauties of the minuter parts of creation are not more hidden from our unassisted sight, than the ends and purposes of their œconomy from slight and superficial observation; the microscope does not more amaze and charm as with a discovery of the first, than the application of our faculties in investigating the latter.
The name of INSECT has been appropriated to these small animals on account of the sections or divisions that are observable in the bodies of the greatest part of them; though, perhaps, it is impossible to find any precise term that shall embrace the whole genera, as many particulars must be described before we can attain an exact notion of these animals and their structure.
An insect is now generally defined to be, an animated being whose head is furnished with antennæ; that is destitute of bones, but which, instead thereof, is covered with a very hard skin; that has six or more feet; and that breathes through spiracula, or pores placed in the side of the body.
To be more particular, quadrupeds, birds, and fishes have an internal skeleton of bones, to which the muscles are affixed; but the whole interior body of insects is composed of soft flesh, and the muscles are attached to an external skeleton, serving the double purpose of skin and bone.
Insects are by most writers considered as divided into four principal parts: the caput, or head; the thorax, or trunk; the abdomen, or belly; and artus, or limbs. A perfect knowledge of these parts, and their several subdivisions, is requisite for those who are desirous of forming accurate ideas of these minute animals, or who wish to arrange them in their proper classes.
The head is affixed to the thorax by a species of articulation or joint; it is the principal seat of the senses, and contains the rudiments of the brain;[50] it is furnished with a mouth, eyes, antennæ, a forehead, a throat, and stemmata. In the greater part of insects the head is distinctly divided from the thorax, but in others it coalesces with it. The head of some insects is very large compared with the size of their bodies; the proportion between the head of the same insect is not always similar; in the caterpillars with horny heads it is generally small, before they moult or change their skin, but much larger after each moulting. The hardness of the exterior part of the head prevents its growth before the change; it is, consequently, in proportion to the body very small; but when the insect is disposing itself for the change, the internal substance of the head retires inwards to the first ring of the neck, where it has room to expand itself; so that when the animal quits the skin, we are surprized with a head twice the former size; and, as the insect neither eats nor grows while the head is forming, there is this further circumstance to be remarked, that the body and the head have each their particular time of growth: while the head expands and grows, the body does not grow at all; when the body increases, the head remains of the same size, without any change. The heads of all kinds of insects, and their several parts, form very pleasing, as well as most diversified objects for the opake microscope.
[50] Fabricius Philos. Entomolog. p. 18.
Os, the mouth, is a part of the insect to which the naturalist will find it necessary to pay a very particular attention; Fabricius goes so far as to assert that, without a thorough knowledge of the mouth, its form, and various appendages, it will be impossible ever to discriminate with accuracy one insect from another. In the structure of the mouth considerable art and wisdom is displayed; the diversity of the figure is almost as great as the variety of species. It is usually placed in the forepart of the head, extending somewhat downwards; in the chermes, coccus, and some other insects, it is placed under the breast. In some insects, the mouth is forcipated, to catch, hold, and tear the prey; in others, aculeated, to pierce and wound animals, and suck their blood; in others, strongly ridged with jaws and teeth, to gnaw and scrape out their food, carry burdens, perforate the earth, nay the hardest wood, and even stones themselves, for habitations and nests for their young. Others are furnished with a kind of tube or tongue, at one time moveable, at another fixed; with this they suck the juices of the flowers: in some again the tongue is so short, as to appear to us incapable of answering the purpose for which it was formed, and the oestri appear to have no mouth.
Maxillæ, the jaws, are generally two in number; in some, four; in others, more. They are sometimes placed in an horizontal, sometimes in a transverse direction; the inner edge is serrated, or furnished with small teeth, as in the cicada, nepa, notonecta, cimex, (bug,) aphis, and remarkably so in some curculeones.
The rostrum, or proboscis, is in general a very curious and complicated organ; it is the mouth drawn out to a rigid point. In many insects of the hemiptera class, it is bent down towards the breast and belly. It has by some writers been considered as serving at once the different purposes of mouth, nose, and windpipe, enabling the insect to extract the juices of plants, communicate the sensation of smelling, and convey air to the body.
Lingua, the tongue, is a taper and compact instrument, by which the insect obtains the juices of plants. Some can contract or expand it, others roll it up with dexterity; in some it is inclosed within a sheath. It is taper and spiral in the butterfly, tubular and fleshy in the fly; in all affording agreeable amusement for the microscope. To exemplify which in one or two instances, while it relieves the reader from the tediousness of narration, will, it is hoped, animate him to farther researches on the subject.
OF THE PROBOSCIS OF THE BEE.
Every day’s experience shews that the more we penetrate into the hidden recesses and internal parts of natural bodies, the more we find them marked with perfection in form and design; of the truth of which observation the minute apparatus now to be described will, no doubt, ensure conviction. Swammerdam, when speaking thereof, breaks out into this pious and humble confession: “I cannot refrain,” says he, “from confessing to the glory of the Immense and Incomprehensible Architect, that I have but imperfectly described and represented this small organ; for, to represent it to the life in its full perfection, as truly most perfect it is, far exceeds the utmost efforts of human knowledge.”
From what has here been said, it will be easy to perceive, that the limits of these Essays will not permit our entering largely into a description of the minute parts of the proboscis of the bee; for an ample account of which recourse must be had to the works of Swammerdam and Reaumur. The last writer, like a skilful workman who takes to pieces a watch which he himself has made, exhibits to you the several parts of which it is composed, and explains their fitness, their adjustments, their uses, the play of the pivots, springs, and pillars; for all these parts, and many more, are to be found in the proboscis of a bee.
It is by this small instrument that the bee procures the food necessary for its subsistence. In a general view, it may be considered as consisting of seven pieces; one of these, i i, b c, Fig. 3. Plate XIII. is placed in the middle; this is supposed to be pervious, and to constitute what may be properly called the tongue; the other six smaller parts or sheaths, disposed in three pairs, are placed on each side of the former: they not only assist in extracting and gathering the honey from the flowers, but they also protect and strengthen the part. The proboscis itself is very curiously divided; the divisions are elegant and regular, and are beset all round with shaggy triangular fibres or villi, distributed in beautiful order: these divisions, though very numerous, appear at first sight as a number of different articulations. The tongue, considered with respect to its length, may be said to have three articulations; one with the head, then a kind of cylindrical horny substance, which forms as it were a base for the true tongue, which is not horny, but soft, fleshy, and pliable.[51]
[51] Philos. Trans. for 1792, Part I.
The two pieces a a of the exterior sheath are of a substance partly between bone and horn, and partly membranaceous; they are set round with fibres, and are furnished with air vessels, which are distributed through their whole texture; the upper ends f f of this sheath appear to be a little bent, but can be straitened by the bee when they are applied to the proboscis. At d d are two articulations, by means of which the pieces a a may be occasionally bent. The joints contribute towards bending the proboscis downwards, or rather underneath, against the head. These sheaths, together with two interior ones e e, assist in defending, covering, and protecting it from injuries; it is also probable that they promote the descent of the honey, by pressing the proboscis. The parts k k of this sheath have been called by some writers the root.
The two parts e e of the interior sheath are placed higher than those of the exterior one; they originate at g g on the proboscis itself, and near that part or articulation, by which the bee can upon occasion bend the proboscis; this sheath, therefore, always moves with the middle part i i, and is carried forward by it, the exterior sheath being left behind, because its attachments and origin are below that of the proboscis. The pieces e e are very similar in structure to those of a a, only that each of them has on the upper part three joints, the lower one is much longer than the other two; they are all of them surrounded with short fibres. The smaller articulated pieces never lie close to the proboscis, nor cover it, but are only placed near it, the two upper joints projecting outwards, as in this figure, even when the whole apparatus is shut up as much as possible. Swammerdam thinks these joints are of essential use to the bee, acting as it were in the manner of fingers, and assisting the proboscis, by opening the leaves of the flowers, and removing other obstructions from it; or like the two fore feet of the mole, by the help of which it pushes the earth from the sides both ways, that it may be able with its sharp trunk to search for its food more conveniently. There are two smaller pieces or sheaths, m m, near the bottom of the proboscis; these cannot be well seen without removing the sheath e e.
The proboscis is partly membranaceous, and partly of a gristly nature; the lower part is formed in such a manner, that it will swell out considerably, by which means the internal cavity may be prodigiously enlarged, and rendered capable of receiving a very large quantity of native and undigested honey, and larger than might be expected from its size. When the proboscis is shut up and inactive, it is very much flattened, and is three or four times broader than it is thick. The edges are always round; it grows tapering, though very gradually, towards the extremity. The lower and membranaceous part of the trunk has no fibres or villi on it, but is covered with little protuberant transparent pimples, that are placed in regular order, and at equal distances from each other, resembling the little risings observable on the skin of birds when the feathers have been plucked off. They are probably glandules, and may have a considerable share in changing or preparing the honey that is swallowed or taken up by the proboscis. Down the middle of the proboscis there is a tube of a much harder nature than the sides, it grows gradually smaller towards the top; at this place the tongue itself is extremely villous, having some very long villi at the point; whether they are open tubes, or whether they only serve as so many claws, to keep it in its proper place while in action, has not been determined; Mr. John Hunter conceives them to act somewhat like capillary tubes.
The proboscis terminates in a small cylinder c, at the top of which there is a little globule or nipple; the bee can contract this cylindrical part, and the little membrane in which the villi are fixed, into a much smaller compass, and draw it inwards. The exterior sheaths lap over each other on the upper part, so that the outside of the proboscis is protected by a very strong double case, a covering that was unnecessary for the under part; because when this instrument is in use the sheaths are opened, but when it is inactive, it is so folded that the under part is protected by the body of the bee. Withinside the exterior sheath, and near the bottom q, are two levers, which are fixed to the end of the proboscis, and by which it is raised and lowered.
Swammerdam thinks that the honey is, as it were, pumped or sucked up by the bee through the hole at the end b of the tongue; he does not seem to have discovered the apertures which are on the cylindrical part, near the end b. But Reaumur is of opinion that it is used to lap up the fluid, which is then conveyed down between the sheath to the mouth of the bee. To ascertain this, he placed a bee in a glass tube, the inside of which was rubbed over with honey, and little pieces thereof placed in different parts; the bee placed the tongue on the honey; stretching the end beyond the piece thereof, she bent it into the form of a bow, and inserted the most convex part of the bow into the honey; by rubbing the glass backwards and forwards with this part, she soon cleaned that portion to which it was applied, conveying the honey afterwards to the throat by the vermicular motion of the tongue.
If you attentively observe a bee, when it has placed itself on a full-blown flower, the activity and address with which it uses this apparatus will be very conspicuous. It lengthens the end, and applies it to the bottom of the petals or leaves of the flower, moving it continually in a vast variety of different directions; lengthening and shortening, bending and turning it in every possible way, to adapt it to the form, &c. of the leaves of the flower. These various movements are executed with a promptitude that surpasses all description.
The whole of this curious apparatus can be folded up into a very small compass under the head and neck. The larynx, or that part next to the head, falls back into the neck, which brings the extreme end of the first portion of the proboscis within the upper lip, or behind the two teeth; then the whole of the second part is bent down upon and under the first part, and the two last sheaths or scales are also bent down over the whole; so that the true tongue is inclosed laterally by the two second horny sheaths, and over the whole lie the two first.
OF THE PROBOSCIS OF THE BUTTERFLY.
From the tongue of the bee, let us now direct our attention to that of the butterfly. This is a spiral substance, somewhat resembling the spring of a watch when wound up, consisting of eight rounds; by means of a pin you may gently pull it out to its full length; it grows gradually tapering from the base, at the end it divides or separates into two tubes, each furnished with little organs of suction; probably, it is by these that it extracts the juices on which it feeds, and not by the extreme ends of the tongue. As the butterfly has no mouth, the proboscis is the only alimentary organ; when separated from the insect, it will often unroll itself, then wind and coil itself up again, continuing these motions at intervals for a considerable time.
OF THE PROBOSCIS OF THE CULEX OR GNAT.
The proboscis of the gnat consists of a great number of extremely delicate pieces, all concurring to one purpose; this is the instrument with which it strikes the flesh, and sucks the blood of animal bodies. The only part exhibited to the naked eye is the sheath, which contains all the other pieces. This sheath is a cylindrical tube, which is slit in such a manner, that the insect can separate it from the dart, and bend it more or less in proportion as the dart is plunged into the wound. From this tube the sting is darted, which consists of five or six blades or lancets of exquisite minuteness, lying one over the other; some of these are sharpened like a two-edged sword, while others are dentated and barbed at their extremities like the head of an arrow. The instant the gnat lances this bundle of darts into the flesh, and penetrates a vein, a drop or two of fluid is by it insinuated into the wound, by which the blood is attenuated, and the blades acting as so many capillary tubes, the blood ascends in them, and is conveyed into the body of the gnat. The injected fluid also by its fermentation causes that disagreeable and teazing sensation of itching, to which most persons are subjected, after having sustained an attack from one or more of these little animals.[52]
[52] To some persons the gnat (culex pipiens) is so truly formidable, that, during the Summer season, they constantly dread the approach of evening, that being the time when these blood-thirsty marauders sally forth in great numbers, pursue them wherever they go, and exempt no part of the face, hands, or even the legs from their depredations; the consequences of which are, violent, though happily only local and temporary inflammation, attended with insupportable itching, succeeded by tumors very similar to those occasioned by a scald; when these have discharged the pellucid fluid they contain, the symptoms subside. Instances have been known in the vicinity of London, where for several days the eyes of the sufferers have been closed, the nose and lips violently swelled, the fingers of both hands so affected as to prevent their motion, and the legs equally affected. It is remarkable, that in general those who thus suffer are not conscious of the moment when they receive the injury, but are soon made sensible of it by the effect it produces. The approach of the enemy is, however, always known by the singing or humming noise they make; the peculiar note of which, though rendered very familiar by daily repetition, is never esteemed sufficiently musical to render it pleasant or agreeable to the destined victims. Amongst the variety of remedies which have been recommended for the cure of this temporary evil, Barbut mentions the immediate application of volatile alkali, or scratching the part newly stung, and washing it with cold water; he likewise asserts, that rubbing the part at night with fuller’s earth and water abates the inflammation. As preventives are certainly more acceptable than curatives, I wish I were enabled to recommend such in the present case: in one instance, the application of vinegar every evening before sun-set produced a happy effect; possibly washing the parts exposed with extract of saturn properly diluted might prove effectual.
In the Philosophical Transactions for the year 1767, is an account of uncommonly numerous swarms of gnats which made their appearance at Oxford, during the months of July, August, and September of the preceding year. So many myriads sometimes occupied the same part of the atmosphere in contiguous bodies, that they resembled a very black cloud, greatly darkened the air, and almost totally interrupted the solar rays. The repeated bites of these malignant insects were so severe, that the legs, arms, heads, and other parts of many persons were swelled to an enormous size. The colour of the parts was red and fiery, perfectly similar to that of some of the most alarming inflammations. Some of these gnats had their bodies greatly distended by the uncommon quantities of blood which they had imbibed.
In short, there is no species of insects more troublesome to mankind than the gnat; others give more pain with their stings, but it is only when they are attacked, or by accident, that we are stung by them; but the gnats thirst for our blood, and follow us in whole companies to attack us. In marshy places of this country the limbs of the inhabitants are kept swelled during the whole season. In warmer climates, particularly the West Indies, they are, under the denomination of musquetoes, still more formidable.
Hooke, in his Micrographia, pleads in justification of these terrible little insects, that they do not wound the skin and suck the blood out of enmity or revenge, but through mere necessity, and to satisfy their hunger:--it may be so; and on this account we cannot annex the criminality to them which appertains to such of the highest rank in the scale of the animal creation, who, though not urged by the same powerful motive, pursue a somewhat similar conduct; but those who have experienced their assaults, will scarcely admit this plea as a sufficient apology, or feel themselves amicably disposed towards them; as, from whatever cause their attacks may proceed, the effect is so very unpleasant, as almost to justify the sufferers in addressing them in the language of the frogs in the fable to the boys, “Consider, I beseech ye, that though this may be sport to you, it is death to us,” and ejaculating a wish, that they might be enabled to gratify their rapacious appetites by some other means. EDIT.
OF THE PROBOSCIS OF THE TABANUS OR OX-FLY.