Part 17

Plate XVI. Fig. 1. is a microscopic view of the proboscis of a tabanus, with which it pierces the skins of horses and oxen, and nourishes itself with their blood; Fig. 2. the same of the natural size. The singular and compound structure, together with the wonderful form and exquisite beauty of this apparatus, discovers such a view of the wisdom, power, and greatness of its infinite composer, as must strike with admiration every contemplative observer, and lead him to reflect on the weakness, impotence, and nothingness of all human mechanism, when compared with the immense skill and inimitable finishing displayed in the subject before us. The whole of this formidable apparatus is composed of six parts, exclusive of the two guards or feelers a a, all of which are inclosed in a fleshy case, which in the figure is totally removed, as it contained nothing remarkably different from that of other insects with two wings. The guards or feelers a a, are of a spungy or fleshy substance, and are grey, covered with short hairs or villi; they are united to the head by a little joint of the same texture, which in this view of the object could not be shewn. These guards are a defence to the other parts of the apparatus, as they are laid upon it side by side, whenever the animal stings, and by that means preserve it from external injury. The two lancets b b and B, evidently open the wound, and are of a delicate and tender structure, formed like the dissecting knife of the anatomist, with a sharp point and slender edge, but gradually increasing to the back. The two instruments, c c and C, appear as if intended to enlarge the wound, by irritating the parts round it; to accomplish which, they are jagged or serrated; they may also serve, from their hard and horny texture, to defend the tube e E, which is of a softer nature and tubular to admit the blood, and convey it to the stomach; this delicate part is inclosed in a case d D, which entirely covers it. These parts are drawn separately at B, C, D, E. De Geer observes, that it is only the female that sucks the blood of animals; and Reaumur declares, that having made one disgorge itself, the blood it threw up, appeared to him to be more than the whole body of the insect could have contained.

Many other instances of the variety and curious fabrication of this little organ in different insects, may be found in the works of Reaumur and De Geer; enough has been said to shew that its mechanism not only eludes the human eye, but far surpasses every work of man; I shall therefore proceed, in the next place, to notice

THE ANTENNÆ OF INSECTS.

The antennæ are fine slender horns consisting of several articulations, moveable in various directions, and constituting one of the discriminating characteristics of insects. They are beautiful in form, and of a very delicate structure, so finely articulated, and so minutely jointed, as to be instantaneously moveable in every direction. They are situated on the fore part of the head.

The shape, the length, the number, and kind of articulations, not only vary in different species, but the antennæ of the male generally differ from those of the female. The greater number of insects have only two antennæ, but the oniscus, the pagurus, and astacus have four. Regular rows of minute holes are said to have been discovered in the antennæ. Several insects cover their eyes with them while they sleep.

We are far from being certain of the use of this organ; some writers have conjectured that they were the organs of smell and hearing, others have supposed them appropriated to a delicate species of feeling, sensible to the least motion or disturbance in the circumambient fluid in which they move.[53] The following observations throw some light on this obscure subject. When a wingless insect is placed at the end of a twig, or in any other situation where it meets with a vacuity, it moves the antennæ backward and forward, elevates and depresses them from side to side, and will not advance further lest it should fall. Place a stick or any other substance near the antennæ, and the insect immediately applies them to this new object, seems to examine whether it be sufficient to support its weight, and then proceeds on its journey. From these observations it would appear that the antennæ assist the insect in judging of the vicinity of objects, and probably enable them to walk with safety in the dark.

[53] Some have thought them intended to defend the eyes, but though this might seem probable in regard to the short plumose ones, it can never hold good in those that are slender and smooth, which can be of no such service. Others have thought them made for wiping and cleaning the eyes, but for this purpose they are totally unfit; the fore legs of the insect are much better calculated for this use by the hairs or fibrilla with which they are covered. Possibly they may be the organs of smelling, since we evidently find that many insects possess this sense in a very exquisite degree, and yet we see no external organs except these to serve that purpose. EDIT.

That these observations are not, however conclusive, appears from an experiment of a very ingenious naturalist: being desirous of ascertaining the nature and use of the antennæ and proboscis of a butterfly, he gently approached one that was flying about in search of food; he observed that it turned the antennæ about every way, till coming within scent of a flower, it kept them fixedly bent toward that object, directing its course by their guidance, till it arrived at the flower; there they appeared to act as an organ of smell, and that the minute holes with which it is furnished assisted in promoting this operation. When the creature had reached the flower, it hovered over it as with rapture, poising itself quietly upon its wing, like a kite or hawk in the air; it then dropped suddenly, till it was on a level with the flower, when it began to agitate its wings briskly and to unroll its spiral trunk, thrusting it to the bottom of the flower; in a little time the trunk was rolled up, and again in a moment unrolled; these operations it repeated till the flower yielded no more juices, the butterfly then sought for and alighted on another.[54]

[54] After all, this subject must for the present remain undecided. Indeed, the bodies of insects are throughout formed of parts so different from ours, that we can probably conceive no more idea of the use of some of their organs, than a man born blind or deaf can of the senses of vision or hearing. They may have senses different from ours, and these may be the organs of them. EDIT.

The differences in the form, &c. of the antennæ are characterized by naturalists under the following names:

Setaceæ; are those that, like a bristle, grow gradually taper towards the point or extremity, as in many of the phalenæ. Filiformes; thread-shaped, and of an uniform thickness. Moniliformes; these are filiform like the preceding, and of a regular thickness, but consist of a series of round knobs, like a necklace of beads, as in the chrysomela. Clavatæ; formed like a club, increasing gradually from the base to the extremity, as in the papilio, butterfly. Capitatæ; these are also formed like a club, but the last articulation is larger than the rest, finishing with a kind of capital or head. Fissiles; these are like the former, only that the capitulum or head is divided longitudinally into three or four parts or laminæ, as in the scarabæi. Perfoliatæ; are also capitated, but have the capitulum divided horizontally, and the laminæ connected by a kind of thread passing through their center, as in the dermestes and dytiscus. Pectinatæ; so called from their similitude to a comb, though they more properly resemble a feather, as in the phalenæ and elateres; this is most obvious in the male. Aristatæ; such as have a lateral hair, which is either naked, or furnished with smaller hairs, as in the fly.

Besides the foregoing terms, the antennæ are called breviores, or short, when they are shorter than the body; mediocres, or middling, when they are of the same length; and longiores, when they are longer.

Near the mouth there is also a species of small filiform articulated antennæ, called the palpi, or feelers; they are generally four in number, sometimes six; they are placed under and at the sides of the mouth, which situation, together with their size, sufficiently distinguish them from the antennæ; they are in continual motion, the animal thrusting them in every matter, as a hog would its nose, when in search of food. Some have supposed them to be a kind of hand to assist in holding the food when it is near the mouth.

OF THE EYES OF INSECTS.

The structure of the eye has always been considered as a wonderful piece of mechanism; the admirable manner in which those of the human species are formed, and the nature of vision, are speculations which cannot but excite the attention of every inquisitive mind. The eyes of insects, though they differ considerably in their construction from those of other animals, are no less objects of our admiration. Indeed, among the exterior parts of insects, none are more worthy of minute investigation, and very few persons are to be found, who can be insensible to the beauties of this organ when exhibited under the microscope, as that instrument alone points out to us the prodigious art employed in their organization, and evidently shews how many wonders escape the unassisted sight.

The construction of the eye in insects is not only distinct from that of other animals, but also differs in different species. They vary in number, situation, connection, and figure. In other creatures the eyes are moveable, and two in number, one on each side of the head: in insects, the genus of cancri excepted, the eyes are fixed; they have no eye-brows, but the outer coating is hard and transparent.

The greater part of insects have two eyes; in the monoculus they approach so near to each other, as to appear like one; the gyrinus has four eyes, the scorpion six, the spider eight, and the scolopendra three.

Of the eyes of insects, some have them single, that is, placed at a small distance from each other; while others are furnished with an indefinite number, all placed in one common case or socket; the latter are generally termed the reticulated eyes.

OF THE RETICULATED EYES OF INSECTS.

The microscope does not disclose greater wonders, when it exhibits to us millions of animals invisible to the naked eye, where we should suppose nothing living existed, than when it discovers to us hidden beauties in those, which, though they are large enough to be seen by our natural eye, yet in their several minute parts are no ways discernible, but by the assistance of glasses.

Thus we readily discern those protuberances on the heads of insects, which are formed by a congeries of eyes; we can even perceive that they consist of a number of lines crossing each other with great regularity and exactness at some little distance, like the meshes of a net. By this we know that they are reticulated substances; but in what manner they are so, can only be shewn by the microscope.

The eyes of the libellula, on account of their size, are peculiarly well adapted for microscopical examination; and, by the assistance of the instrument, you will find that they are divided into a number of hexagonal cells, each of which forms a complete eye. The external parts of these eyes are so perfectly smooth, and so well polished, that, when viewed as opake objects, they will, like so many mirrors, reflect the images of all the surrounding objects. The figure of a candle may be seen on their surface multiplied almost to infinity, shifting its beam to each eye, according to the motion given to it by the hands of the observer. Other creatures are obliged to turn their eyes towards the object, but insects have eyes directed thereto, on whatsoever side it may appear: they more than realize the wonderful accounts of fabulous history: poets gave to Argus an hundred eyes; insects are furnished with thousands, having the benefit of vision on every side with the utmost ease and speed, though without any motion of the eye or flexion of the neck.

Each of these protuberances, in its natural state, is a body cut into a number of faces; like an artificial multiplying glass; but with this superiority in the workmanship, that as there, every face is plane, here, every one is convex, immensely more numerous, and contained in a much smaller space. If one of these protuberant substances be nicely taken from the head of the insect, washed clean, and placed before the microscope, its structure is elegantly seen, and it becomes an object worthy of the highest admiration. You will find that each of the eyes is an hexagon, varying in its size according to its situation in the head, and that each of them is a distinct convex lens, and has the same effect in forming the image of an object placed before it. Of this you will be convinced, by turning the mirror of the microscope so as to bring the picture of some well-defined object under the eye; thus, turn it towards a house, and in the eye of the insect you will perceive the house diminished to a box, but multiplied into a city; turn it towards a soldier, and you will have an army of pigmies performing every motion at the same instant of time; again, turn the mirror towards a candle, and you will have a beautiful and resplendent blaze from multitudes of regular flames.

Hooke, Catalan, &c. have shewn that these small eyes are furnished with every requisite of vision, and that each of them has the use, the power, and properties of an eye. But we must have recourse to the works of Swammerdam for a full account of the astonishing organization of the eyes of insects. Among other things, he has shewn, that under each facet there is a pyramid of fibres broad at the base, and growing smaller as it proceeds inwards; the pyramid has the same number of sides as the eye, and there are as many hexagonal pyramids, as there are small facets or eyes in the insect. An innumerable number of pulmonary tubes ascend these fibres, terminating in a white fibrous convex membrane; under these membranes there is another, still more delicate and transparent; beneath this, a second species of fibres is transversely applied, like so many beams to support the pyramids that are laid upon them. Still we cannot determine with certainty, how these numerous inlets to sight operate for the service of the animal; they may increase the field of view, augment the intensity of light, and be productive of advantages of which we can have no conception.

Hooke computed 14000 of these facets in the two eyes of a drone; Leeuwenhoek reckoned 6036 in the two eyes of a silkworm, when in its fly state; in the eyes of the libellula he reckoned 12544 hexangular lenses.

Swammerdam covered the reticulated eyes of certain insects with black paint; in this state they flew at random, and seemed to be deprived of their strength; when they settled, they did not avoid the hand that was going to take hold of them. Reaumur made similar experiments on the eyes of bees, which concurred with those of Swammerdam.

Some ephemera flies have four reticulated eyes, two of which are placed as in the common fly; the other two are placed, one beside the other, upon the upper part of the head, and have the appearance of a kind of mushroom, the head extended somewhat beyond the stalk. The first pair are of a brown colour, those of the mushroom form are of a very beautiful citron colour.

In some of the fly class, these reticulated eyes are little inferior in colour and brilliance to the brightest gem. The colour varies in different species; in some you find it green, in others red, &c. some have a most elegant changeable colour thrown over them, partly purple, partly green, and partly of that brassy hue, which is seen on the backs of some of our beetles, and which is not equalled by any other production of art or nature.

Fig. 3. Plate XVI. is a representation of a small part of the cornea of a libellula, as seen by the microscope; the sides of the hexagons in some positions of the light, appear of a fine gold colour, and divided into three parallel borders. Fig. 4. the same object of its natural size.

Fig. 5. Plate XVI. represents a small portion of the cornea of a lobster; here each of the eyes are small squares, not hexagons; a conformation which admits a smaller number in the same surface; so great a number was not necessary in this instance, as the eyes of the lobster are moveable. Fig. 6. the same of its natural size.

OF THE EYES OF THE MONOCULUS POLYPHEMUS.

The monoculus polyphemus, or king crab, has four eyes, two large and two small ones; the large eyes are formed of a great number of transparent amber-like cones, the small ones of a single cone,

“The internal surface of the large eyes, examined with the microscope, is found to be thick set with a great number of small transparent cones, of an amber colour, the bases of which stand downward, and their points upward next the eye of the observer. The cones in general have an oblique direction, except some in the middle of the cornea, about thirty in number, the direction of which is perpendicular. The center of every cone being the most transparent part, and that through which the light passes, on that account the perpendicular or central cones always appear beautifully illuminated at their points. In a word, they are all so disposed, as that a certain number of them receive the light from whatever point it may issue, and transmit it to the immediate organ of sight, which we may reasonably suppose is placed underneath them. The cones are not all of the same length; those on the edges of the cornea are the longest, from whence they gradually diminish as they approach the center, where they are not above half the length of those on the edges.

“The structure of the small eyes being less elaborate, their internal appearance, when placed in the microscope, will be described in a few words. They consist of an oval transparent horny plate, of an amber colour, in the center of which stands a single cone, through which and the oval plate the light passes.”[55]

[55] See Mr. André’s paper with a plate, in the Phil. Trans. for 1782, page 440.

OF THE EYES OF A SPIDER.

Though the form of this insect is naturally disgusting, yet the eyes make a beautiful object for the microscope. They have generally eight; two on the top of the head, that look directly upwards; two in the front, a little below the foregoing, to discover what passes before it; on each side a couple more, whereof one points sideways forward, the other sideways backward; so that the spider can nearly see all around. These eyes are immoveable, and seem to be formed of a hard transparent horny substance. A portion of each sphere projects externally beyond the socket, the largest part is sunk within it. There is round each eye a circular transparent membrane. Mr. Baker placed the eye of a spider over a pin-hole made through a piece of card, and then applied it as a lens to examine objects; he found it magnified the objects greatly, but that it did not exhibit them distinctly; this he however attributed to the length of time the spider had been dead whose eye he used. The number of eyes is not the same in all species of the spider.

OF THE STEMMATA.

It might be imagined, that as every fly has two reticulated eyes, they could not have occasion for more; but so it has not appeared to that GREAT BEING who formed them, for many are furnished besides with other eyes, differing in form and construction from those that are reticulated.

These were first noticed by M. de la Hire; they are three lucid protuberances placed on the back part of the head of many insects: their surface is glossy, of an hemispheric figure, and a coal black colour. They are transparent, and disposed in a triangular form; by modern naturalists they are termed stemmata.

Reaumur made experiments on these eyes, similar to those he had made on the reticulated ones, and found that when the stemmata were covered with dark varnish, the insects flew but to a small distance, and always at random.

No insect is, I believe, found with both kind of eyes, unless in its perfect state: there are many species which are not furnished with stemmata, gnats and tipulæ are without them.

We are apt to suppose that nature has lavished all her bounty upon her larger creatures, and left her minims of existence, as Shakspeare phrases it, unfinished; with what different ideas must those be impressed, who find the apparatus for vision in these small creatures so various and so wonderful in their structure, and who must perceive so much design and order manifested in the position, construction, and number of these delicate and useful organs.

OF THE BODY OF INSECTS.

The trunk or body of the insect is situated between the head and abdomen. Naturalists divide it into three parts; the thorax, scutellum, and sternum.

The thorax is the upper part of the body, it is of various shapes and proportions; the sides and back of it are often armed with points.

The scutellum, or escutcheon, is the lower part of the body, and is generally of a triangular form; though it adheres to the thorax, it is easily distinguished therefrom by its figure, and often by an intervening suture. It seems intended to assist in expanding the wings.

The sternum is situated on the under part of the thorax; in some species it is pointed behind, as in the elateres; in others, bifid, as in some of the dytisci.

OF THE ABDOMEN OF INSECTS.

The abdomen, or under part of the body, contains the stomach, the intestines, the air vessels, &c. It is composed of several rings or segments, so that it may be moved in various directions, or lengthened and shortened at pleasure; in some it is formed of one piece only. It is perforated with spiracula, or breathing holes, and is terminated by the tail.

The spiracula are small oblong holes or pores placed singly one on each side of every ring of the abdomen; these are the means or instruments of respiration, supply the want of lungs, and form a peculiar characteristic of insects.

OF THE LIMBS OF INSECTS.

By the limbs are here meant the instruments used by the insect both for motion and defence. They are, alæ, the wings; halteres, the poisers; pedes, the legs; cauda, the tail; and aculeus, the sting.

OF THE WINGS OF INSECTS.

The wings are those organs by which the insect is enabled to fly; some have only two, others are furnished with four, two on each side; these are, in some, of the same size; in others, the superior ones are much larger than the inferior: Linnæus has made them the foundation of the order into which he has divided this numerous class of beings. The variety in the form and structure of the wings is almost infinite; the beauty of their colouring, the art with which they are connected to the body, the curious manner in which some are folded up, the fine articulations provided for this purpose, by which they are laid up in their cases when out of use, and yet are ready to be extended in a moment for flight; together with the various ramifications, by which the nourishing juices are circulated, and the wing strengthened, afford a fund of rational investigation highly entertaining; exhibiting, particularly when examined by the microscope, a most wonderful display of divine wisdom and power. The more delicate and transparent wings are covered and protected by elytra, or cases, which are generally hard and opake. The wings of moths and butterflies are mostly farinaceous, covered with a fine dust; by the assistance of the microscope, we discover that this dust is a regular assemblage of organized scales, which will be more particularly noticed hereafter.