Читать книгу British Butterflies - William S. Coleman - Страница 8

Table of Contents

"COMING OUT"—ICHNEUMONS—THE BUTTERFLY PERFECTED—ITS WINGS—LEPIDOPTERA—MEANING OF THE WORD—MICROSCOPIC VIEW—NEW BEAUTIES—MAGNIFIED "DUST"—THE HEAD AND ITS ORGANS—THE TONGUE—THE EYES—THE ANTENNÆ—THEIR USES—INSECT CLAIRVOYANCE—AN UNKNOWN SENSE—FORMS OF ANTENNÆ—THE LEGS.

We now arrive at the last stage, the consummation of all this strange series of transformations; for veritable transformations they are to all intents and purposes; though some learned naturalists have discovered—or imagined so—that the butterfly, in all its parts, really lies hid under the caterpillar's skin, and can be distinguished under microscopical dissection; and that, therefore, the so-called transformations are merely the throwing off of the various envelopes or husks, as they become in turn superfluous, as a mountebank strips off garment after garment, till lastly the sparkling harlequin is discovered to view; or, in more exact language, they consider these changes in the light rather of successive developments and emancipations of the various organs than as their actual transformations. Still, it seems to me, the difference is chiefly one of terms. The real wondrous fact remains undiminished and unexplained; that a creeping wormlike creature, in process of time, is changed into a glorious winged being, differing from the former in form, habits, food, and every essential particular, as widely as any two creatures can well differ, as widely as a serpent from a bird, for instance.

As the imprisoned butterfly approaches maturity, a change is observable in the exterior of the chrysalis, the skin becomes dry and brittle, usually darkens in colour, and if the enclosed butterfly be a strongly marked one, the pattern of its wings shows through, often quite distinctly.

When the fulness of time arrives, the creature breaks through its thin casings, which divide in several places, and the freed insect crawls up into some convenient spot to dry itself, and allow the wings to expand.

All the organs are at first moist and tender, but on exposure to the air soon acquire strength and firmness.

At the moment of emergence, the wings are very miniature affairs, sometimes hardly one-twentieth of their full size when expanded; but so rapid is their increase in volume, that they may actually be seen to grow, as the fluids from the body are pumped into the nervures that support the wing-membrane, and keep it extended.

In the more strongly marked, or richly coloured species, it is a wonderfully beautiful sight to watch this expansion of the wings, and to see the various features of their painted devices growing under the eye and developing gradually into their true proportions.

Generally within an hour the development is complete, and the wings, having gained their full expanse and consistency by drying in the sun, are ready for flight, and the glad creature wings his way to the fields of air, and enters on that life of sunshine and hilarity which is associated with the very name of "Butterfly."

But not every chrysalis arrives at this happy consummation of its existence. Supposing that you have reared and watched a caterpillar to apparently healthy maturity, that it has duly become a chrysalis, and you are awaiting its appearance in butterfly splendour—peeping into your box some morning to see if the bright expected one is "out," be not surprised if in its stead you find the box tenanted by a swarm of little black flies—an impish-looking crew. Whence came all these? Why they and the empty chrysalis shell are all that remains of your cherished prize; so look no more for the fair sunny butterfly, devoured ere born by that ill-favoured troop of darklings who have just now issued from the lifeless shell.

The truth is, that long since, perhaps in early larva-hood, the creature's fate was sealed; a deadly enemy to his race is ever on the alert, winging about in the shape of a small black fly, in search of an exposed and defenceless caterpillar. Having selected her victim, she pierces his body with a sharp cutting instrument she is armed with, and in the wound deposits an egg; the caterpillar winces a little at this treatment, but seems to attach little importance to it. Meanwhile his enemy repeats her thrusts till some thirty or forty eggs, germs of the destroyers, are safely lodged in his body, and his doom is certain beyond hope. The eggs quickly hatch into grubs, who begin to gnaw away at the unhappy creature's flesh, thus reducing him gradually, but by a profound instinct keeping clear of all the vital organs, as if knowing full well that the creature must keep on feeding and digesting too, or their own supply would speedily fail; as usurers, while draining a client, keep up his credit with the world as long as they can.

Weaker grows the caterpillar as the gnawing worms within grow stronger and nearer maturity. Sometimes he dies a caterpillar, sometimes he has strength left to take the chrysalis shape, but out of this he never comes a butterfly—the consuming grubs now finish vitals and all, turn to pupæ in his empty skin, and come out soon, black flies like their parent.

But, supposing that it has escaped this great danger, we now see the creature in its completest form, as the

IMAGO, OR PERFECT BUTTERFLY.

The first term, Imago, is a Latin one, merely signifying an image, or distinct unveiled form; as distinguished from the previous larva, or masked state, and the pupa, or swathed and enveloped state. The word imago then, in works on entomology, always means the perfect and last stage of insect life, and is applied to all insects with wings—for it must be borne in mind that no insect is ever winged till it reaches the last stage of its existence.

If the progressive development of these lovely beings is so marvellous, no less so is their structure when perfected, and of this some general description must now be attempted.

In contemplating a butterfly, one feels that the mind is first engaged by that ample spread, and exquisite painting of the wings that form the creature's glory; let therefore these remarkable organs have our first attention.

Wherein do these wings chiefly differ from all other insect wings? Certainly in being covered thickly with a variously coloured powdery material, easily removed by handling. This apparent dust is composed, in reality, of a vast number of regularly and beautifully formed scales—feathers they are sometimes called, but they are more comparable to fish scales than to any other kind of natural covering. The general term Lepidoptera, applied to all butterflies and moths, is derived from these scaly-wings; Lepis^[2] being the Greek for a scale, and ptera meaning wings in the same language.

The use of a tolerably powerful pocket lens will afford some insight into the exquisite mode of painting

employed in these matchless pieces of decoration; but the possessor of a regular microscope may, by applying it to some of our commonest butterflies, open for himself a world of beauty, and feast his eyes on a combination of refined sculpture with splendour of colouring; now melting in softest harmony, then relieved by boldest contrast—a spectacle, the first sight of which seldom fails to call forth expressions of wonderment and warm delight; and, truly, little to be envied is the mind untouched by such utter beauty as here displayed.

As an example of the method by which this admirable effect is produced, let us take a small portion of the wing of the Peacock, a very beautiful, though an abundant species, and one admirably adapted for microscopic examination, and to illustrate the subject, from the great variety of rich tints brought together in a small space, the part selected being the eye-like spot at the outer corner of each upper wing. Even to the naked eye this appears as a very splendidly coloured object, yet but little of its exquisite mechanism can be discovered by the unassisted organ. Something more is brought out by a moderately strong lens: we then see the colours disposed in rows, reminding us of the surface of Brussels carpet, or of certain kinds of tapestry work.

Now let us place the wing on the stage of a good microscope, with the root of the wing pointing towards the light (that is the best position for it); we shall then first perceive that the whole surface is covered, or, so to speak, tiled over with distinct, sharply cut scales, arranged as in fig. 16, Plate II., with the outer or free edges of one row overlapping the roots of the next. These roots being all planted towards the base of the wing, if we place that end next the light (as above directed), the free edges of the scales throw a strong shadow on the next row, which brings out the imbricated effect most strikingly.

Beginning our observations at the outer edge of the wing, we first notice a delicate fringe of scales or plumes, more elongated and pointed than the surface scales, and of a quiet brown colour. This tint is continued inwards for a short space, gradually lightening, when (as we shift the field of view towards the centre of the wing) the colour of the scales suddenly changes to an intense black; then a little further, and the black ground is all spangled with glittering sapphires, then strewed deep with amethyst round a heap of whitest pearls. Golden topaz—(jewels only will furnish apt terms of comparison for these insect gems)—golden topaz ends the bright many-coloured crescent, and in the centre is enclosed a spot of profoundest black, gradating into a rich unnameable red, whose velvet depth and softness contrast deliciously with the adjacent flashing lustre; then comes another field of velvet black, then more gold, and so on till the gorgeous picture is complete.

Subject a piece of finest human painting to the scrutiny of a strong magnifying glass, and where is the beauty thereof? Far from being magnified, it will have wholly vanished: its cleverest touches turned to coarse, repulsive daubs and stains.

Now, bring the microscope's most searching powers to bear upon the painting of an insect's wing, and we find only pictures within pictures as the powers increase; the very pigments used turn out to be jewels, not rough uncut stones, but cut and graven gems, bedded in softest velvet.

If by gentle rubbing with the finger-tip the scales be removed from both sides of the wing (for each side is scale-covered, though generally with a very different pattern), there remains a transparent membrane like that of a bee's or fly's wing, tight stretched between stiff branching veins, but bearing no vestige of its late gay painting, thus showing that the whole of the colouring resides in the scales, the places occupied by the roots of the latter being marked by rows of dots.

Hitherto we have been looking at these scales as the component parts of a picture, like the tesseræ of mosaic work; but they are no less interesting as individual objects, when viewed microscopically. To do this, delicately rub off a little of the dust or scales with the finger; then take a slip of glass, and pressing the finger with the adhering dust upon it, the latter will come off and remain on the glass, which is then to be placed under the microscope. These scales may be treated either as opaque or transparent objects, and in both conditions display exceeding beauty, some of these single atoms showing, by aid of the microscope, as much complexity of structure as the whole wing does to the unassisted vision.

A few of the highly varied forms they present are shown on Plate II. Figs. 23 to 38 are selected from among the commoner forms, as seen by a comparatively low power. The small stalk-like appendage is the part by which the scale is affixed to the wing: it may be called the root. Figs. 17, 18, 19, 20, 21, show some very remarkable forms, which are, so far as has been ascertained, peculiar to butterflies of the male sex, though the use or reason of this masculine badge, only visible to highly magnifying optics, is neither known nor probably to be known at present; but singularly beautiful and curious they are to look at. The little balls at the end of threads are the root portion, and fit into cup-like sockets, placed here and there among the ordinary scales. The surface of these scales is beautifully ribbed and cross-ribbed, and at the upper end is a plume-like tuft of delicate filaments. The curious scale aptly called, from its shape, the Battledore scale, and shown at fig. 22, also belongs to the male of various butterflies, especially those pretty little ones known as the "Blues." Its surface is most curiously ornamented with rows of bead-like prominences.

Probably one would imagine that in such wee specks as are these scales, one single layer of substance would suffice for their whole thickness (if we can talk of thickness, with objects almost immeasurable in their thinness). But such is not the case, for when scales have been injured by rubbing we now and then find a part with the sculptured surfaces torn off on each side, showing a plain central layer, so that at least three layers—two ornamented and one plain—go to form a filmy body, only a small fraction of the thickness of paper.

But there are other portions of a butterfly to claim our interest besides its wondrous wings.

On the creature's head are grouped together some most beautiful and important organs. The most peculiar of these is the long spiral "sucker," which extracts the honied food from the blossoms to which its wings so gracefully waft it. This organ is shown, slightly magnified, at fig. 8, Plate II., and a most delicate piece of animal mechanism it is. Any human workman would, to a certainty, be not only puzzled, but thoroughly beaten, in an attempt to construct a tube little thicker than a horse-hair, yet composed throughout its length of two distinct pieces, capable of being separated at pleasure, and then joined again so as to form an air-tight tube. This redoubtable problem, however, is solved in the construction of this curious little instrument that every butterfly carries.

The junction of the two grooved surfaces that form the tube is effected by the same contrivance that reunites the web of a feather when it has been pulled apart. We all know how completely it is made whole again, and on examining by what means this result is brought about, we find that it is by the interlacing of a number of small fibres or hairs, just as, on a larger scale, a pair of brushes adhere when pressed face to face; and so in the butterfly's sucker, the two edges that join to form the tube are closely set with minute bristles that, when brought together, interlock so closely as to make an air-tight surface.

Fig. 9, Plate II., is a transverse section taken near the base of the sucker, the small opening at the top being the food passage, those at the side the air-tubes that supply air for respiration and perhaps assist in suction.

The tube is probably made with separable parts in order that if its interior should become at any time clogged by grosser particles drawn up with the flower nectar, it may be opened and cleansed by the insect; otherwise, the tube once rendered impassable, the insect would speedily starve, as this narrow channel is the only inlet for the creature's nourishment—its only mouth, in fact, for no butterfly possesses jaws to bite with, or can take any but the liquid food pumped up by suction through this pipe.

At the end of the proboscis—or, as it is called scientifically, the Haustellum^[3]—there are visible in some butterflies a number of small projections, of the form shown at fig. 10, Plate II., which is a highly magnified figure of the end of the Red Admiral's proboscis. These appendages are generally supposed to be organs of taste, and to aid in the discrimination of food when the pipe is unrolled and thrust down deep into the nectary of a flower.

The compound eye of a butterfly, wonderful as its structure is, does not greatly differ from that of many other insects, being like them composed of an immense number of little lenses set together to form a hemisphere large in comparison with the insect's head. A portion of one of these eyes forms a pretty and interesting object for the microscope, presenting a honey-comb appearance, the hexagonal lines that mark the division of the lenses being most beautifully geometrical and regular in their arrangement. More than seventeen hundred of these lenses have been counted in a single eye, and each of these is considered to possess the qualities of a complete and independent eye. If this be true, the butterfly may be said to be endowed with at least thirty-four thousand eyes!

There exist also, as in other insects, two simple eyes, placed on the top of the head, but so buried in down and scales as to be neither visible, nor useful for vision as far as we can perceive; probably the creature finds that his allowance of thirty-four thousand windows to his soul lets in as much light as he requires.

Every one looking at a butterfly must have remarked its long horns, called antennæ,^[4] which project from above the eyes, like jointed threads, thickening—in some species gradually, in others suddenly—into a club or knob at the extremity; a peculiarity which, it will be remembered, was pointed out at the commencement, as a prominent mark of distinction between butterflies and moths.

Very graceful appendages are these waving antennæ, and evidently of high importance to their owner; but still, their exact office or function is unknown, notwithstanding that many guesses and experiments have been made with a view of settling that question.

Investigators have perhaps erred, by assuming at the outset that these antennæ must be organs of some sense that we ourselves possess; whereas, I think that there is much evidence to show that insects are gifted with a certain subtle sense, for which we have no name, and of which we can have as little real idea, as we could have had of the faculty of sight, had all the world been born blind.

For example; if you breed from the chrysalis a female Kentish Glory Moth, and then immediately take her—in a closed box, mind—out into her native woods, within a short space of time an actual crowd of male "Glories" come and fasten upon, or hover over, the prison-house of the coveted maiden. Without this magic attraction, you might walk in these same woods for a whole day and not see a single specimen, the Kentish Glory being generally reputed a very rare moth; while as many as some 120 males have been thus decoyed to their capture in a few hours, by the charms of a couple of lady "Glories," shut up in a box.

Now, which of our five senses, I would ask—even if developed into extraordinary acuteness in the insect—would account for such an exhibition of clairvoyance as this?

May not, then, this undiscovered sense, whatever may be its nature, reside in the antennæ? for it is a remarkable fact, that the very moths, such as the Eggers, the Emperor, the Kentish Glory, &c., which display the above-mentioned phenomenon most signally, have the antennæ in the males amplified with numerous spreading branches, so as to present an unusually large sensitive surface. This seems to point to some connexion between those organs and the faculty of discovering the presence, and even the condition, of one of their own race, with more, perhaps, than a mile of distance, and the sides of a wooden box, intervening between themselves and their object.

Whilst writing this, the current number of the "Entomologist's Weekly Intelligencer" has arrived, and I there read that Dr. Clemmens, an American naturalist, has been lately experimenting on the antennæ of some large American moths, for the purpose of gaining some information as to their function. The article, though very interesting, is too long for quotation here; but it appears that with the moths in question, a deprivation of the whole, or even part of the antennæ, interferes with, or entirely annihilates the power of flight, so that the creature when thus shorn, but not otherwise injured, if thrown into the air seems to have no idea of using his wings properly, but with a purposeless flutter tumbles headlong to the earth. Still this merely goes to prove that the antennæ are the instruments of some important sense, one of whose uses is to guide the creature's flight; but as many wingless insects have large antennæ, this evidently is not their only function.

The antennæ are also often styled the "feelers;" but with our present incomplete knowledge of their nature, the former term is preferable, as it does not attempt to define their use as the word "feelers" does.

Considerable variety of form exists in the clubbed tip of the antennæ in various butterflies, as will be seen by reference to Plate II., where three of the most distinct forms are shown considerably magnified. Fig. 12 is the upper part of the antenna of the High-brown Fritillary (Argynnis Adippe), the end suddenly swelling into a distinct knob. Fig. 13 is that of the Swallow-tail Butterfly (Papilio Machaon), the enlargement here being more gradual; and fig. 14 is that of the Large Skipper Butterfly (Pamphila Sylvanus), distinguished by the curved point that surmounts the club. These differences in the forms of the antennæ are found to be excellent aids in the classification of butterflies, and I shall therefore have occasion to refer to them more minutely in describing the insects in detail.

The stems of these organs are found to be tubular, and at the point of junction with the head the base is spread out (as shown at fig. 15), forming what engineers call a "flange," to afford sufficient support for the long column above.

The legs are the last portions of the butterfly framework that require especial notice, on account of a peculiar variation they are subject to in different family groups.

It may be laid down as an axiom, that all true insects have six legs, in one shape or another; and butterflies, being insects, are obedient to the same universal rule, and duly grow their half-dozen legs; but in certain tribes the front pair, for no apparent reason, are so short and imperfect as to be totally useless for walking purposes, though they may possibly be used as hands for polishing up the proboscis, &c. So the butterfly in this case appears, to a hasty observer, to have only four legs.

This peculiarity is a constant feature in several natural groups of butterflies, and therefore, in conjunction with other marks, such as the veining of the wings and the shape of the antennæ, its presence or absence is a most useful mark of distinction, in classifying or searching out the name and systematic place of a butterfly.