Читать книгу British Butterflies: Figures and Descriptions of Every Native Species - W. S. Coleman - Страница 4

Table of Contents

INTRODUCTION.

WHAT IS A BUTTERFLY—BUTTERFLIES AND MOTHS—BUTTERFLY LIFE—THE EGG STAGE—SCULPTURED CRADLES—BUTTERFLY BOTANY—THE CATERPILLAR STAGE—FEEDING UP—COAT CHANGING—FORMS OF CATERPILLARS—THE CHRYSALIS—MEANING OF PUPA, CHRYSALIS, AND AURELIA—FORMS OF CHRYSALIDES—DIFFICULTIES OF TRANSFORMATION—INFLUENCE OF TEMPERATURE.

Occasionally a missive arrives from some benevolent friend, announcing the capture of a "splendid butterfly," which, imprisoned under a tumbler, awaits one's acceptance as an addition to the cabinet. However, on going to claim the proffered prize, the expected "butterfly" turns out to be some bright-coloured moth (a Tiger moth being the favourite victim of the misnomer), and one's entomological propriety suffers a shock; not so much feeling the loss of the specimen, as concern for the benighted state of an otherwise intelligent friend's mind with regard to insect nomenclature.

It is clearly therefore not so superfluous as it might at first otherwise seem, to commence the subject by defining even such a familiar object as a butterfly, and more especially distinguishing it with certainty from a moth, the only other creature with which it can well be confounded.

The usual notion of a butterfly is of a gay fluttering thing, whose broad painted wings are covered with a mealy stuff that comes off with handling. This is all very well for a general idea, but the characters that form it are common to some other insects besides butterflies. Moths and hawk-moths have mealy wings, and are often gaily coloured too; whilst, on the other hand, some butterflies are as dusky and plain as possible. Thus the crimson-winged Tiger, and Cinnabar moths get the name of butterflies, and the Meadow brown butterfly is as sure to be called a moth. So, as neither colouring nor mealy wings furnish us with the required definition, we must find some concise combination of characters that will answer the purpose. Butterflies, then, are insects with mealy wings, and whose horns (called "antennæ") have a clubbed or thickened tip, giving them more or less resemblance to a drum-stick. So the difference in the shape of the antennæ is the chief outward mark of distinction between butterflies and moths, the latter having antennæ of various shapes, threadlike or featherlike, but never clubbed at the tip.

Having thus settled how a butterfly is to be recognized at sight, let us see what butterfly life is: how the creature lives, and has lived, in the stages preceding its present airy form.

In like manner with other insects, all butterflies commence their existence enclosed in minute eggs; and these eggs, as if shadowing forth the beauty yet undeveloped whose germ they contain, are themselves such curiously beautiful objects, that they must not be passed over without admiring notice. It seems, indeed, as if nature determined that the ornamental character of the butterfly should commence with its earliest stage; form, and not colour, being employed in its decoration, sculpture being here made the forerunner of painting.

Some of these forms are roughly shown on Plate II. (figs. 1-7), but highly magnified; for as these eggs are really very tiny structures, such as would fall easily through a pin-hole, the aid of a microscope is of course necessary to render visible the delicate sculpture that adorns their surface. The egg (fig. 1, Plate II.) of the common Garden white butterfly (Pieris Brassicæ) is among the most graceful and interesting of these forms, and also the most easily obtained. It reminds us of some antique vessel, ribbed and fluted with consummate elegance and regularity.

Others—such as those of the Large Heath butterfly (fig. 3), and the Queen of Spain Fritillary (fig. 2), simulate curious wicker-work baskets. The Peacock butterfly has an egg like a polygonal jar (fig. 4), while that of its near ally, the large Tortoise-shell (fig. 5), is simply pear-shaped, with the surface unsculptured and smooth (fig. 5). The eggs of the Meadow Brown (fig. 6), and the Wood Argus (fig. 7), are globular—the former with lines on its surface like the meridian lines on a geographical globe, and a pretty scalloping at the top that gives a flower-like appearance to that portion; the latter has the whole surface honey-combed with a network of hexagonal cells. Such are a few of the devices that ornament the earliest cradle of the butterfly; but probably those of every species would well repay their examination to any one who possesses a microscope.

Prompted by a most remarkable instinct, and one that could not have originated in any experience of personal advantage, the female butterfly, when seeking a depository for her eggs, selects with unerring certainty the very plant which, of all others, is best fitted for the support of her offspring, who, when hatched, find themselves surrounded with an abundant store of their proper food.

Many a young botanist would be puzzled at first sight to tell a sloe-bush from a buckthorn-bush. Not so, however, with our Brimstone butterfly: passing by all the juicy hedge-plants, which look quite as suitable, one would think, she, with botanical acumen, fixes upon the buckthorn; either the common one, or, if that is not at hand, upon another species of rhamnus—the berry-bearing alder—which, though a very different looking plant, is of the same genus, and shares the same properties. She evidently works out the natural system of botany, and might have been a pupil of Jussieu, had she not been tutored by a far higher Authority.

This display of instinct would seem far less wonderful did the mother butterfly herself feed on the plant she commits her eggs to. In that case, her choice might have appeared as the result of personal experience of some peculiar benefit or pleasure derived from the plant, and then this sentiment might have become hereditary; just as, for example, the acquired taste for game is hereditary with sporting dogs. Whereas the fact is, that a butterfly only occasionally, and as a matter of accident rather than rule, derives her own nectareous food from the flowers of the plant, whose leaves nourish her caterpillar progeny. So that this, as well as numberless other phenomena of instinct, remains a mystery to be admired, but not explained by any ordinary rule of cause and effect.

Having thus efficiently provided, as far as board and lodging are concerned, for the welfare of the future brood, the mother seems to consider them settled for life, takes no further care of them, nor even awaits the opening of the sculptured caskets that contain their tiny life-germs; but, trusting them to the sun's warmth for their hatching, and then to their own hungry little instincts to teach them good use of the food placed within their reach, she sees them no more.

But though abandoning her offspring to fate in this manner, it must not be imagined that the butterfly mother takes her pattern of maternity from certain human mothers, and in a round of "butterfly's balls," and such like dissipations, forgets the sacred claims of the nursery. No, she has far other and better excuses for absenting herself from her family; one of which is, that she usually dies before the latter are hatched; and if that is not enough, that the young can get on quite as well without her; for probably she could not teach them much about caterpillar economics, unless, indeed, she remembered her own infantile habits of lang syne, so totally different from those of her perfected butterfly life.

The space of time passed in the egg state varies much according to the temperature—from a few days when laid in genial summer weather, to several months in the case of those laid in the autumn, and which remain quiescent during the winter, to hatch out in the spring.

The eggs of butterflies, in common with those of insects in general, are capable of resisting not only vicissitudes, but extremes of temperature that would be surely destructive of life in most other forms. The severest cold of an English winter will not kill the tender butterfly eggs, whose small internal spark of vitality is enough to keep them from freezing under a much greater degree of cold than they are ever subjected to in a state of nature. For example, they have been placed in an artificial freezing mixture, which brought down the thermometer to 22° below zero—a deadly chill—and yet they survived with apparent impunity, and afterwards lived to hatch duly. Then as to their heat-resisting powers, some tropical insects habitually lay their eggs in sandy, sun-scorched places, where the hand cannot endure to remain a few moments; the heat rising daily to somewhere about 190° of the thermometer—and we know what a roasting one gets at 90° or so. Yet they thrive through all this.

For a short time previous to hatching, the form and colour of the caterpillar is faintly discoverable through the semi-transparent egg-shell. The juvenile Caterpillar, or Larva, gnaws his way through the shell into the world, and makes his appearance in the shape of a slender worm, exceedingly minute of course, and bearing few of the distinctive marks of his species, either as to shape or colouring. On finding himself at liberty, in the midst of plentiful good cheer, he at once falls vigorously to work at the great business of his life—eating; often making his first meal—oddly enough—off the egg-shell, lately his cradle. This singular relish, or digestive pill, swallowed, he addresses himself to the food that is to form the staple fare during the whole of his caterpillar existence—viz. the leaves of his food-plant, which at the same time is his home-plant too.

At this stage his growth is marvellously rapid, and few creatures can equal him in the capacity for doubling his weight—not even the starved lodging-house "slavey," when she gets to her new place, with carte blanche allowance and the key of the pantry; for, in the course of twenty-four hours, he will have consumed more than twice his own weight of food: and with such persevering avidity does he ply his pleasant task, that, as it is stated, a caterpillar in the course of one month has increased nearly ten thousand times his original weight on leaving the egg; and, to furnish this increase of substance, has consumed the prodigious quantity of forty thousand times his weight of food—truly, a ruinous rate of living, only that green leaves are so cheap.

But the life of a caterpillar, after all, is not merely the smooth continual feast he would doubtless prefer it to be; it is interrupted, several times in its course, by the necessity nature has imposed upon him of now and then changing his coat—to him a very troublesome, if not a painful affair.

For some time previous to this phenomenon, even eating is nearly or quite suspended,—the caterpillar becomes sluggish and shy, creeping away into some more secluded spot, and there remaining till his time of trouble is over. Various twitchings and contortions of the body now testify to the mal-aise of the creature in his old coat, which, though formed of a material capable of a moderate amount of stretching, soon becomes outgrown, and most uncomfortably tight-fitting, with such a quick-growing person inside it: so off it must come, but it being unprovided with buttons, there's the rub. However, with a great deal of fidgeting and shoulder-shrugging, he manages to tear his coat down the back, and lastly, by patient efforts, shuffles off the old rag; when, lo! underneath is a lustrous new garment, somewhat similar, but not exactly a copy of the last, for our beau has his peculiar dress for each epoch of his life,—the most splendid being often reserved for the last.

This change of dress ("moulting," it is sometimes called) is repeated thrice at least in the creature's life, but more generally five or six times. Not only does the outer husk come off at these times, but, wonderful to relate! the lining membrane of all the digestive passages, and of the larger breathing tubes, is cast off and renewed also.

After each moult, the caterpillar makes up for his loss of time by eating more voraciously even than before, in many instances breaking his fast by making a meal of his "old clo'"—an odd taste, first evinced, as we have seen, in earliest infancy, when he swallowed his cradle.

On Plate I. are shown the chief varieties of form taken by the caterpillars of our British butterflies, and a glance at these will give, better than verbal descriptions, a general idea of their characteristics.

Their most usual shape is elongated and almost cylindrical, or slightly tapering at one or both ends. Of these, some are smooth, or only studded with short down or hairs; such are the caterpillars of the Swallow-tail butterfly (fig. 1), of the Brimstone (fig. 2), Clouded Yellows, and Garden, and other white butterflies. Others, of the same general form, are beset with long branched spines, making perfect chevaux-de-frise; such are those of the Peacock, Red Admiral, Painted Lady, and the Silvery Fritillaries.

The caterpillars of another large section have the body considerably thicker in the middle (rolling-pin shaped), and the tail part two-forked, or bifurcate. This form belongs to the numerous family that includes the Meadow-brown (fig. 3), the Ringlets, and many others.

The bizarre personage, at fig. 4, turns to the graceful White Admiral butterfly.

The Purple Emperor begins his royal career in the curious form shown at fig. 5—a shape unique among British butterflies, as beseems that of their sovereign; and he carries a coronet on his brow already.

All those beautiful little butterflies called the Hair-streaks (fig. 9), the Blues (fig. 10), and the Coppers, have very short and fat caterpillars, that remind one forcibly of wood-lice—a shape shared also by that small butterfly with a big name, the Duke of Burgundy Fritillary (fig. 8), an insect very distinct from the Fritillaries above mentioned with thorny caterpillars.

The legs of a caterpillar are usually sixteen in number, and composed of two distinct kinds, viz. of six true legs, answering to those of the perfect insect, and placed on the foremost segments of the body; and of ten others, called "prolegs;" temporary legs, used principally for strengthening the creature's hold upon leaf or branch.

Like the rest of its body, the caterpillar's head widely differs in structure from that of the perfect insect, being furnished with a pair of jaws, horny and strong, befitting the heavy work they have to get through, and shaped like pincers, opening and shutting from side to side, instead of working up and down after the manner of the jaws in vertebrate animals. This arrangement offers great convenience to the creature, feeding, as it is wont to do, on the thin edge of a leaf. It is a curious sight to watch a caterpillar thus engaged. Adhering by his close-clinging prolegs, and guiding the edge of the leaf between his forelegs, he stretches out his head as far as he can reach, and commences a series of rapid bites, at each nibble bringing the head nearer the legs, till they almost meet; then stretching out again the same regular set of mouthfuls is abstracted, and so on, repeating the process till a large semi-circular indentation is formed, reaching perhaps to the midrib of the leaf; then shifting his position to a new vantage ground, the marauder recommences operations, another sweep is taken out, then another, and soon the leaf is left a mere skeleton.

But a change, far more important than mere skin-shifting, follows close upon the animal's caterpillar-maturity, complete as soon as it ceases to grow.

The form and habits of a worm are to be exchanged for the glories and pleasures of winged life; but this can only be done at the price of passing through an intermediate state; one neither of eating, nor of flying, but motionless, helpless and death-like.

This is called the Chrysalis or Pupa state.

Pupa is a Latin word, signifying a creature swathed, or tied up; and is applied to this stage of all insects, because all, or some, of their parts are then bound up, as if swathed.

The term Chrysalis is applicable to butterflies only, and, strictly, only to a few of these—Chrysalis^[1] being derived from the Greek χρυσός (chrysos), gold—in allusion to the splendid gilding of the surface in certain species, such as the Vanessas, Fritillaries, and some others.

In the older works on entomology we frequently meet with the term Aurelia applied to this state, and having the same meaning as chrysalis, but derived from the Latin word Aurum, gold.

Here the reader is again referred to Plate I. for a series of the principal forms assumed by the chrysalides of our native butterflies, and as these for the most part represent the next stage of the caterpillars previously figured, an opportunity is afforded of tracing the insect's form through its three great changes; the whole of the butterflies in their perfect state being given in their proper places in the body of the work.

The complicated and curious processes by which various caterpillars assume the chrysalis form, and suspend themselves securely in their proper attitudes, have been most accurately and laboriously chronicled by the French naturalist, Réaumur; but his memoirs on the subject, which have been frequently quoted into the larger entomological works, are too long for insertion here in full, and any considerable abbreviation would fail to convey a clear idea of the process, on account of the intricacy of the operations described. So I can only here allude to the difficult problems that the creature has to solve, referring the reader to the above-mentioned works for a detailed description of the manner of doing so; or, better still, I would recommend the country resident to witness all this with his own eyes. By keeping a number of the caterpillars of our common butterflies, feeding them up, and attentively watching them when full-grown, he will now and then detect one in the transformation act, and have an opportunity of wondering at the curious manœuvres of the animal, as it triumphs over seeming impossibilities.

By reference to the figures of chrysalides on Plate I. it will be seen that there are two distinct modes of suspension employed among them; one, by the tail only, the head hanging down freely in the air:—in the other, the tail is attached to the supporting object; but the head, instead of swinging loosely, is kept in an upright position by being looped round the waist with a silken girdle.

To appreciate the difficulty of gaining either of the above positions, we must bear in mind that, before doing so, the caterpillar has to throw off its own skin, carrying with it the whole of its legs, and the jaws too—leaving itself a mere limbless, and apparently helpless mass—its only prehensile organs being a few minute, almost imperceptible hooks on the end of the tail; and the required position of attachment and security is accomplished by a series of movements so dexterous and sleight-of-hand like, as to cause infinite astonishment to the looker-on, and, as Réaumur justly observes, "It is impossible not to wonder, that an insect, which executes them but once in its life, should execute them so well. We must necessarily conclude that it has been instructed by a Great Master; for He who has rendered it necessary for the insect to undergo this change, has likewise given it all the requisite means for accomplishing it in safety."

If we examine a chrysalis we are able to make out, through the thin envelope, all the external organs of the body stowed away in the most orderly and compact manner. The antennæ are very conspicuous, folded down alongside of the legs; and precisely in the centre will be seen the tongue, unrolled and forming a straight line between the legs. The unexpanded wings are visible on each side—very small, but with all their veinings distinctly seen; and the breathing holes, called spiracles, are placed in a row on each side of the body.

The duration of the chrysalis stage, like that of the egg, is extremely variable, and dependent on difference of temperature. As an instance of this, one of our common butterflies has been known to pass only seven or eight days in the chrysalis state; this would be in the heat of summer. Then, in the spring, the change occupies a fortnight; but when the caterpillar enters the chrysalis state in the autumn, the butterfly does not make its appearance till the following spring. Furthermore, it has been proved by experiment, that if the condition of perpetual winter be kept up by keeping the chrysalis in an icehouse, its development may be retarded for two or three years beyond its proper time; while, on the other hand, if in the middle of winter the chrysalis be removed to a hothouse, the enclosed butterfly, mistaking the vivifying warmth for returning summer, makes its début in ten days or a fortnight.