Fig. 10. – The Four Stages of the Large White Butterfly (Pieris Brassicæ).
a, larva; b, pupa; c, imago; d, egg.
Neither can we easily impute to the insect an acquired knowledge of the nature and wants of her offspring, or an acquaintance with botany sufficient to enable her to distinguish plant forms. Our only solution of the problem (which is really no solution at all) is to attribute the whole thing to that inexplicable quality which we are pleased to term natural instinct. It is to be observed, however, that it is not all butterflies and moths that display this unerring power. Some few seem to deposit their eggs indiscriminately on all kinds of herbage. But, I believe, the larvæ of these species are generally grass feeders, and would seldom have to travel far from any spot without meeting with an acceptable morsel.
But we must now pass on to a brief consideration of the other stages of the insect's existence. After a time, varying from a few days to several months, the young caterpillars or larvæ make their appearance. They soon commence feeding in right earnest. Their period of existence in this state varies from a few weeks to several months, and even, in some cases, to years. During this time their growth is generally very rapid, and they undergo a series of moults or changes of skin, of which we shall have more to say in a future chapter. Then, when fully grown, they prepare for an apparently quiescent form, which we speak of as the pupa or chrysalis, and in which they again spend a very variable period, extending over a few days, weeks, or months. Now, inclosed in a protective case, each pupa is undergoing a remarkable change. Some of its old organs are disappearing, and others are developing; and, after all the parts of the future insect have been developed as far as its narrow shell will permit, it bursts forth into the world as a perfect insect or imago.
Its wings at first are small, shapeless, and crumpled in a most unsightly fashion; but it is not long before they assume their full size, beautiful form, and gorgeous colouring. Then, in about another hour or two, the wings, at first soft and flaccid, have become sufficiently dry and stiff to bear their owner rapidly through the air.
We have thus observed some of the more striking features in the structure of the butterfly and moth in its most perfect state; and alluded in a very brief manner to the various stages through which these creatures must necessarily pass before finally reaching this stage. But now we must study these earlier stages more closely, and watch the insects during the marvellous transitions they are destined to undergo. This we shall do in the following chapters.
CHAPTER II
THE EGG
I suppose you are all acquainted with the general structure of the hen's egg, having dissected several, in your own way, many a time.
Its outer covering, which you speak of as the 'shell,' you have observed is hard and brittle. It is composed of a calcareous or limy substance, known chemically as carbonate of lime. If you put some pieces of it into an egg cup, and throw over them a little vinegar or any other liquid acid, you will see them gradually dissolve away, and small bubbles of carbonic acid gas will rise into the air. Then again, if you take a long and narrow strip of the shell, and hold one end of it in a gas or lamp flame, after a short time that end will become softer, and will glow brightly in the flame, for it is converted into lime – the same substance that is used by the builders for making their mortar – and the bright glow is really a miniature lime light, such as is always produced when a piece of lime is made intensely hot.
Just inside this shell you have seen a thin membrane or skin that is easily peeled off the substance of the egg itself. Next to this comes the 'white' of the egg, which is really colourless while liquid, but turns white and more or less solid in the cooking. Last of all, in the centre of this, you have noticed the oval yellow mass that is termed the 'yoke' or 'yolk,' and which contains the embryo of the future chick.
Now if you imagine this egg to be reduced in size till two or three dozen of them would be required to form a single line about one inch long, the outer calcareous shell to be entirely removed, the skin or membrane to be converted into a firmer substance of a horny nature, and, finally, the yolk to be absent and the whole internal space to be filled with the 'white,' you will then have some idea of the nature of the egg of a butterfly or moth.
To put the matter more briefly, then, we will say that the eggs of these insects are simply little liquid masses, usually of a colourless substance, surrounded by a horny and flexible covering.
Such a description may certainly give you some idea of the nature of the eggs of insects, but no amount of book reading will serve the purpose so well or be so pleasant as the examination of the eggs themselves. During the summer months very little difficulty will be experienced in finding some eggs in your own garden. Turn over some leaves and examine their under surfaces, choosing especially those plants which show, by their partially eaten leaves, that they are favourites with the insect world. Or you may amuse yourself by catching a number of butterflies – common 'Whites' are as good for the purpose as any – and temporarily confine them in a wooden or cardboard box, containing a number of leaves from various plants, and covered with gauze. In this way you are sure to obtain a few females that have not yet laid all their eggs; and if you watch your prisoners you will soon see them carefully depositing the eggs on the under surfaces of leaves, bending their abdomens round the edges if there is not sufficient room to get themselves completely under. And then, when you are satisfied with the number of eggs thus obtained for your examination, you can have the pleasure of seeing all your liberated captives flying joyfully in the free air.
In giving these simple instructions I have assumed that the reader has not yet learnt any of the characters by which female butterflies are to be distinguished from their lords and masters; but I hope that he will know soon, at least with regard to a good many species, from which individuals he may most reasonably expect to obtain eggs, and so be able to avoid the imprisonment, even though only temporary, of insects which cannot satisfy his wants.
Again, it is not necessary, after all, that butterflies should be captured for the purpose of obtaining eggs. Watch them as they hover about among your flowers. Some, you will observe, are intent on nothing but idle frolicking; and you may conclude at once that these have no immediate duty to perform. Others are flying without hesitation from flower to flower, gorging themselves with the sweets of life: these are not the objects of your search. But you will descry certain others, flying round about the beds and borders with a steadier and more matronly air, taking little or no notice of their more frivolous companions, and paying not the slightest heed to the bright nectar-producing cups of the numerous flowers. These are seriously engaged with family affairs only. Watch one of them carefully, and as soon as she has settled herself on a leaf, walk steadily towards her till you are near enough to observe her movements. She will not move unless you approach too closely, for, like busy folk generally, she has no time to worry about petty annoyances. You will now actually witness the deposition of the eggs exactly as carried on in the perfect freedom of nature; and the eggs themselves may be taken either for examination or for the rearing of the caterpillars.
Some species of Lepidoptera lay some hundreds of eggs, and it is seldom that the number laid by one female is much below a hundred.
As already stated, the under surfaces of leaves are generally chosen for the deposit of eggs, but a few of the insects we are considering always select the upper surface for this purpose. Thus the Puss Moth (page 235), and two or three others resembling it, though much smaller, known as the Kittens (page 234), invariably lay them on the upper surface. And this is the more surprising since the eggs of these moths are brown or black, and consequently so conspicuous on the green leaves as to be in danger of being sighted by the numerous enemies of insects.
The Hairstreak Butterflies (page 183) afford another exception to the general rule, for their eggs are deposited on the bark of the trees and shrubs (birch, sloe, elm, oak, and bramble) on which their larvæ feed.
At the moment each egg is laid it is covered with a liquid sticky substance, so that it is immediately glued to the leaf or stem as soon as it is deposited. The sticky substance soon dries, causing the egg to be so firmly fastened in its place that it is often impossible to force it off without destroying it completely.
Some of the Lepidoptera deposit their eggs singly, or in small irregular clusters; but by far the larger number set them very regularly side by side, in so compact a mass that it would be impossible to place them on a smaller area without piling one on top of another. This is not accomplished with the aid of the sight, for the insect performing her task with such precision often has her head on one side of a leaf or stem while arranging her eggs on the other. If you take the trouble to watch her, you will see that she carefully feels out a place for each egg by means of the tip of her abdomen immediately before laying it.
The eggs are laid by moths and butterflies at various seasons of the year. In some cases they are deposited early in the spring, even before the buds of the food plants have burst; and the young larvæ, hatched a few weeks later, commence to feed on the young and tender leaves. Then, throughout the late spring, the whole of the summer and autumn, and even till the winter frosts set in, the eggs of various species are being laid.
Those deposited during the warm weather are often hatched in a few days, but those laid toward the autumn remain unchanged until the following spring.
In this latter case the frosts of the most severe winter are not capable of destroying the vitality of the eggs. In many instances the perfect insect or the larva would be killed by the temperature of an average winter day, but the vitality of the eggs is such that they have been subjected to a temperature, artificially produced, of fifty degrees below the freezing point, and even after this the young larvæ walked out of their cradles at their appointed time just as if nothing unusual had occurred.
Experiments have also been performed on the eggs with a view of determining how far their vitality is influenced by high temperatures. We know that the scorching midsummer sun has no destructive influence on them, but these experiments prove that they are not influenced by a temperature only twenty degrees below the boiling point – actually a considerably higher temperature than is necessary to properly cook a hen's egg.
Let us now examine a number of eggs of different species, that we may note some of the many variations in form and colour.
With regard to colour, we have already observed that the eggs of a few species are black; but more commonly they are much lighter – pearly white, green, yellow, and grey being of frequent occurrence.
The great variety of form, however, will provide a vast amount of enjoyment to anyone who possesses a good magnifying lens or a small compound microscope. Some are globular, others oval; while many others represent cups, basins, and domes. Then we have miniature vases, flasks, bottles with short necks, and numerous figures that must remind a juvenile admirer of the sweet cakes and ornamental jellies that have so often gladdened his longing eyes.
Again, the beautifully sculptured surfaces of a large number are even more striking than their general shapes. Some are regularly ribbed from top to bottom with parallel or radiating ridges, and at the same time marked with delicate transverse lines. Others are beautifully pitted or honeycombed, some ornamented with the most faithful representation of fine wicker-work, while a few are provided with a cap, more or less ornamental, that is raised by the young larva when about to see the world for the first time. A few of these beautiful forms are here illustrated and named, and another has already appeared on page 14, but an enthusiastic young naturalist may easily secure a variety of others for his own examination.
Fig. 11. – Egg of the Meadow Brown Butterfly.
Fig. 12. – Egg of the Speckled Wood Butterfly.
Fig. 13. – Egg of the Vapourer Moth.
It may be surmised from the accompanying illustrations that the form of the egg is always the same for any one species. This is really the case, and consequently an experienced entomologist can often decide on the name of the butterfly or moth that deposited a cluster of eggs he happens to find in his rambles and searchings; but in such decisions he is always greatly assisted by a knowledge of the food plants of the various insects, and sometimes also by the manner in which the eggs are arranged.
We have seen that the period during which the Lepidoptera remain in the egg stage is very variable, and depends largely on the season in which they were laid; but it is often possible to tell when to expect the young larvæ by certain changes which take place in the appearance of the egg. As the horny covering of the egg is transparent, the gradual development of the caterpillar from the clear fluid can be watched to a certain extent; but if you have a microscope, and would like to witness this development to perfection, proceed as follows.
Arrange that some butterflies and moths shall lay their eggs on strips of glass of convenient dimensions for microscopic work – three inches long by one wide is the usual size for this kind of work. This is easily accomplished by placing a proper selection of female insects in a rather small box temporarily lined with such 'slips.' When a few eggs have thus been secured, all you have to do is to examine them at intervals with your microscope, always using the reflector so as to direct a strong light through the eggs from below.
But even without such an arrangement some interesting changes are to be observed. As a rule, the colour of the egg turns darker as the time for the arrival of the infant larva approaches, and you will often be able to see a little brown or black head moving slightly within the 'shell.' You may know then that the hatching is close at hand, and the movements of the tiny creature are well worth careful watching. Soon a small hole appears in the side of the case, and a little green or dark cap begins to show itself. Then, with a magnifier of some kind, you may see a pair of tiny jaws, working horizontally, and not with an up-and-down motion like our own, gradually gnawing away at the cradle, till at last the little creature is perfectly free to ramble in search of food.
Strange to say, the young larva does not waste a particle of the horny substance that must necessarily be removed in securing its liberty, but devours it with an apparent relish. Indeed, it appreciates the flavour of this viand so highly that it often disposes of the whole of its little home, with the exception of the small circular patch by which it was cemented to the plant. When the whole brood have thus dispensed with their empty cradles, there remains on the stem or leaf a glittering patch of little pearly plates.
After the performance of this feat the young caterpillar starts off in life on its own account with as much briskness and confidence as if it had previously spent a term in the world under the same conditions; but we must reserve an account of its doings and sufferings for our next chapter.
CHAPTER III
THE LARVA
In almost every case the young caterpillar, on quitting the 'shell' of the egg, finds itself standing on and surrounded by its natural food, and immediately commences to do justice to the abundant supply. It will either nibble away at the surface of the leaf, removing the soft cellular substance, so that the leaf exhibits a number of semi-transparent patches when held up to the light, or it will make straight for the edge, and, closing its horizontal jaws on either side, bite the leaf completely through, and thus remove a small piece each time.
Fig. 14. – The Caterpillar of the Clouded Yellow Butterfly.
Several naturalists have amused themselves by performing experiments and making calculations on the efficiency of the masticating and digesting powers of the caterpillar. The illustrious Réaumur, for example, proved that some of the cabbage eaters disposed of more than twice their own weight of food in twenty-four hours, during which time their weight increased one-tenth. Let us see what this would be equivalent to in human beings: A man weighing eleven stone would devour over three hundred pounds of food in a day, and at the end of that day weigh about fifteen pounds more than he did at the beginning!
So the young caterpillar eats, and rests, and grows, till, while still young, its body has become too large for the already tightened skin. It evidently feels very uncomfortable. Its appetite fails, and it remains for a time perfectly quiet in one spot, having previously spun a little carpet of silk to form a firm foothold during its temporary indisposition. Its colours have also become dingy, and anyone, not understanding the character of its growth, might easily be led to suppose that the poor creature was displaying the earlier symptoms of a serious and perhaps fatal illness.
But soon an encouraging symptom is observed. The caterpillar begins to get restless. Its front segments are turned alternately right and left, and are also made to swell out much beyond their normal size. Then in a very short time – often less than a minute from the first appearances of restlessness – the skin, which has become somewhat dry and brittle, splits along the back over the second, third and fourth segments, revealing a new and bright coat beneath. The caterpillar continues its struggles and, in addition to the previous movements, causes the swelling to move backward along the body. This, acting like a wedge, causes the rent in the old coat to extend in that direction.
Fig. 15. – The Caterpillar of the Dark Green Fritillary Butterfly.
The caterpillar now draws its head backward, and, with a few convulsive struggles, pulls the front segments out of their old skin, and passes its head out of the rent in the back. With its foremost segments thus rendered perfectly free, it walks straight out of the old garment, which is left still fixed by the legs to the silken carpet.
The larva, although now fresh and smart in its appearance, is exhausted by these struggles and its prolonged fast. The new skin, moreover, is very soft and tender, even to the cases of the head and legs, which are normally very hard. But a short period of rest suffices to dry its skin and sharpen its appetite, and then it eats more vigorously than ever.
We will now leave the caterpillar for a moment while we look at its cast-off clothes. They are still clinging to a stem so firmly that they can scarcely be removed without injury. The hard shell that covered the head and jaws is perfect in form, and so are the claws and cases of the legs. All the hairs or spines that happened to adorn the previous owner still retain their positions; and the whole skin, although always more or less shrivelled, is sometimes so slightly altered in form that it might be mistaken for a living caterpillar if not closely examined.
But this is not all. For, according to the accounts of some authoritative observers, the lining of the digestive organs, which is really a continuation of the outer skin, is cast off (or rather cast out) at the same time, as are also the linings of the larger breathing tubes which are presently to be described.
Fig. 16. – The Caterpillar of the Purple Emperor Butterfly.
We have seen that some caterpillars, on quitting their egg cases (which may really be regarded as the first moult), make their first meal of the old covering. So also some of them, in their future moultings, exhibit an apparently useless economy (seeing that they are surrounded by an abundance of their natural vegetable diet) by devouring their old coats! In the face of this fact we can hardly describe them as strict vegetarians.
Having thus passed through its first hardship, the caterpillar has by no means seen the end of the troubles and dangers that beset it; for, during its existence in the larval state, it has to go through a series of three, four, five, or even six moults, all of which are periods of considerable inconvenience, and perhaps even pain, and frequently prove fatal. And it is by no means an uncommon thing to meet with the lifeless body of an unfortunate individual who, as shown by its shabby appearance and the silken carpet under its feet, has evidently fallen a victim to the dangerous process of ridding itself of an old garment.
But this is only one of the many dangers to which caterpillars are exposed. Throughout every hour of the day the sharp and hungry eyes of the numerous insect-eating birds are searching the leaves for such delicacies to satisfy the wants of themselves and their broods. The lively little lizards, too, during the sunny hours are busily engaged in searching them out among the foliage of heaths and banks.
Very formidable enemies also exist in the form of Ichneumon and other species of flies, which pierce the skins of caterpillars with their sharp ovipositors, and lay their eggs within the bodies of the unfortunate victims. As soon as the young larvæ are hatched from these eggs, they commence feeding on the fatty substance stored beneath the caterpillar's skin. They carefully avoid, at first, attacking the vital organs of their host's body, and in this way secure for themselves a more lasting supply of fresh food. When the fatty substance is nearly all gone, they eat their way into the more important structures, of course steadily growing all the time; and so, even though the body of the caterpillar is rapidly diminishing, the total bulk shows often no very appreciable decrease in size. When the larvæ of the flies are fully fed, they either change to the pupa within the carcase of their host, or eat their way out of its body and construct for themselves a cocoon in which to undergo the transformation.
Fig. 17. – An Ichneumon Fly (Cryptus Migrator).
Fig. 18. – Another Ichneumon Fly (Pimpla Instigator).
As for the caterpillar itself, it sometimes dies before the time for its metamorphosis has arrived; but it often changes to the chrysalis before its fate is sealed. In this latter case, a number of flies, having undergone their final transformation within the chrysalis shell (there being but little else than shell remaining of the victim's body), break forth from the remains of the carcase somewhere about the time at which the butterfly or moth should have appeared.
Caterpillars have also their nocturnal enemies and devourers, among which may be mentioned frogs, toads, newts, and insect-eating mammals.
We must now learn something of the structure of caterpillars; and then become acquainted with their habits, and the change to the chrysalis or pupa.
Take a caterpillar from your garden, preferably a full-grown one of a rather large species, that is not very densely covered with hair, and examine it carefully as we note the main points in its structure. The first point that strikes our notice is the division of its body into segments or rings, separated from each other by a more or less distinct line or slight constriction of the body.
There are thirteen of these segments, reckoning, as is usual, the head as the first.
The head is usually very hard, and often of a much darker colour than the rest of the body. It is also frequently divided into two lobes by a couple of oblique lines, between which the parts of the mouth are situated. The two powerful horizontal jaws, to which we have already referred, are very hard and sharp, and curved like a sickle, and therefore splendidly adapted for biting from the edges of leaves. The head is also provided with a pair of antennæ, usually very short and inconspicuous and protected by a horny covering.
Unlike the perfect insect, the caterpillar has no large compound eyes, but twelve very small simple eyes, situated on the cheeks, very near the mouth – six on each side.
Fig. 19. – The Caterpillar of the Angle Shades Moth (Meticulosa).
If you examine them with a magnifier, you see that each one is provided with a small and very convex lens – a lens of very short focus, such as would be used for the examination of small objects held very near to the eye. From this arrangement we should be inclined to conclude that the caterpillar can see only those objects that are close to its mouth; and this idea is strengthened if you place one in a box containing a number of leaves, one of which is that of its own food plant. It will wander about the box, apparently looking at every part of every leaf it passes, after the manner of a very short-sighted individual, and never taking a general look round. A butterfly or a moth can see a flower in the distance, for it flies unhesitatingly from one to another in the straightest and shortest path, but if you place a caterpillar in the centre of a ring composed of a leaf of its food plant and nine others from other plants, the chances are (nine to one) that it will not walk towards what it would like to have.
Again, the eyes are situated on the lower part of the cheek, directed slightly downward, and are therefore adapted for seeing what is just under its jaws as it walks along. Had we no knowledge whatever of the caterpillar's twelve little eyes, we should probably have thought that it sought out its food by some sense other than that of vision.
Another important and interesting feature of the head is the silk-spinning apparatus, situated under cover of the lower lip. This consists of two tubular glands, corresponding to our own salivary glands, the special purpose of which is to secrete a viscid fluid that solidifies on exposure to air. The opening by which the fluid escapes is so situated that the caterpillar can easily apply it to the surface of any object over which it is walking, and then, by drawing or turning away its head, cause a silken fibre to be produced.
Some caterpillars make use of this spinning apparatus only on a few special occasions, but others, more especially some of the smaller species, seem to have it always in use, so that if at any time you suddenly start them into the air by giving a smart tap to the plant or twig on which they rest, they invariably fall slowly on the end of a growing web, the spinning of which they stop as soon as they consider they have fallen far enough. Sometimes, as you are walking through a wood, you will see hundreds, nay, thousands of little caterpillars thus suspended, swinging gently in the breeze. Not long since, after only a few minutes' walk among the trees of Epping Forest, I found I was decorated with several dozens of these swingers with which I had come into collision – in this case consisting chiefly of the larvæ of the Green Tortrix Moth (Tortrix viridana).
Fig. 20. – Walking Leg of a Caterpillar.
Now let us examine the caterpillar's limbs. Attached to each of the second, third, and fourth segments is a pair of true walking legs, corresponding with those of the perfect insect. These are covered with a hard and shining substance, and are also each provided with a hook. The fifth and sixth segments have no limbs at all, nor have the eleventh and twelfth, but some or all of the others (seventh, eighth, ninth, tenth, and thirteenth) are furnished with a pair of claspers which we shall presently describe.
First, as regards the number of claspers, it will be seen from what has just been said that this is not always the same. Some caterpillars possess five pairs, thus making up the total number of walking appendages to sixteen. In fact, we must regard this as the usual number. But there are at least a few hundred exceptions to the rule. Many of the Bombyces (page 217), for example, have only four pairs of claspers; and in others of the same group the fifth pair is present, but only partially developed, and quite useless for walking.
Fig. 21 – Larva of the Yellow Underwing Moth (Pronuba).
Fig. 22. – Larva of the Crimson Speckled Moth (Pulchella).
Fig. 23. – Larva of the Lobster Moth (Fagi).
Look at the peculiar caterpillar of the Lobster Moth (fig. 23) – a creature that differs from most other caterpillars not only in its claspers, but in many other respects too. Observe its long and slender legs, its humped middle segments, and its upturned hindermost segment, of enormous size and mounted with a pair of clubbed 'horns.' This last segment you will observe, has no claspers.
Another allied caterpillar is that of the Iron Prominent Moth (fig. 24). This one also has humped segments, and the claspers of the thirteenth segment are imperfectly developed.
A large number of other exceptions to the general rule are to be found in the caterpillars of the Geometer Moths (page 268), one of which is here represented. These have generally only two pairs of claspers, one pair on each of the tenth and last segments, so that there is a distance equal to the combined length of six segments between the hindermost true leg and the first pair of claspers. But even among the Geometers there are variations to be observed in the number of claspers, and some of these will be pointed out in our brief descriptions of the commoner species.
Fig. 24. – Caterpillar of the Iron Prominent Moth (Dromedarius).
Fig. 25. – Larva of the Brimstone Moth (Luteolata).
These limbs which we have been calling claspers are known by several other names. Thus they are termed 'pro-legs,' 'temporary legs,' 'false legs,' and 'abdominal legs;' but if you watch a caterpillar as it walks up a stalk or along the edge of a leaf, you will certainly agree that the term 'clasper' is everything that could be desired. But why not call them legs, seeing that they are used in walking? The reason is that they differ in many respects from the three foremost pairs of limbs as regards structure, persistency, and function. The true legs, as we have called them, continue to exist, though concealed, in the chrysalis state, and again appear, far more perfectly developed, in the butterfly or moth, but the claspers are no more to be seen after the caterpillar has passed into the quiescent stage. We have noticed, too, that the true legs are pointed and clawed, also that they are protected by a hard and horny covering; but examine a large caterpillar, holding it between the fingers and thumb with its under side uppermost, and you will soon see that the claspers are not at all hard, but soft and fleshy; not pointed, but often terminating in a broad flat circular surface. You will also observe, as the creature struggles to escape from your grasp, and tries to get a hold on something with its claspers, that these limbs, if we may so call them, are retractile, and are sometimes completely drawn into the body. Finally, examine the broad end of a clasper with a magnifier, and you will see it surrounded by a circle of little hooks, turning in all directions. You will no longer wonder how it is that a caterpillar can hold so tenaciously to a piece of twig that it is often almost impossible to remove it without injury.