Butterflies and Moths Part 2

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 ill.u.s.trated 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.

[Ill.u.s.tration: FIG. 11.--EGG OF THE MEADOW BROWN b.u.t.tERFLY.]

[Ill.u.s.tration: FIG. 12.--EGG OF THE SPECKLED WOOD b.u.t.tERFLY.]

[Ill.u.s.tration: FIG. 13.--EGG OF THE VAPOURER MOTH.]

It may be surmised from the accompanying ill.u.s.trations 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 b.u.t.terfly or moth that deposited a cl.u.s.ter of eggs he happens to find in his rambles and searchings; but in such decisions he is always greatly a.s.sisted 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 larvae by certain changes which take place in the appearance of the egg. As the h.o.r.n.y 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 b.u.t.terflies and moths shall lay their eggs on strips of gla.s.s 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 'sh.e.l.l.'

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 h.o.r.n.y 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 'sh.e.l.l' 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.

[Ill.u.s.tration: FIG. 14.--THE CATERPILLAR OF THE CLOUDED YELLOW b.u.t.tERFLY.]

Several naturalists have amused themselves by performing experiments and making calculations on the efficiency of the masticating and digesting powers of the caterpillar. The ill.u.s.trious Reaumur, 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 appet.i.te 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.

[Ill.u.s.tration: FIG. 15.--THE CATERPILLAR OF THE DARK GREEN FRITILLARY b.u.t.tERFLY.]

The caterpillar now draws its head backward, and, with a few convulsive struggles, pulls the front segments out of their old skin, and pa.s.ses 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 appet.i.te, 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 sh.e.l.l 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.

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.

[Ill.u.s.tration: FIG. 16.--THE CATERPILLAR OF THE PURPLE EMPEROR b.u.t.tERFLY.]

Having thus pa.s.sed 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 larvae 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 larvae 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 coc.o.o.n in which to undergo the transformation.

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 sh.e.l.l (there being but little else than sh.e.l.l remaining of the victim's body), break forth from the remains of the carcase somewhere about the time at which the b.u.t.terfly or moth should have appeared.

[Ill.u.s.tration: FIG. 17.--AN ICHNEUMON FLY (_Cryptus Migrator_).]

[Ill.u.s.tration: FIG. 18.--ANOTHER ICHNEUMON FLY (_Pimpla Instigator_).]

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 antennae, usually very short and inconspicuous and protected by a h.o.r.n.y 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.

[Ill.u.s.tration: 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 pa.s.ses, after the manner of a very short-sighted individual, and never taking a general look round. A b.u.t.terfly 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 larvae of the Green Tortrix Moth (_Tortrix viridana_).

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.