Butterflies and Moths (British). Furneaux William Samuel
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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