Читать книгу Butterflies and Moths (British) - Furneaux William Samuel - Страница 4

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.