British Butterflies

by Alexander Morrison Stewart

The caterpillars are ravenous eaters; you will not notice this fact particularly at first, because they are then such tiny creatures, but in proportion to their size their eating capacity is enormous. They grow at an exceedingly rapid rate and to such an extent that they literally burst their skins! In a very short time—three or four days—the old skin bursts and out comes Mr. Caterpillar with a brand-new one. And this is the manner of their growth; several times (five or six) this skin-shedding process is repeated. And then the creature prepares for the last and final change before turning into a butterfly.

There are one or two more points I would ask you to notice about our caterpillar ere we pass on to consider his next stage. The legs are generally sixteen in number. There are six true legs, one pair on each of the first three body-segments behind the head; four more pairs near the anal end, and the last segment carries another pair, known as the “anal claspers.” The first six may be said to represent the same legs in the perfect insect. Note also the breathing holes, or spiracles, placed in a row along either side of the larva. The head seems to carry very large eyes, but it does not really do so; the real eyes are very minute, and it requires a good strong pocket-lens to make them out. There are twelve of them all told, and they are not all of equal size. There are six on either side of the mouth, and the three larger ones on each side are not very difficult to find. The mouth is furnished with strong mandibles for biting and chewing food, and also contains the spinneret for the production of the silk used on various occasions. All these details should be carefully noted—the head, the eyes, the breathing spiracles, the mandibles, the fore-legs and claws, and the hind- or pro-legs. Mark the totally different types of feet which terminate these two sets of legs. You will need to use your lens for this observation, and to enable you to see the beautiful structure of the pro-leg foot, it will be necessary for you to examine it through a compound microscope. It is well for the young entomologist to know these more prominent features of a caterpillar’s economy, if for no other reason than to be able to answer the questions that are sure to be put to him on these and many other points.

But only a small percentage of the larvæ that are born into the world live to become butterflies; some seasons a larger number than usual may escape, and then we have a butterfly year, but the relentless ichneumon flies soon restore the balance. They, too, have their young to provide for, and a strange mode of existence they have. Once you get to know these ichneumons at sight, you will be astonished at the number of them. All the summer through you will find them hawking about the trees, bushes, nettles, and heather, and, indeed, wherever larvæ are to be found, there, too, you will find these flies. There are many species of them. Once a female has discovered a larva its doom is sealed. The ordinary larva has very few defensive weapons; he may wriggle and squirm and look terrifying, but all the same the ichneumon sets about her task of placing one or two, and in many cases a dozen or two, of her eggs either upon or under his skin. These eggs soon hatch, and the little white maggots pass their existence inside the doomed creature, eating all the tissues away, at first avoiding the vital organs, which they leave until the last. When they have reached their allotted span, and are about to change to the pupa state themselves, they soon finish off their victim, and all that remains of what might have been a brilliant butterfly is a little shrivelled bit of skin and a host of little—or it may be a few big—black, brown, or grey flies. Sentiment apart, these parasitic flies are extremely useful. When you consider the large number of eggs laid by a single female butterfly or moth—from two to six hundred is a fair average—you will realize that if this enormous progeny were to survive and go on increasing without any check, the vegetation of the world would very soon prove quite inadequate to support the vast army of caterpillars, to say nothing of you and me.

You may at some time find a dozen or two larvæ of some particular species of butterfly or moth, and at the time of collecting them they may seem healthy and all right, but weeks afterwards you may discover that only a very small number will change to chrysalids, the ichneumons having had the rest. If you can catch and induce a female butterfly to give you a batch of eggs in captivity, then you may be sure, providing your treatment of them has been right, that all your brood will arrive at the perfect state.

The next stage we have to consider we will pass over briefly. The change from the larva to the chrysalis is always a very fascinating performance to watch, not that one could sit and see the whole performance right through from start to finish, the time occupied is too long for that. Generally the process lasts a day or two, but by watching at frequent intervals, where several individuals are engaged at the same operation and each at its own stage of the work, it is not difficult to follow the whole process of the transformation. Try it with the larva of the Large Garden White butterfly, perhaps the commonest, and therefore the easiest to procure; you will gather plenty of “stung” or “ichneumoned” examples, but still a sufficient number should be clean to serve your purpose.

We will not enter into all the details of the “spinning-up” process and describe how an attachment is secured at the anal extremity, and how our little friend “loops the loop.” Some species, such as the Tortoiseshell, get over this part of their difficulty by omitting the loop altogether, and therefore hang head downward, suspended only by the hooks and silk at the tail. Concealment during this stage is the creature’s only hope and chance of survival; other defence they have none. Their colour may occasionally protect them by virtue of making them harmonize beautifully with their surroundings. The ichneumons seldom molest them during the chrysalis stage; but birds and small animals have sharp eyes when foraging for food, so it is usually far more difficult to discover these chrysalids than to find the feeding caterpillars.

The time passed as a chrysalis is very variable; ten days to a fortnight in summer is sufficient for many species; others pass over the whole winter, like the spring brood of our common white butterflies, so that these can be sought for during the winter months under the overhanging portion of palings, walls, outhouses, and in similar situations. The cold does not seem to injure them; it may, and generally does, retard their emergence, and possibly has some effect on the colours of the wings, but it cannot change their ultimate pattern. Experiments have been tried with various chrysalids, part of a brood being hatched out after being submitted to a very low temperature, and another part of the same brood after being treated with a high temperature. Speaking generally, the coloration of those subjected to the cold treatment was brightened and intensified, and Nature does the same thing in her own way. The early summer butterflies, which pass through the winter as chrysalids, are almost invariably larger and brighter than the midsummer or autumn brood of the same species.

But suppose our caterpillar to have successfully run the gauntlet—ichneumon, bird, beast, and beetle—and to have become a healthy pupa, and that the time has arrived when he must make the last and greatest transformation in his short and interesting career. Several days prior to his exit as a butterfly taking place, a noticeable change occurs in the apparent colour of the chrysalis.

As a matter of fact it is not the chrysalis shell which is changing colour, but the developing insect, the colours of which are beginning to show through it, at first rather faintly; but latterly the pattern of the wings can be distinctly seen, and the whole body surface gets darker. When this stage is reached, the advent of our butterfly is not long delayed. The hour chosen is usually early in the morning, so that by the time the sun is high and the fresh perfumed flowers are nodding in the breeze, our little butterfly has expanded and dried his wings, and is now quite prepared for the beautiful and consummating act in the wonderful drama of his existence.