Page:Hardwicke's Science-Gossip - Volume 1.pdf/90

We shall now describe in a few words the being produced from the little worm which sprung from the Pieris' egg (page 32).

The body, which is entirely clothed with hairs perceptible to the naked eye, consists of three segments, which are separated from each other by well-marked constrictions; these three are: the head, the chest constrictions; these three are: the head, the chest or thorax, and the belly or abdomen. The head is small, and has attached to it in front two long, horny, jointed, club-shaped feelers, or antennæ, which had no existence in the caterpillar. The little simple eyes are still present; but in addition, a large round mass with a trellised surface may be seen on each side. These masses are the compound eyes, each facette of which is a true organ of vision; and since, according to the calculations of several naturalists, there are no less than thirty thousand of these facettes, it follows that the animal has this enormous number of distinct eyes. The mouth, which now, instead of being used for division and mastication, is only employed for suction, is admirably adapted to its new office. We can hardly find a trace of the upper and lower lips or the mandibles. The jaws have been immensely elongated; their horny tissue has disappeared, and flexor and extensor muscles have been developed in its place. Each jaw is penetrated by nerves and tracheæ, and is grooved deeply on its inner surface. These organs, when placed side by side and soldered to each other, constitute a sort of tube twice as long as the whole body, and continuous with the mouth. From having been organs of mastication, they are now converted by this process of fusion into a proboscis, which the insect rolls or unrolls at will, and which can penetrate the deepest calyx and allow it to suck up the juices of the flower as perfectly as if a syphon had been employed.

The chest or thorax bears the legs and wings. The first correspond to the horny feet of the caterpillar, but are very unlike them in appearance. The limbs of the caterpillar were short and massive, those of the butterfly are long and slender. Besides, their structure is entirely different. In the butterfly's limb there are five distinct parts, and the last or tarsus is itself composed of five joints and a pair of hooklets. The four wings are attached in pairs to each side of the back. Each one is united to the solid structures through the intervention of a chain of horny pieces, connected together by strong ligaments and supplied with powerful muscles, and to these are due the suppleness and force which are exhibited in the movements of flight. From this basal portion spring four main nervures, which diverge from each other, and by their ramifications support the wing-membrances, which are thus stretched upon a horny frame. Notwithstanding their solid appearance, these nervures are really hollow in their interior, and are pierced by tracheæ, or air-tubes, which extend almost to their extremities. Moreover they are covered by two exquisitely delicate, transparent membrances, which, firmly united together, clothe the upper and under surfaces. It is to these that the little scales are attached which give the peculiar colours to this and other butterflies, being implanted in the membrane somewhat in the same manner as a bird's feathers are implanted in its skin. The caterpillar did not present the slightest trace of these wings or their appendages.

The abdomen, which corresponds to the hinder portion of the caterpillar's body, has lost all its false feet; but with that exception has undergone very little alteration. The general form has been slightly modified, the colour is no longer the same; but the abdomen is invariably divided into seven rather distinct rings.

These moultings, metamorphoses, and alterations, although apparently occurring spasmodically, do not do so in reality. Beneath the veil of skin, which will be thrown off in course of time, even within those parts which will eventually disappear or be transformed into others, the new integument is being gradually formed; the general plan of the future animal is being drawn out, and the various organs which will afterwards be required are being organized. The old garment alone is cast off, both at the period of moulting and of metamorphosis. If we cautiously detach the old but still vital skin, a few days before the moulting takes place, we may see its successors lying beneath it. By doing the same with the caterpillar, a few days before it passes into the chrysalis condition, we shall discover the rudimentary wings and antennæ. If at this period we cut off the little scaly feet, we shall find that, when the butterfly emerges from the case, its legs will be imperfect.

Laying aside the internal changes which we have already alluded to, we shall confine ourselves to the study of the metamorphosis of some of the large apparatus, and more especially of the digestive tube and nervous centres.

The digestive apparatus which this caterpillar presents when it first springs from the egg, and even when it commences its transformation, is very simple in character. The alimentary canal begins as a very short and wide æsophagus, and ends in an intestine, somewhat of the same kind, but which can hardly be said to consist of two regions. Between these organs we find a stomach disproportionately large, which fills almost the entire cavity of the body. In addition to these, two salivary glands, composed of