Page:Hardwicke's Science-Gossip - Volume 1.pdf/91

1, 1865.] long tortuous tubes, are attached to them in front, and six well-formed biliary canals, which represent the liver, are connected with them behind. Opening into the mouth and the spinning apparatus which we alluded to before, may be seen two peculiar organs, which extend ina tortuous manner along the entire length of the stomach, and whose office is the secretion of the fluid from which the silk is formed. Every portion of this digestive apparatus is calculated to extract the nutritious materials from the crude masses of unsubstantial food which has been imperfectly prepared by the action of the jaws.

Even on the second day after the caterpillar has been converted into the chrysalis, very considerable alterations may be observed. The æsophagus is narrowed and elongated; the intestine, similarly modified, is now divided into two well-marked regions; the stomach has been diminished by about one-half of its breadth and a quarter of its length; the salivary and biliary glands have been shortened, and the organs which secrete the silk have become smaller. On the eighth day, the entire digestive tube is exactly like a spindle one-half of which is covered with thread, and which is loaded with lead in order to balance it. The æsophagus is narrowed and elongated; the intestine, similarly modified, is now divided into two well-marked regions; the stomach has been diminished by about one-half of its breadth and quarter of its length; the salivary and biliary glands have been shortened, and the organs which secrete the silk have become smaller. On the eighth day, the entire digestive tube is exactly like a spindle one-half of which is covered with thread, and which is loaded with lead in order to balance it. The æsophagus represents the upper portion of the spindle; the stomach corresponds to the middle, which is covered with thread; the small intestine to the thin portion, and the large intestine to the loaded part. At the same period the salivary glands and biliary cæca are reduced by about two-thirds, and the silk-secreting organs appear as two very slender threads.

During the entire winter—that is to say, for five or six months of the year—these operations are suspended; but they are recommenced in fine weather, and continued till the insect is fully formed. In a short time the silk-forming canals have entirely disappeared; hardly a trace of the salivary glands is to be found; and the stomach, though preserving its former shape, has decreased in size; but to compensate for this, it has developed a new cavity called the crop, which is destined to assist in sucking up the juices of the flowers, and retaining them till required by the insect. Moreover, the two intestinal regions have become more distinct, and the large intestine has developed an accessory pouch, not a trace of which existed hitherto.

We come now to the nervous system. In insects in every condition, this system is composed of two distinct portions. The brain is placed in the head just above the æsophagus. The other nervous masses, or ganglia, are situate below the digestive tube, where they constitute a ganglionic chain. The brain is united to the first ganglion, this to the second, and so on, by a series of nervous filaments, technically termed commissures. In each ring of the caterpillar's body there is a distinct ganglion; consequently there are in all twelve of these structures, equally distant from each other, with the exception of the two first, which are more closely approximated than the rest. The brain itself is very small, and is composed of two smooth lobes which are obliquely united, and give off a few slender nervous filaments.

Two days after the caterpillar has been converted into the chrysalis, the ganglionic chain has been shortened one-fourth, and various changes of alteration and concentration have begun. Some ganglia are approximated; others, on the contrary, are separated. At about the eighth day the chain has been shortened one-half. On the fourteenth the brain and first ganglion have come so close together that their commissures surround the æsophagus; the fourth and fifth ganglia have been fused together; and the sixth and seventh are hardly perceptible. Now there is a period of rest, brought about by the approach of winter. Then, after the latter season, the operations are recommenced, and when they are again arrested, after the last apparent transformation, there are only eight ganglia to be seen. The second, third, fourth, and fifth, have given rise, by their fusion, to two large masses, which are placed in the chest quite close to each other; the sixth and seventh have completely disappeared, and their former position is alone marked by the origin of a few nervous filaments; the five posterior ones have undergone little or no change. Finally, the brain itself had become twice as large, its lobes have assumed a transverse position, and each of them gives origin to a large optic nerve which travels to one of the compound eyes.

The changes which the organs of circulation and respiration undergo are not by any means as well known as those we have been describing, and this ignorance is due, most likely, to the great simplicity of the first, and the equally complex character of the second. In this caterpillar, as in every other one, the circulation is almost entirely lacunar. In it, as in the butterfly, there is a distant heart, or rather its representation, in the form of a long many-chambered canal, stretching from end to end of the body. When the latter is shortened, this dorsal vessel, as it is called, is also diminished in length, and becomes more and more tortuous in proportion as the regions of the body are mapped out and separated from each other.

This degradation of the circulatory organ is compensated for the by the formation and distribution of a series of respiratory organs, or tracheæ. These open externally at the stigmata to which we have alluded already. They consist, in Pieris, as in all other caterpillars, of two great lateral trunks, reaching from end to end of the body, and giving off hundreds of branches and ramifications, which travel over the whole frame, penetrate the smallest cavities, and supply the most delicate organs. In