INSTINCT
Jules and Emile had put a caterpillar into a glass and brought it to their uncle, who made it the subject of a little talk to his young hearers.
“Examine the creature closely,” said he. “Its skin is delicate, so delicate that even a light touch hurts it; but here on the head, at the point called the skull, it has the hardness of horn, forming a sort of cap or helmet which can without injury endure friction with the hard texture of wood. The head is the part of the creature that opens the way, and it is therefore protected by armor, while the rest of the body, as it follows the head, does not need this casing of horn.”
“I understand,” said Emile; “the creature works its way along by scratching and tunneling with its feet.”
“No, my boy; the feet are not used for boring through wood. The caterpillar has eight pairs. The first three pairs, or those nearest the head, have quite a different shape from the others: they are slender and pointed, and it is they that in the change that takes place later become the butterfly’s legs, though in doing so they grow much longer and take another shape. Hence they are called the true legs. [[291]]The next four pairs are placed toward the middle of the body, and the last pair is at the very end. These five pairs bear the name of false legs because they completely disappear when the caterpillar gives place to the butterfly or moth. They are short and wide, and are furnished beneath with numerous little hooks by which the caterpillar clings to the walls of its abode. The stiff hairs covering the body are also used for locomotion, the caterpillar wriggling and squirming in its tunnel somewhat as does the chimney-sweep in helping himself with knees and back as he makes his way up the inside of a chimney.”
“Then what does the caterpillar use for boring through the wood?” asked Jules.
“The tool for chipping away the wood consists of two curved fangs or teeth, almost black, one on each side of the mouth, which open and shut like a pair of cutting nippers. They are called mandibles and are in reality jaws or, more properly, teeth which, instead of meeting in a vertical plane as do ours, come together horizontally or sidewise. For precision of movement these mandibles are superior to our best cutting nippers, and for hardness they are almost equal to steel. They seize the wood, bit by bit, patiently and untiringly; they cut, saw, tear away a little at a time, and so bore a tunnel just large enough for the caterpillar to pass through.”
“And what becomes of the wood-dust?” Jules further inquired. “I should think it would block the way, the passage being so narrow.” [[292]]
“The wood-dust passes through the creature’s body; the caterpillar eats it, and after digestion has taken from it the very small amount of nourishment it contains, it is ejected behind, molded into tiny pellets. Digestion in a caterpillar is soon accomplished. Just think of it: wood is an extremely meager fare, and so the worm must keep on eating its way forward, cutting, gnawing, digesting. To acquire the fatness necessary for the coming change the creature must have a good-sized pear-tree limb or lilac trunk to work on.
“The wormhole dust left behind by the boring worm sometimes betrays the insect to its enemies. Whenever you see any of this dust left by digestion coming out at some little orifice in the bark of a pear-tree, apple-tree, or other tree, you may know the borer is at work, and the branch where he is at work should be cut off immediately, to prevent more serious harm. If the caterpillar has not gone too far, a pointed iron wire may be thrust into the opening and an attempt made to kill the creature in its hole. But as the passage is very winding, this method is by no means certain of success.”