The Story-Book of Science Chapter 33


“WE frequently see, at the ends of pine branches, voluminous bags of white silk intermixed with leaves. These bags are, generally, puffed out at the top and narrow at the bottom, pear-shaped. They are sometimes as large as a person’s head. They are nests where live together a kind of very velvety caterpillars with red hairs. A family of caterpillars, coming from the eggs laid by one butterfly, construct a silk lodging in common. All take part in the work, all spin and weave in the general interest. The interior of the nest is divided by thin silk partitions into a number of compartments. At the large end, sometimes elsewhere, is seen a wide funnel-shaped opening; it is the large door for entering and departing. Other doors, smaller, are distributed here and there. The caterpillars pass the winter in their nest, well sheltered from bad weather. In summer they take refuge there at night and during the great heat.

“As soon as it is day, they set out to spread themselves on the pine and eat the leaves. After eating their fill they reënter their silk dwelling, sheltered from the heat of the sun. Now, when they are out on a campaign, be it on the tree that bears the nest, or on the ground passing from one pine to another, these caterpillars march in a singular fashion, which has given them the name of processionaries, because, in fact, they defile in a procession, one after the other, and in the finest order.

“One, the first come—for amongst them there is perfect equality—starts on the way and serves as head of the expedition. A second follows, without a space between; a third follows the second in the same way; always thus, as many as there are caterpillars in the nest. The procession, numbering several hundreds, is now on the march. It defiles in one line, sometimes straight, sometimes winding, but always continuous, for each caterpillar that follows touches with its head the rear end of the preceding caterpillar. The procession describes on the ground a long and pleasing garland, which undulates to the right and left with unceasing variation. When several nests are near together and their processions happen to meet, the spectacle attains its highest interest. Then the different living garlands cross each other, get entangled and disentangled, knotted up and unknotted, forming the most capricious figures. The encounter does not lead to confusion. All the caterpillars of the same file march with a uniform and almost grave step; not one hastens to get before the others, not one remains behind, not one makes a mistake in the procession. Each one keeps its rank and scrupulously regulates its march by the one that precedes it. The file-leader of the troop directs the evolutions. When it turns to the right, all the caterpillars of the same line, one after the other, turn to the right; when it turns to the left, all, one after the other, turn to the left. If it stops, the whole procession stops, but not simultaneously; the second caterpillar first, then the third, fourth, fifth, and so on until the last. They would be called well-trained troops that, when defiling in order, stop at the word of command and close their ranks.

“The expedition, simply a promenade, or a journey in search of provisions, is now finished. They have gone far away from their nest. It is time to go home. How can they find it, through the grass and underbrush, and over all the obstacles of the road they have just traveled? Will they let themselves be guided by sight, obstructed though it be by every little tuft of grass; by the sense of smell, which wafted odors of every sort may put at fault? No, no; processionary caterpillars have for their guidance in traveling something better than sight or smell. They have instinct, which inspires them with infallible resources. Without taking account of what they do, they call to their service means that seem dictated by reason. Without doubt, they do not reason, but they obey the secret impulse of the eternal Reason, in whom and through whom all live.

“Now, this is what the processionary caterpillars do in order not to lose their way home again after a distant expedition. We pave our roads with crushed stone; caterpillars are more luxurious in their highways: they spread on their road a carpet of silk, they walk on nothing but silk. They spin continually on the journey and glue their silk all along the road. In fact, each caterpillar of the procession can be seen lowering and raising its head alternately. In the first movement, the spinneret, situated in the lower lip, glues the thread to the road that the procession is following; in the second, the spinneret lets the thread run out while the caterpillar is taking several steps. Then the head is lowered and lifted again, and a second length of thread is put in place. Each caterpillar that follows walks on the threads left by the preceding ones and adds its own thread to the silk, so that in all its length the road passed over is carpeted with a silky ribbon. It is by following this ribbon conductor that the processionaries get back to their home without ever losing their way, however tortuous the road may be.

“If one wishes to embarrass the procession, it suffices to pass the finger over the track so as to cut the silk road. The procession stops before the cut with every indication of fear and mistrust. Shall they go on? Shall they not go on? The heads rise and fall in anxious quest of the conductor threads. At last, one caterpillar bolder than the others, or perhaps more impatient, crosses the bad place and stretches its thread from one end of the cut to the other. A second, without hesitating, passes over on the thread left by the first, and in passing adds its own thread to the bridge. The others in turn all do the same. Soon the broken road is repaired and the defile of the procession continues.

“The processionary caterpillar of the oak marches in another way. It is covered with white hairs turned back and very long. One nest contains from seven to eight hundred individuals. When an expedition is decided on, a caterpillar leaves the nest and pauses at a certain distance to give the others time to arrange themselves in rank and file and form a battalion. This first caterpillar has to start the march. Following it, others place themselves, not one after another, like the processionaries of the pine, but in rows of two, three, four, and more. The troop, completed, begins to move in obedience to the evolutions of its file-leader, which always marches alone at the head of the legion, while the other caterpillars advance several abreast, dressing their ranks in perfect order. The first ranks of the army corps are always arranged in wedge formation, because of the gradual increase in the number of caterpillars composing it; the remainder are more or less expanded in different places. There are sometimes rows of from fifteen to twenty caterpillars marching in step, like well-trained soldiers, so that the head of one is never beyond the head of another. Of course the troop carpets its road with silk as it marches, so as to find its way back to its nest.

“The processionaries, especially those of the oak, retire to their nests to slough their skins, and these nests finally become filled with a fine dust of broken hairs. When you touch these nests, the dust of the hairs sticks to your hands and face, and causes an inflammation that lasts several days if the skin is delicate. One has only to stand at the foot of an oak where the processionaries have established themselves, to receive the irritating dust blown by the wind, and to feel a smart itching.”

“What a pity the processionaries have those detestable hairs!” Jules exclaimed. “If they hadn’t—”

“If they hadn’t Jules would much like to see the caterpillars’ procession. Never mind; after all, the danger is not so great. And then, if one had to scratch one’s self a little, it would not be a serious matter. Besides, we will turn our attention to the processionary of the pine, less to be feared than that of the oak. At the warmest part of the day we will go and look for a caterpillars’ nest in the pine wood; but Jules and I will go alone. It would be too hot for Emile and Claire.”

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