HE silk-worm which gives us the silk of commerce, has been domesticated for centuries in China. Because of this domestication, it is willing to be handled and is reared successfully in captivity, and has thus come to be the source of most of our silken fabrics. However, we have in America native silk-worms which produce a silk that is stronger and makes a more lustrous cloth than does that made from the Chinese species. But we have never had the time and the patience, here in America, to domesticate these giant silk-worms of ours, and so they are, as yet, of no commercial importance.
The names of our common native silk-worms are: The cecropia, promethea, polyphemus, and luna. In all of these species the moths are large and beautiful, attracting the attention of everyone who sees them. The caterpillars are rarely found, since their varied green colors render them inconspicuous among leaves on which they feed. None of the caterpillars of the giant silk-worms occur in sufficient numbers to injure the foliage of our trees to any extent; they simply help nature to do a little needful pruning. All of the moths are night flyers and are, therefore, seldom seen except by those who are interested in the visitors to our street lights.
The cecropia is the largest of our giant silk-worms, the wings of the moth expanding sometimes six and one-half inches. It occurs from the Atlantic Coast to the Rocky Mountains.
The cecropia cocoon is found most abundantly on our orchard and shade trees; it is called by the children the "cradle cocoon," since it is shaped like a hammock and hung close below a branch, and it is a very safe shelter for the helpless creature within it. It is made of two walls of silk, the outer one being thick and paperlike and the inner one thin and firm; between these walls is a matting of loose silk, showing that the insect knows how to make a home that will protect it from winter weather. It is a clever builder in another respect, since at one end of the cocoon it spins the silk lengthwise instead of crosswise, thus making a valve through which the moth can push, when it issues in the spring. It is very interesting to watch one of these caterpillars spin its cocoon. It first makes a framework by stretching a few strands of silk, which it spins from a gland opening in the lower lip; it then makes a loose net-work upon the supporting strands, and then begins laying on the silk by moving its head back and forth, leaving the sticky thread in the shape of connecting M's or of figure 8's. Very industriously does it work, and after a short time it is so screened by the silk, that the rest of its performance remains to us a mystery. It is especially mysterious, since the inner wall of the cocoon encloses so small a cell that the caterpillar is obliged to compress itself in order to fit within it. This achievement would be something like that of a man who should build around himself a box only a few inches longer, wider and thicker than himself. After the cocoon is entirely finished, the caterpillar sheds its skin for the last time and changes into a pupa.
Very different, indeed, does the pupa look from the brilliant colored, warty caterpillar. It is compact, brown, oval and smooth, with ability to move but very little when disturbed. The cases which contain the wings, which are later to be the objects of our admiration, are now folded down like a tight cape around the body; and the antennæ, like great feathers, are outlined just in front of the wing cases. There is nothing more wonderful in all nature than the changes which are worked within one of these little, brown pupa cases; for within it, processes go on which change the creature from a crawler among the leaves to a winged inhabitant of the air. When we see how helpless this pupa is, we can understand better how much the strong silken cocoon is needed for protection from enemies, as well as from inclement weather.
In spring, usually in May, after the leaves are well out on the trees, the pupa skin is shed in its turn, and out of it comes the wet and wrinkled moth, its wings all crumpled, its furry, soft body very untidy; but it is only because of this soft and crumpled state that it is able to push its way out through the narrow door into the outer world. It has, on each side of its body just back of the head, two little horny hooks that help it to work its way out. It is certainly a sorry object as it issues, looking as if it had been dipped in water and had been squeezed in an inconsiderate hand. But the wet wings soon spread, the bright antennæ stretch out, the furry body becomes dry and fluffy, and the large moth appears in all its perfection. The ground color of the wings is a dusky, grayish brown while the outer margins are clay colored; the wings are crossed, beyond the middle, by a white band which has a broad outside margin of red. There is a red spot near the apex of the front wing, just outside of the zigzag white line; each wing bears, near its center, a crescent-shaped white spot bordered with red. But though it is so large, it does not need to eat; the caterpillar did all the eating that was necessary for the whole life of the insect; the mouth of the moth is not sufficiently perfected to take food.
When the cecropia caterpillar hatches from the egg, it is about a quarter of an inch long and is black; each segment is ornamented with six spiny tubercles. Like all other caterpillars, it has to grow by shedding its horny, skeleton skin, the soft skin beneath stretching to give more room at first, then finally hardening and being shed in its turn. This first molt of the cecropia caterpillar occurs about four days after it is hatched, and the caterpillar which issues looks quite different than it did before; it is now dull orange or yellow with black tubercles. After six or seven days more of feeding, the skin is again shed and now the caterpillar appears with a yellow body; the two tubercles on the top of each segment are now larger and more noticeable. They are blue on the first segment, large and orange-red on the second and third segments, and greenish blue with blackish spots and spines on all the other segments except the eleventh, which has on top, instead of a pair of tubercles, one large, yellow tubercle, ringed with black. The tubercles along the side of the insect are blue during this stage. The next molt occurs five or six days later; this time the caterpillar is bluish green in color, the large tubercles on the second and third segments being deep orange, those on the upper part of the other segments yellow, except those on the first and last segments, which are blue. All the other tubercles along the sides are blue. After the fourth molt it appears as an enormous caterpillar, often attaining the length of three inches, and is as large through as a man's thumb; its colors are the same as in the preceding stage. There is some variation in the colors of the tubercles on the caterpillars during these different molts; in the third stage, it has been observed that the tubercles, usually blue, are sometimes black. After the last molt the caterpillar eats voraciously for perhaps two weeks or longer and then begins to spin its cocoon.
References—Moths and Butterflies, Ballard; Moths and Butterflies, Dickerson; Caterpillars and their Moths, Elliot and Soule.
Leading thought—The cecropia moth passes the winter as a pupa in a cocoon which the caterpillar builds out of silk for the purpose. In the spring the moth issues and lays her eggs on some tree, the leaves of which the caterpillar relishes. The caterpillars are large and green with beautiful blue and orange tubercles.
Method—It is best to begin with the cocoons, for these are easily found after the leaves have fallen. These cocoons if kept in the schoolroom should be thoroughly wet at least once a week. However, it is better to keep them in a box out of doors where they can have the advantage of natural moisture and temperature; and from those that are kept outside the moths will not issue, until the leaves open upon the trees and provide food for the young caterpillars when the eggs hatch.
1. How does the cocoon look on the outside? What is its general shape? To what is it fastened? Is it fastened to the lower or the upper side of a twig? Are there any dried leaves attached to it?
2. Where do you find cecropia cocoons? How do they look on the tree? Are they conspicuous?
3. Cut open the cocoon, being careful not to hurt the inmate. Can you see that it has an outer wall which is firm? What lies next to this? Describe the wall next to the pupa. How does this structure protect the pupa from changes of temperature and dampness?
4. Is the outside covering easy to tear? What birds are strong enough to tear this cocoon apart?
5. Are both ends of the cocoon alike? Do you find one end where the silk is not woven across but is placed lengthwise? Why is this so? Do you think that the moth can push out at this end better than at the other? Do you think the caterpillar, when it wove the cocoon, made it this way so that the moth could get out easily?
1. Take a pupa out of a cocoon carefully and place it on cotton in a wide-mouthed fruit jar where it may be observed. Can the pupa move at all? Is it unable to defend itself? Why does it not need to defend itself?
2. Can you see in the pupa the parts that will be the antennæ and the mouth?
3. Describe how the wing coverings look. Count the rings in the abdomen.
4. Why does the pupa need to be protected by a cocoon?
1. What is the first sign which you discover that the moth is coming out of the cocoon? Can you hear the little scratching noise? What do you suppose makes it? How does the moth look when it first comes out? If it were not all soft and wet how could it come out from so small an opening?
2. Describe how the crumpled wings spread out and dry. How does the covering of the wings change in looks?
3. Make a water-color drawing or describe in detail the fully expanded moth, showing the color and markings of wings, body and antennæ.
4. Do the moths eat anything? Why do they not need to eat?
5. If one of the moths lays eggs, describe the eggs, noting color, size and the way they are placed.
1. On what do you find the cecropia caterpillar feeding? Describe its actions while feeding.
2. What is the color of the caterpillar? Describe how it is ornamented.
3. Can you see the breathing pores, or spiracles, along the sides of the body? How many of these on each segment? How do they help the caterpillar to breathe?
4. Describe the three pairs of true legs on the three segments just back of the head. Do these differ in form from the prolegs along the sides of the body? What is the special use of the prolegs? Describe the prop-leg which is the hindmost leg of all.
5. Do you know how many times the cecropia caterpillar sheds its skin while it is growing? Is it always the same color?
6. Watch the caterpillar spin its cocoon, describe how it begins and how it acts as long as you can see it. Where does the silk come from?