To provide for this great change in size, the larva moults five times, but the time between these moults is not always the same; there is usually about ten days between the first four moults and about twenty between the fourth and fifth. The larva stops eating a day before the moult, spins a few threads upon the leaf to which it attaches its hind legs, and waits for the transformation, which usually takes place in the afternoon. The larva, when mature and ready to spin its cocoon, is about three inches long. It is sometimes influenced in its color by the food plant; the normal larva being of a golden green, although it has been known to show more yellow coloring when found on red maple.
A short time before beginning its cocoon the larva ceases to eat and selects a place for its cocoon. These cocoons are usually found upon the ground among the leaves, but are frequently attached to twigs. After about a half day's work the larva spreads over the inside of the cocoon a gummy, resinous substance, which binds together the threads. After four or five days more of almost continuous work, another coating is smeared over the inside, which renders the cocoon practically air-tight. The silk fibres become considerably finer as the cocoon nears completion and the supply of silk begins to run low. For this reason the inner layers of the cocoon are only about half as strong as the outer ones. The larva, as the supply of silk diminishes in the silk glands, becomes perceptibly reduced in size. It has been estimated that the larva, in attaching the continuous thread of its cocoon, makes two hundred and fifty-four thousand back and forward movements. The cocoons are very strong and dense, of a dirty white color and generally coated with a white powder, the female being the larger.
There is but a single brood in the north, while in the south there are two.
In order to see if the pupa needed air, Mr. Trovelot sealed up some cocoons over winter in shellac, but the moths emerged in due time after being in an air-tight space for nine months. He also delayed the emergence of the moth till twenty-one months after entering the cocoon by placing it upon ice.
The silk in the spinning glands before it is spun is a clear, transparent fluid. These glands seem to be of excessive size when compared with that of the larva, since, when fully expanded, they reach the great length of twenty-five inches, or about eight times the length of the full-grown larva. These glands are paired, one being found on each side of the body, are considerably folded and taper at each end. The ducts leading from the anterior end of the glands unite to form a single duct which opens below the mouth. The thread is double, being really composed of two different fibres, one from each gland, as may be shown by separating them. The silk in these glands is prepared and sold as silk "gut" to anglers. On account of its transparency when in water, it becomes invisible and thus aids in deluding the wary fish, who does not see any connection between the line and the baited hook. The "gut" is prepared as follows: Larvae which are ready to spin their cocoons are cut open and placed in strong vinegar for eighteen hours; the glands are then taken out, stretched and dried in the shade.
Six or eight days after beginning the cocoon, the larval skin is moulted and the real chrysalic or pupal stage begins. This stage normally lasts till the following spring or summer. A few days before the time of emergence a pair of glands which open into the mouth become very active and secrete an acidulated fluid which escapes and wets the fore end of the cocoon, causing the resinous material binding together the fibres to become soft. Even cocoons sealed up in shellac and starch have been dissolved by this fluid, and thus the moths have been able to escape. When the cocoon has become sufficiently soft, the moth pushes its way between the fibres, but in doing so often breaks some of the threads, thus making the silk of such cocoons useless for commercial purposes. The moth at the time of emergence, with its folded and crumpled wings, is quite a forlorn-looking object. These wilted wings soon begin to fill up with fluids from the body, which is very large at this time. In some cases, the fluid is driven into the wings with so much force that they swell up, and if such a wing is punctured, thus allowing some of the fluid to escape, the mature wing will be of a smaller size than one from which no fluid has been lost. It must be remembered that it is possible to inflate a butterfly or moth's wing, because the wings of insects are not composed of a single layer, but are sacs of two layers which are closely applied. It is thus possible to split the wing into upper and lower halves, but this can only be done at the time of emergence, when these two layers are not so firmly cemented together as they are in a few hours after emergence.
The enemies of Polyphemus are numerous. Birds prey upon the larvae, in addition to numerous parasitic insects which are very similar to those which destroy Cecropia. The cocoon itself is not a complete protection because rats and squirrels plunder them. We thus see that the life of even an insect is full of dangers, and that it is really a wonder that so many are able to become mature and reproduce.
The silk-worm moths are excellent illustrations of what is called complete metamorphosis in insects. An insect like the grasshopper, when it hatches from the egg, is very much like the adult insect in its general form and appearance; the most evident difference being the lack of wings. An insect which shows such slight changes in its growth to maturity is said to have an incomplete metamorphosis. It is incomplete in the sense that the change is not of a very radical nature. But in the case of the silk worm moths, and moths and butterflies in general, the larva which hatches from the eggs has not even the most superficial resemblance to the adult insect, the fully-developed moth. This necessitates a complete change or metamorphosis in the form and structure of the insect before it can become mature. This great change is accomplished during the quiet pupal stage in the cocoon. Because the pupa is apparently passive when viewed from the exterior, one must not conclude that it is so internally; far from it; the digestive organs of the larva must be completely made over from those of a chewing leaf eater to those of a moth which can only take liquid food.
Charles Christopher Adams.