The eggs of Polyphemus are very much flattened, about the size of those of Cecropia, and are deposited on leaves and twigs singly or in small groups. These hatch in about ten days and usually in the morning. The young larva often devours the shell which a few moments before afforded it shelter. This larva feeds upon oak, hickory, apple, maple, elm and a variety of other trees, and thus has a larger range of food plants than the Luna larva. The rate of growth is prodigious, as has been shown by Mr. Trovelot. When the larva hatches it weighs about one-twentieth of a grain; in ten days it weighs one-half of a grain, or ten times its original weight; in twenty days it weighs three grains, or sixty times its original weight; when a month old it weighs thirty-one grains, or six hundred and twenty times its original weight, and has consumed about ninety grains of food; after fifty days it weighs two hundred and seven grains, or over four thousand times the original weight. At fifty-six days the larva has eaten eighty-six thousand times its original weight in food! It is therefore not surprising that these larvae can often be easily detected upon trees by the large number of leaves which they have devoured.

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 days 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 airtight 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.