The Polyphemus moth possesses a transparent window on each of its four wings. (Photos by Mary Holland)

Now is the time to keep an eye on the ground for cocoons of the Polyphemus moth. Any day, the adult moths will be emerging.

Most insects reach adulthood in one of two ways. Some, like grasshoppers, emerge from their eggs as miniature, though flightless, versions of the adults, and are referred to as nymphs by entomologists. These three developmental stages—egg, nymph, adult—are collectively referred to as incomplete metamorphosis. Moths, on the other hand, experience complete metamorphosis. They go through four life stages—egg, larva, pupa, and adult. The larva often looks so unlike the adult that, unless you knew differently, you would probably assume they were two different species.

One of Harvard's (and North America's) largest moths is the Polyphemus moth, Antheraea polyphemus. Pieter Cramer, the entomologist who discovered this member of the giant silk moth family, Saturniidae, named it after the son of Poseidon, an enormous one-eyed Cyclops from Greek mythology named Polyphemus. The adult Polyphemus moth possesses a small eye-shaped, transparent "window" on each of its four wings, perhaps the feature for which it was named.

This time of year you are most likely to encounter the pupal stage of the Prometheus moth. Last fall the Polyphemus larvae, or caterpillars, found leaves that they curled up in before spinning a cocoon. Some of these caterpillars descended from the trees, whose leaves they had been eating (a wide variety including oaks, maples, birches, and willows), climbing down the trunk and disappearing into the leaf litter where they spun their cocoons. Others spun their cocoons in leaves that were still on the tree and dropped to the ground when the leaves fell. A few cocoons remained attached to tree branches through the winter.

The Polyphemus larva spends about half a day creating the cocoon that will house its overwintering pupal stage. After all the silk has been spun and the cocoon constructed, the larva spends the next four or five days spreading a gummy, resinous substance it produces over the inside of the cocoon, which binds the silk threads together. When that has dried, it smears a second coating over the inside, making it practically airtight. Six to eight days after beginning the cocoon, the larva molts its skin for the final time, and the pupal stage begins.

The Polyphemus pupa remains in this tough, thick cocoon through the winter. It's about 1½ inches long, but is easily overlooked because it is so well camouflaged within a leaf. Apparently Native Americans in the west have trained their eyes to see them, however. They fill empty Polyphemus cocoons (and those of other silk moths) with sand or pebbles, tie them together on a stick or around their ankles, and use them as musical instruments or as rattles in ritualistic ceremonies.

In early May, a few days before the time of emergence, the moth secretes a fluid through glands in its mouth that softens the interior resinous coating at one end of the cocoon. When this area has become sufficiently soft, the moth pushes its way between the fibers and emerges into the world. After pumping fluid from its abdomen into its wings and letting them dry, the moth flies off with its impressive 5½-inch wingspan.

The male Polyphemus moth, with his feathery antennae, immediately seeks out the pheromone-emitting female, and mating ensues. After remaining coupled for as much as 12 hours, they part, and the female proceeds to lay her oval, squat, sesame seed-size, pearly white eggs on trees whose leaves the larvae will consume.

The larval stage of a moth is the only stage in which it possesses mandibles capable of chewing, and the moths take full advantage of this. The Polyphemus larva, at 56 days old, will weigh 86,000 times its original weight. This 3½-inch, nearly fluorescent green caterpillar with yellow stripes and red and silver spots is a real eye-catcher. If it isn't eaten by a predator, such as a bird or small rodent, and avoids being parasitized by a wasp, it will eventually continue the cycle and begin spinning its protective, silken, winter house before the snow flies.

Mary Holland is the author of Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England. She blogs about natural history at www.naturallycuriouswithmaryholland.wordpress.com.