Naturally Curious: Season of the Giant Moths

Monday, June 17, 2013
P erhaps because of their size, or because we don’t see them that often, giant silk moths cause quite a stir when discovered on a June morning, clinging to a screen door or window near an outdoor light that was on overnight. These large moths of the family Saturniidae are among the largest moths in North America, measuring up to six inches from wing tip to wing tip. They have just emerged from their cocoons, and have only a week or two at most to mate and lay eggs before dying. In fact, their adult lives are so short that they have greatly reduced mouth parts and no digestive tract. Eating is strictly a larval occupation – reproduction is their task as adults.

You may be familiar with giant silk moths without realizing that they are part of this group – they include the beautiful green Luna moth, the hairy-scaled Cecropia moth, the eye spot-winged Polyphemus and the Promethea and Columbia silk moths. These moths share several characteristics in addition to their large size: heavy bodies covered in hair-like scales, reduced mouth parts and small heads. It is fairly easy to determine the sex of a giant silk moth by looking at its antennae — generally males have large, broad, feathery antennae (with which they detect pheromones released by females) while the females’ are more modest in size.

The adult moths emerge from their silken cocoons in June. Female moths, including giant silk moths, produce chemicals called pheromones that can signal alarm, breeding opportunity and a number of other messages to moths of the same species. The pheromones that a Cecropia moth female releases attract males from miles around. Male Cecropia moths are able to detect these pheromones with sensitive receptors located on the tips of their antennae. According to the entomologist Stephen Marshall, one millionth of a gram of the pheromone released by female moths is, in theory, enough to attract one billion male moths.

The adult silk moths mate and lay up to 200 round, slightly flattened eggs on the host tree or shrub whose leaves their larvae will eat. Some species of larvae that hatch from these eggs are quite well camouflaged, but others are as stunningly colored as the adults, and often bear spines or barbed horns. The Cecropia caterpillar, for instance, is a pale greenish-blue, and has two rows of red, yellow, and blue spiny tubercles on its body. Not only are they colorful, but giant silkworm larvae, in general, are huge — several inches long, and as big around as a nickel. Although harmless to humans, Luna and Polyphemus larvae will produce clicking sounds with their mandibles when disturbed.

For the next two or three months, the larvae do little but eat. The Polyphemus larva can eat 86,000 times its initial weight in this amount of time. Depending on the species, the larva will molt four to six times, and be two to four and a half inches long before stopping its growth and spinning a cocoon in which to pupate over winter. As you would imagine, the cocoons are proportionately as large as the caterpillars, reaching a good three inches in length.

Although they are called silk moths, these moths don’t spin the cocoons from which silk fabric is made. The true silkworms that are used to produce silk for human use are in another family of moths, Bomb)ycidae. Each of these caterpillar’s cocoons is made up of a double silk strand about half a mile long, thousands of which are required to make a pound of silk. There have been attempts to establish a silkworm industry in North America, at least one of which had dire consequences. In the mid-1800s a Harvard astronomer imported eggs of the gypsy moth in an attempt to cross this species with a distantly related true silkworm. His experiment failed, and we are still living with the consequences of a few of his escaped gypsy moths.

Giant silk moths are most abundant in the tropics, but northern New England has five species. Take advantage of the few short weeks when these spectacular moths can be seen in our woodlands. It’s the one time of year when I allow myself to keep the porch light burning all night for one night, in hopes of finding an adult silk moth in the morning.

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” and “Milkweed Visitors.” She has a natural history blog which can be found at

Their adult lives are so short that they have greatly reduced mouth parts and no digestive tract. Eating is strictly a larval occupation – reproduction is their task as adults.

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