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They Might Be Giants

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Published on: Aug. 2, 1998

Last revision: Nov. 2, 2010

Saturniid species-including the cecropia, luna and Io moth-spin silken cocoons with material produced by special glands; hence the name giant silk moth. The cocoon provides a cloth that protects the pupa through the winter. Cocoons may hang from twigs, be attached lengthwise to a branch, dangle from one end, or be tucked into the fold of a few adjoining leaves.

Other larvae drop to the ground beneath their host tree and spin their cocoons under the cover of dead leaves. Luna moth cocoons, for example, are always found in the leaf litter below the trees or shrubs where they spent the summer eating. The cocoon of the polyphemus, another giant moth found in Missouri, rests in both trees and the leaf litter on the ground. A few Saturniid species spend the winter in a burrowed chamber in the soil.

After a cold winter hanging high in a tree or nestled in a chamber just underground, how do the giant moths know it's time to come out of the cocoon? Generally, the wintering pupae respond to external stimuli or changes in the environment. The pupa of the luna moth, for example, is thought to have a clear "window" feature on its head that allows it to measure length of daylight.

Other saturniids respond to temperature changes. Still others, raised in a laboratory, were found to respond little to changing light and temperature, appearing to rely instead on a punctual internal clock. At the right moment the moth breaks open its cocoon and crawls to a nearby safe spot to spread its wings.

A moth's legs, antennae and body are fully developed. Only its wings require attention. The moth immediately anchors itself to a nearby branch and allows its wings, which resemble small flaps, to hang down and harden. The moth has to accomplish this in minutes, or the wings harden into useless, semi expanded flaps. If this happens, the moth will never fly, and its chances of mating or laying eggs is next to none.

People often say the new moth is giving its wings time to "dry out." In fact, they don't emerge from the cocoon wet. When dragonfly nymphs molt, for example, their exoskeletons actually harden under water. It is more accurate to describe a Saturniid's wings as "hardening" rather than drying.

Fully formed, a giant silk moth looks like you could pet it. But don't! Their soft velvety wings are made up of thousands of tiny, fragile scales. Under a microscope, they appear to be layered like roof tiles. Even the striking "fur" on their bodies is made up of elongated scales.

Some Saturniids in Missouri are active during the day. However, the best time to try to see the biggest and most dramatic species-the cecropia, luna or polyphemus-is in the wee hours between 3 a.m. to just before sunrise.

Whether a day or night-time flyer, a giant silk moth must accomplish its life's work-procreation-in about one week. That's how long the velvety adult Saturniid moths live. None of that precious time is spent eating because, unlike most lepidoptera, the adult Saturniids have no functioning mouth parts. There's no time for eating when you have to negotiate pheromones, locate a mate, find the right host plant, lay hundreds of eggs, be so beautiful and die-all within a few days.

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