They Might Be Giants

By Charlotte Overby | August 2, 1998
From Missouri Conservationist: Aug 1998

We don't even have a jar big enough to catch it in." Those were the first words out of my mother's mouth when she saw the giant moth. About 5 or 6 inches wide in wingspan, it was fluttering against a tiled wall-in our bathroom, of all places. It appeared to be covered with something resembling green fur, and it was huge. It finally stopped moving and clung to the shower curtain. My sister and I caught it in a shoe box and let it go outside.

I later learned the moth we saw that night was a luna moth, one of about 13 species of giant silk moths found in Missouri. Members of the Saturniidae family, they have silky, carpet plush wings, delicate legs and inviting furry bodies. "Velvety" may be the best word to describe them. The Saturniids are the largest members of the Order Lepidoptera, or butterflies and moths, and are among the most spectacular animals found in North America's forests and neighborhoods.

The majority of the world's 1,500 species of Saturniids live in the tropics. About a dozen species are found in Europe, including the cold reaches of the North Cape of Norway and part of Siberia. About 50 species reside in North America. No Saturniids have been known to become extinct, but many are endangered, especially the tropical inhabitants.

In Missouri, we are most likely to see giant silk moths after midnight in late May or June. Many people wait years to see one, in part because the adult stage of these colorful and largest of all moths lasts for only about one week.

The giant silk moths begin life like most Lepidoptera. Usually at night, females emit pheromones to attract mates. Males "smell" the pheromone up to miles away with the help of sensitive receptors located at the tips of their featherlike antennae. Males often fly great distances to reach females and, once united, couples remain together about a day while they mate.

Female moths usually lay eggs the next day. They search for trees or shrubs that provide nutritious leaves for the new larvae. The largest Saturniid found in North America, the Cecropia moth, may lay more than 100 eggs, usually on the undersides of leaves on oak, hickory and other hardwoods. Cecropias may have a wing span of up to 7 inches. Females fly from leaf to leaf, sometimes tree to tree, dispersing their eggs. Female Saturniids die about a day after laying eggs. Males often mate again, but also die soon.

Seven to 14 days later, depending on air temperature, a battalion of tiny larvae, each one about the size of a mosquito, emerges. Many don't live to take their first bite of leaf; young larvae are food for birds, spiders and other insects. Those that survive soon develop well-defined heads and mouth parts for chewing. All Saturniids feed on the leaves of trees and shrubs. None of them bore into stems, stalks or fruits, and almost none feed on grasses, herbs or wildflowers. Most have poorly developed eyes and are almost blind.

Behind the head, the body has three segments that later form the adult-stage thorax. Each segment sports a small pair of legs-called true legs-used for mobility. The abdomen, just behind the thorax segments, holds several pair of fleshy legs, called prolegs. These usually are equipped with small suckerlike pads ringed with hooked claws-tools that help the caterpillar anchor itself to a leaf as it eats.

Although Saturniid larvae, like the fully grown adults, appear soft and squishy, they have a firm, external skeleton. Because this exoskeleton restricts growth, larvae must stop feeding and molt at regular intervals. Spinning a silken pad on a twig, a larva firmly anchors its legs to the pad and sits tight for about two days. The old "skin" splits apart, and the "new" larva pulls itself out, shaking its head side to side to free itself from the old head capsule. A Saturniid larva usually molts four to six times before it is mature enough to molt to a pupa and spin a cocoon.

Saturniid larvae are diverse in shape and color and each species is different. They may be green, clear, smooth or fuzzy. The Io moth larva, native to Missouri's hardwood forests, grows to 2 inches long and is covered with branched, green spines with black tips. If you handle one, you'll likely feel a painful sting. They don't have stingers, but their "hairs" break and dispatch a mild toxin.

Other Saturniid larvae depend on camouflage for survival and mimic the seed pods of their host tree or shrub. Still others have no camouflage at all and rely, instead, on simply tasting bad. They eat leaves from their host plants that make them gustatorily unappealing to predators.

Eventually the larvae molt to the pupal stage-a dormant stage between larvae and adulthood. Almost all 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.

This Issue's Staff

Editor - Tom Cwynar
Assistant Editor - Charlotte Overby
Managing Editor - Jim Auckley
Art Editor - Dickson Stauffer
Designer - Tracy Ritter
Artist - Dave Besenger
Artist - Mark Raithel
Photographer - Jim Rathert
Photographer - Cliff White
Staff Writer - Jim Low
Staff Writer - Joan McKee
Composition - Libby Bode Block
Circulation - Bertha Bainer