The Giant Silk Moths

/ / Real Science

While visiting a hardware store four years ago, I saw a large, beautiful moth resting on the wall near a light. Though at the time I didn’t know what kind of moth it really was, I was amazed. I later found out that it was a cecropia moth (Hyalophora cecropia) and was in the family Saturniidae, one of the largest and most spectacular moths in North America.

There are more than 1,000 species of this worldwide family, which contains most of the largest and most beautiful moths on earth. The largest one is the atlas moth (Attacus atlas), native to India and Sri Lanka to China, Malaysia, Indonesia, and Thailand. The second largest is the Hercules moth (Coscinocera hercules), ranging from Papua New Guinea to northern Australia. Both species can have wingspans over twelve inches. In many genera, the adults have long, tail-like hind wings, such as the spectacular luna moth (Actias luna), which is native to the eastern United states and southeastern parts of Canada. All Saturniidae have a reduced or absent proboscis (tongue), so they do not feed; and most of their natural life spans last only a few days, just long enough to mate and lay eggs. Like all moths, female giant silk moths release a pheromone (scent) that the males of that species can detect from miles away. In most cases, males have wider antennae for detecting and tracking female pheromones, and females tend to have reduced, smaller antennae for host plant detecting purposes.

As with most moths, many giant silk moths are nocturnal, yet a number of Saturniidae species fly during the day, such as the buck moth (Hemileuca maia), which is native to the eastern part of the US. In many species, male and female wing colors and patterns may be totally different, like the promethea moth (Callosamia promethea), which is native to eastern North America. Others might be closely alike and most species have a transparent patch or eyespot on each wing. Many giant silk moths hibernate over winter one of two ways, either as a pupa or as eggs, but both hatch in the spring. Some species, in warm climates, brood and hatch continuously.

Most Saturniidae larvae are large, and the two largest caterpillars of North America are the royal walnut moth (Citheronia regalis) and the imperial moth (Eacles imperialis). Both of these caterpillars often exceed full-grown lengths of five inches. They can be found throughout the southeastern US. As with all butterflies and moths, giant silk moth caterpillars go through instars (skin molts). Most species go through five instars, but a number of larger species go through six.

Although most giant silk moth caterpillars are harmless, a number of species have stinging spines, such as the genus Automeris. Some species can even become minor tree pests, such as the North American rosy maple worm moth (Dryocampa rubicunda), which can sometimes be so abundant that their larvae strip young maple trees of almost all their foliage. Another occasionally abundant giant silk moth species is the striking great peacock moth (Saturnia pyri), which is widespread throughout central and southern Europe, extending to North Africa and westward to Asia, and is sometimes a minor orchard pest.

Many parasitic insects attack Saturniidae caterpillars every year, including parasitic wasps and tachinid flies. Most giant silk moth caterpillars spin and pupate (the process of larvae forming pupae) in silk cocoons, while other species form their pupae in the ground (a pupa, or chrysalis, is a thin outlined shell in which a moth or butterfly forms).

Many species have been reared specially for silk from their cocoons to make fabric. This sparked an idea in the 1800s to build a silk industry with the species Samia cynthia in the United States, but finding cheap labor to unreel the cocoons proved impossible, and thus the business failed. The moths were simply released and have populated themselves along the north Atlantic coast.

Most people think of moths as dull and worthless “bugs,”but many of them are day flying and, like butterflies, pollinate an abundance of plants by feeding on nectar and are an important part of our ecosystem.

Train up a child in the way he should go and when he is old, he will not depart from it. - Proverbs 22:6