The Caterpillar With the Candy-Cane Stripe

This silk moth symbolizes National Moth Week, which has gone global.

An Io moth caterpillar, which has molted into its last instar. Photo by Andrei Sourakov
An Io moth caterpillar, which has molted into its last instar. Photo by Andrei Sourakov

This is the caterpillar of an Io moth, a species of silk moth that so intrigues entomologist Andrei Sourakov that he’s endured a number of the larva’s bee-like stings while studying various specimens. “I now wear gloves when I work with them,” says Sourakov, the collections coordinator at the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History, “but I used to care less.”

The pain source lies in each of the caterpillar’s little spines. “All of them are full of poison,” says Sourakov, which the larvae synthesize and use as a defense against predators.

During each larval stage (called instars), the caterpillar sheds its old skin and grows a new one over its body and each individual spine. “When [the caterpillars] freshly molt into the next instar, they look like young Christmas trees,” says Sourakov. In later instar stages, the larvae develop a yellow-green hue overlaid with a candy cane-like stripe—warning colors to further advertise their toxicity.

A sixth-instar Io moth caterpillar. Photo by Andrei Sourakov
A sixth-instar Io moth caterpillar. Photo by Andrei Sourakov


Adult Io moths, on the other hand, don’t seem to be toxic, according to Sourakov. But they do have some visual flare: Two eyespots on their lower wings presumably serve to frighten off predators.

The species fascinates Sourakov because it can be “remarkably variable in its color pattern,” and also because the larvae feed on a wide variety of plants, including toxic ones that other species can’t tolerate. Adult moths, meanwhile, don’t feed at all—they have no mouthparts. Mature moths instead rely on energy they built up as larvae, focusing their brief lives (about one to two weeks) primarily on mating and laying eggs.

The Io moth also symbolizes this year’s National Moth Week, which kicked off this past Saturday and continues until July 27th (the event has actually gone international). This is “the year of the silk moth,” says co-founder Dave Moskowitz, and on the day SciFri spoke with him, he singled out the Io as his favorite moth. (“[But] I suspect that if you ask me tomorrow, I might change it,” he added.) The species, which builds paper-thin silk cocoons, is “beautiful—just spectacular,” he says. (Listen to SciFri’s broadcast about Moth Week here.)

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Moskowitz and his co-founder, Liti Haramaty, launched National Moth Week three years ago to boost the public’s interest in these oft-overlooked flyers. “[Many people have] this idea that moths are small and drab and brown and eat clothing,” he says, “but nothing could be further from the truth.”

An Io moth female. Photo by Andrei Sourakov
An Io moth female. Photo by Andrei Sourakov
An Io moth male. Photo by Andrei Sourakov
An Io moth male. Photo by Andrei Sourakov

There are at least 140,000 described species of moths, according to Sourakov, and globally, moths outnumber described butterfly species about 20 to one.

“Moth diversity is extraordinary,” says Moskowitz, who’s working on a Ph.D. in entomology. “They come in every conceivable shape, every conceivable color pattern, [and] they range in size from smaller than a pinhead to as big as your hand.”

While moth variation often goes underappreciated, so does their vital ecological role, says Sourakov. Moth caterpillars are a major avian food source, for instance. “For a lot of the songbirds, [caterpillars form] more than half the diet of what they’re feeding to their young during nesting season,” says avian expert and field guide author Kenn Kaufman, who leads a nighttime moth-seeking field trip at least once a year in Ohio, where he lives.

Despite their importance in the ecosystem, there’s a lot about moths that scientists have yet to uncover. For example, populations of Io moth—which in North America are found from South Florida to Canada and westward to the Rocky Mountains—are declining, but researchers aren’t certain of the causes, according to Sourakov (he suspects a combination of habitat loss and parasitoids, such as the ones introduced to control gypsy moths).

Moskowitz hopes that National Moth Week will encourage citizen scientists to submit photographs and other information to databases that researchers can tap into to better understand moths. And because most species are nocturnal, just stopping to look at what’s fluttering around a porch light can be fruitful, adds Kaufman. “You’re likely to discover that there are some amazing things to see there.”


*This article was updated on July 22, 2014 to reflect the following corrections: Andrei Sourakov is the collections coordinator at the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History, not the collections curator. Also, gypsy moths are not parisitoids, as earlier stated—rather, parisitoids have been introduced to control gypsy moths.

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About Julie Leibach

Julie Leibach is a freelance science journalist and the former managing editor of online content for Science Friday.

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