If you see a banded woolly bear caterpillar inching along, winter is probably around the corner. Also called woolly worms, the fuzzy-looking insects are a familiar autumn sight — and their unique color variations have long sparked curiosity.

Young caterpillars are mostly black, with a small band of lighter-brown hairlike bristles in the middle, said Susan Weller. She is an entomologist and director of the University of Nebraska State Museum. Each time the insects molt, growing a new coat, they turn more brown. According to folklore, the wider the brown band, the milder the winter — an idea later popularized by entomologist Charles Howard Curran. (Entomologists are scientists who study insects.)

While the caterpillars have inspired many contests and festivals, “they don’t predict the weather,” said Joe Boggs. Boggs is an assistant professor at Ohio State University Extension and an educator in the university’s entomology department. Last fall, Boggs collected a random group of woolly bears from a plant-rich area of Ohio. Although the caterpillars were similar sizes, suggesting they were the same age, they had different amounts of brown coloring. If such markings truly predicted the weather, “you would think all the woolly bears would look pretty much the same,” Boggs said.

Winter forecasting skills aside, woolly bears have cool qualities.

Insect 'antifreeze'

The caterpillars we see crawling in the fall hatched from eggs during the summer. We notice them in the autumn because they’re bigger, said Boggs, and because they’re moving a lot to find shelter before winter. Spaces that are buffered against the cold, such as under logs and leaf piles, make good hiding spots.

As winter approaches, the worms stop feeding and empty their digestive and excretory systems by defecating — pooping — out any contents. That ensures they “get rid of anything that could cause ice crystals to form,” Weller said.

An insect “antifreeze” — made of sugars, proteins and alcohol — is produced by the insects internally and circulates in their blood, helping woolly bears survive temperatures far below zero degrees.

Once warmer weather returns, the antifreeze breaks down and the caterpillars begin moving again. They pupate, forming cocoons, and eventually emerge in their adult form as Isabella tiger moths. Those adults find mates and lay eggs that become a new generation of caterpillars.

Outsmarting predators

The woolly bear’s stiff hairs are an important defense against predators such as yellow jackets and other wasps. By curling into a ball, caterpillars position their bristles on the outside and protect vulnerable organs.

“The wasp has to be really hungry to want to tackle a bristly caterpillar,” Weller said.

Some species have developed defenses against parasites, which attack from inside caterpillars’ bodies. Normally woolly bears avoid eating plants with bitter compounds, called pyrrolizidine alkaloids, because they can stunt growth. But scientists found that when flies laid eggs on caterpillars and the larvae started feeding on them, the caterpillars began to “self-medicate” by eating plants with bitter compounds. Entomologists are studying how this behavior helps the caterpillars.

“We’re not exactly sure whether it’s like drinking chicken soup, which helps your immune system be strong, or if it’s actually like taking an antibiotic that’s killing the disease inside of you,” said Weller. Either way, this adaptation is good news for caterpillars, which survive and grow up to be moths.

Fun facts

• The adult form of the woolly bear caterpillar, the Isabella tiger moth, does not eat and lives only for about one week. Its job is to quickly find a mate and lay eggs.

• Woolly bears can survive temperatures as low as minus-40 degrees.

• Woolly bears don’t sting or bite, but their bristles can cause a mild reaction in some people. It’s best to observe the caterpillars in nature, because they’re not adapted to survive indoors.

• Anything you do to encourage pollinators such as bees and butterflies will help woolly bears, too. Planting native wildflowers or creating a butterfly garden in your backyard can support these insects.

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