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Move Over Groundhog, Woolly Caterpillars Forecast Winter Too!

brown and black woolly bear caterpillar
This brown and black woolly bear caterpillar is inching its way across the straw. About 4 or 5 of its bands appear to be brown, which would mean a mild winter. Photo by Katkami/Getty Images

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Between Punxsutawney Phil and pig spleens, there's no shortage of folksy ways people use to predict the weather. One longstanding legend says that woolly bears, those fuzzy caterpillars wriggling along on your local sidewalks, often know how severe a winter will be.

Woolly bears, also called woolly worms, or hedgehog caterpillars because of the way they curl up and play dead when touched, are the larvae stage of the tiger moth. These common caterpillars have distinctive black and rusty brown bands, and they're often spotted in autumn months as they prepare for winter. As the folklore goes, the wider the black bands, the harsher the upcoming winter will be. The wider the brown bands, the milder the winter.

There are festivals all over the eastern side of America celebrating the woolly bear and its weather skills. Officiants count to see how many of the worm's 13 segments are black (or brown) and announce the number to the crowd.

The legend of the woolly bear is an old one. But in 1948 Dr. Howard Curran, an entomologist from the American Museum of Natural History in New York, set out to observe the little worms near Bear Mountain State Park (New York) and see if there was any merit to the claim. For eight years, he collected as many caterpillars as he could in a day and sized up their brown segments. He found that on average the brown band took up more than a third of the woolly bear's body. Such a wide band would mean a mild winter and the eight winters when Curran did his experiment were mild. So, he concluded it might be true, though with such a small sample size, he didn't take the study seriously. Nevertheless, the results were published in the New York Herald Tribune, which gave the furry worms a lot of publicity.

So, do these woolly creatures have any prognosticating abilities? The reality is that scientists haven't established any correlation between the banded worms and the weather. Caterpillars from the same area – and even from the same hatch – often have very different banding. The banding is more a reflection of a caterpillar's age and how much feeding it has done.

"I find the folklore predictions interesting, but their accuracy low at predicting the winter weather," says Seth Nagy, Caldwell County extension director at the North Carolina Cooperative Extension by email. "I'm not aware of any scientific reason for a woolly worm's colors to be a prediction of the weather, but sometimes they get lucky and get it right."

You'd be better served, he says, by observing certain types of environmental factors. For example, the ocean temperature and snow cover in Siberia do have measurable impacts on weather patterns.

Even if they're not-so-reliable forecasters, woolly bears have some impressive physical feats. While most caterpillars retreat into cocoons during winter, the woolly bear braves blizzards head on.

Shunning protective warmth, it remains in cold areas and literally begins to freeze. Its little heart stops beating and it will display no signs of life. Yet its body produces an antifreeze substance, called glycerol that shields its vital organs and cells from the worst effects of the cold, reportedly to as low as -90 Fahrenheit (-67 Celsius). When spring weather arrives, the little guys thaw out and go on their merry way, soon to pupate and emerge as moths.

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