The black and brown bands of the larval form of the Isabella tiger moth – also known as the woolly worm, woolly bear, or hedgehog caterpillar – are known from North Carolina to Pennsylvania as a way to predict the severity of the coming winter.
The caterpillars emerge in great numbers during early fall and forage in preparation for surviving winter, providing amateur entomologists plenty of chances to study the creatures for meteorological data.
Some say that the animal's 13 bands of color correspond to the 13 weeks of winter, with black indicating more severe weeks and brown meaning milder weeks.
The Old Farmers Almanac takes a different approach, still focused on band colors: "If the rusty band is wide, then it will be a mild winter; the more black there is, the more severe the winter." Other folklore focuses on fuzziness rather than brown vs black, with a thicker coat on the worm meaning a harsher winter for us all. Even a caterpillar's travels may be interpreted as a predictor, with a northward crawl indicating mild weather on the way, while a south-bound worm is wisely trying to escape the coming harsh winter temps.
People have been "reading the worm" for hundreds of years, but the practice became even more popular after Dr. C.H. Curran, curator of insects at the American Museum of Natural History in New York City, began a rather informal, eight-year study of woolly worm coat patterns and the severity of the following winters. His data set did show a correlation between the amount of brown striping on the average woolly worm and the warmth of the winter to follow, yet even Curran himself admitted that his study was much too small to be conclusive. His Original Society of the Friends of the Woolly Bear was obviously more social than scientific. However, The New York Herald Tribune published the results of Curran's study, and the woolly worm legend was cemented in the public consciousness.
Whether or not the caterpillars actually predict winter weather, they are remarkably adept at surviving it, no matter how harsh. Their "fur" is actually called setae, and it produces a natural antifreeze that allows the caterpillar to freeze little by little. Eventually, everything freezes but the interior of the woolly worm's cells. This allows the creatures to survive winters with temperatures as low as 90 below zero.
Come spring, the caterpillars thaw out, form cocoons, pupate, and emerge as small, yellow-orange tiger moths, which provide food for a number of nocturnal creatures, from spiders to bats.
When inspecting a woolly bear for meteorological data, one need not worry about being bitten: the caterpillars are incapable of nipping humans. Though human skin does not usually react to touching a woolly bear, some people can develop a rash, so it is wise to merely observe the woolly worms rather than pick them up for inspection.
Caterpillar aficionados can even attend the 42nd annual Woolly Worm Festival in Banner Elk, N.C., on Saturday and Sunday, where visitors can enjoy the usual fall festival fare while learning how to read those legendary worms.