How bad will our winter be? A caterpillar can tell you.

Memories of last year's icy winter are etched in our minds. Many are wondering what's in store this winter. Look no further than the woolly bear caterpillar. Like Punxsutawney Phil, the renowned groundhog who predicts the coming of spring, the woolly bear caterpillar is said to predict the winter ahead.

Woolly bears are common in autumn, marching across roads, driveways, and sidewalks. They're furry-looking creatures, about two inches long. There are bands of colors along the body, typically dark brown on the ends and rusty orange in the middle. The width of the bands is variable. And this is where the meteorology comes in.

Legend has it that the wider the orange colored band, the worse the winter will be. A narrower orange band is a predictor of mild winter weather. Are you skeptical? Well, first let's see how this all got started.

In 1948 an entomologist named Dr. Howard Curran, then curator of entomology at the American Museum of Natural History, set out to collect woolly bears in New York. He sampled 15 individual caterpillars and measured the width of their bands. Perhaps on a whim, Curran predicted the winter weather from this “data.” (A sample of 15 is hardly a scientific study.) Somehow the media got wind of this, and a reporter accompanied Curran to his “study” site. From there, the story spread and became legendary.

The truth of the matter is that the bristles of the woolly bear vary for several reasons. The stage, or instar, of the caterpillar is one factor. The older the caterpillar, the more reddish-brown. Rainfall also affects the coloration. The black bands grow wider in wet weather. This has no bearing on the weather to come.

It's fun to watch woolly bears even if you're not looking for weather forecasts. If you pick one up, it will curl up in a ball and play dead. The hairs don't sting, but they do serve as protection. Would-be predators avoid woolly bears because a mouthful of bristles is not very appealing. Black and orange are also warning colors that serve as a “do not touch” sign.

Why are there so many woolly bears this time of year? They're on the march very purposefully, looking for a good protected spot to spend the winter. Unlike other species of moths and butterflies, these caterpillars overwinter as adults. They need a shelter in which they can tough out the winter months. This can be under logs, leaves, or your porch.

Woolly bears are able to survive the below-freezing temperatures of winter thanks to a DIY antifreeze. They produce an antifreeze called a cryoprotectant that lowers the lethal freezing point of their bodies. (Other animals that make cryoprotectants include frogs and toads.)

On the first warm days of spring, the woolly bears rouse and pick up where they left off in the fall. They eat and eat. Then, using those long bristles, they weave a cocoon. After about a month, the adult will make its debut as — ta-da! — an Isabella Tiger Moth.

Thus, when a woolly bear is finished being a woolly bear, it's a moth. A pretty drab one at that. The hind wing of the male is pale orange. The female has a touch of pink in it. You may see the Isabella Tiger Moth at your porch light in the summer.

The adults don't eat. Their sole purpose is to make new Isabellas, or woolly bears. There are generally two generations in our area, but we rarely see the woolly bear caterpillars in the summer. It's late fall when they become obvious in their rush to find a place to spend the winter.

Woolly bears aren't considered pests, although their relatives, the fall webworms, are. The woolly bear's diet is varied and includes both herbaceous and woody plants. Asters and grasses are big items on the menu this time of year.

Take a look at these colorful caterpillars during our last sunny days of fall. They will be out of sight soon, ready for whatever winter weather is to come.

• Valerie Blaine watches woolly bears and other wild beasts in her spare time as a naturalist with the Forest Preserve District of Kane County.

  A woolly bear caterpillar curls up in its defense mode. If it survives the winter, the caterpillar will turn into a tiger moth. Laura Stoecker/
In the spring, woolly bear caterpillars transform into Isabella Tiger Moths. Wikipedia photo
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