Why did the woolly bear cross the road?
To find a hibernaculum! In autumn, woolly bears wander in search of a sheltered place (a hibernaculum) to curl up for the winter. They spend those dark months frozen nearly solid. They produce glycerol to lower their body freezing point and move water out of their cells so that it freezes in the extracellular space and doesn’t rupture cell membranes.
Also known as woolly worms, weather worms, fuzzy wuzzies, and hedgehog caterpillars, the caterpillar’s genus name Arctiinae is from the Greek word for bear. This “fur” is setae, fuzzy bristles that dissuade predators. According to folklore, the coloration of woolly bears’ fuzzy bands predicts the severity of the coming winter, more brown bands mean a mild winter and more black bands point to a harder winter.
The width of the brown bands do tell us about the age of the caterpillar. That can give us some insights into the past winter when looking at woolly bears in the spring. Brown bands are added to the middle every time the caterpillar molts. So, the more brown bands, the older the caterpillar, which suggests that the previous winter didn’t come early since the caterpillar had time to mature fully before curling up for the season.
They can live through several freeze-thaw cycles through the winter, though that takes its toll. The caterpillars become active again in spring, feed for a few days, and make a cocoon with their setae. After about a month, those caterpillars emerge as adult Isabella tiger moths.
Be on the lookout for wandering woolly bears this fall. Compare their brown and black colored segments for yourself and see if you find any patterns!
Diversity of woolly bear caterpillars on the same stretch of road on the same day.
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