REGIONAL—You may have noticed a few brown and black caterpillars on the move in recent weeks. It appears to have been a good year for Pyrrharctica Isabella, a moth species best known for its larval stage, commonly called the woolly bear caterpillar here in the North Country.
The woolly bears are on the move this time of year, looking for a sheltered place to spend the winter. Unlike most caterpillars, which hatch, grow, and pupate into their adult form within a single growing season, the woolly bear typically overwinters before transforming into its adult stage as an Isabella Tiger Moth.
This is a remarkably hardy species, which can survive temperatures as cold as minus 90 degrees F, an ability that has allowed the species (which is found primarily in eastern North America) to spread as far north as the Arctic. In the far north, the species develops very slowly, spending as much as 14 years in its larval stage before becoming an adult moth.
A common myth surrounding the woolly bear is that the length of the caterpillar’s fur and the relative width of the brown and black bands, can foretell the severity of the coming winter. In fact, those characteristics reflect the length and abundance of the previous growing season as well as the age of the caterpillar. If the growing season has been a good one, the caterpillars tend to grow longer and that makes the brownish-orange section appear relatively shorter compared to the black segments. A longer growing season achieves the same thing.
While woolly bears can’t predict the weather, they’re ability to survive the winter’s cold is even more remarkable. The “fur” on the woolly bear’s body doesn’t keep the caterpillar any warmer, but it apparently does help their body’s freeze more controllably. The caterpillars have glycerol in their bodies, a kind of natural antifreeze that keeps the interior of their cells from freezing entirely even in remarkable cold.