Woolly Bear (Isabella Tiger Moth)

Pyrrharctia isabella — the Isabella tiger moth — is best known not for the adult moth but for the caterpillar: the woolly bear, also called the woolly worm or banded woolly bear. The caterpillar is black at both ends and rust-red in the middle, dense with bristles, and is one of the most familiar insect images in North American autumn folklore. American weather lore claims the relative width of the rust-red middle band predicts the upcoming winter's severity (a wider rust band predicts a milder winter), a tradition formalized in the annual Woollybear Festival in Vermilion, Ohio. The folklore is incorrect: band width varies with caterpillar age, last year's growing-season conditions, and individual genetic variation, not with future weather. The species' real biology is more remarkable than the folklore. Woolly bear caterpillars overwinter exposed in leaf litter — not buried, not in a cocoon — and survive temperatures as low as -90°C through the production of glycerol and sorbitol cryoprotectants that prevent ice crystal formation in their cells. They essentially freeze solid for the winter, thaw in spring, finish feeding for a few weeks, then pupate and emerge as adult Isabella tiger moths in early summer. The closely related Arctic woolly bear (Gynaephora groenlandica) takes this strategy further — Arctic summers are so brief that the species takes 7 YEARS as a caterpillar before pupation, surviving multiple winters in deep-frozen suspended animation each year. The Isabella tiger moth as an adult is unremarkable — small, yellow-tan, with a few black spots — and lives only a few weeks.