Every autumn, banded woolly bear caterpillars inch across roads and footpaths and spark the same debate about whether their stripes forecast the coming winter. The folklore is shaky. What the caterpillar does once winter arrives is far more remarkable.
Freezing on purpose
Most animals that live through cold winters try to avoid freezing. The woolly bear does the opposite. It is “freeze-tolerant”, able to survive being frozen and thawed again and again through the season, comparative physiologist Katie Marshall of the University of British Columbia has explained.
“The liquids within their body will turn to ice,” Marshall said. The trick is where that ice forms. Rather than freezing solid, which would kill it, the caterpillar lets ice build up in the spaces between its cells, while a glycerol-based substance, a natural antifreeze, protects what is inside them.
“There aren’t a ton of species that do this,” Marshall said. “They can do things that humans can’t do with our bodies.”
From caterpillar to tiger moth
The common woolly bear is Pyrrharctia isabella, the larva of the Isabella tiger moth. When warmer weather returns, the caterpillar spins a cocoon and later emerges as a pumpkin-orange moth with black spots on its abdomen, active at night through the summer.
The species is thought to be common and is not endangered. Because the adult moths are drawn to lights at night, cutting back on outdoor lighting is one simple way to help them.
And those stripes?
The caterpillars shed their skin six times as they grow, turning rustier and less black with each moult, so the youngest tend to be the darkest. That means the width of the bands likely says more about a caterpillar’s age and when it last moulted than about the weather ahead.
Further reading
- Pyrrharctia isabella — Wikipedia
- Marshall & Sinclair (2015), “Cold-hardening during long-term acclimation in a freeze-tolerant woolly bear caterpillar”, Journal of Experimental Biology
- Layne et al. (1999), “Cold Hardiness of the Woolly Bear Caterpillar”, The American Midland Naturalist
Image: Woolly bear caterpillars by Brocken Inaglory, BY-SA 4.0 via Wikimedia Commons
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