Woolly Bear Caterpillar
Pyrrharctia isabella
The banded larva that freezes solid and lives
This bristly larva, black at both ends and rust-brown between, belongs to the Isabella tiger moth. Folklore reads its bands to forecast winter, but its real talent is chemistry: cryoprotectants let it freeze solid — heart stopped — and thaw out alive.
Key figures
- Size
- Larvae about 4–5 cm long
- Freeze survival
- Overwinters frozen solid, guarded by natural antifreeze chemicals
- Life cycle
- Passes winter as a caterpillar, then pupates and emerges as a moth in spring
- Habitat
- Meadows, roadsides and open woodland of eastern North America
- Diet
- Plantain, dandelion, grasses and other low-growing herbs
- Feature
- Black bristle bands at both ends framing a rusty middle belt
Life cycle
Egg → Larva → Pupa → Adult
Ecological role
A modest herbivore of wayside plants, it converts meadow growth into moth biomass and feeds shrews and birds that brave the bristles.
Did you know
Every autumn Ohio crowns a Woolly Bear Festival, complete with races in which caterpillars dash up a string to "predict" winter.
Range
Eastern and central North America
Status
Common
Anatomy
- Head — A neat dark capsule that gnaws low herbs nonstop through late summer to build reserves
- Body — Thirteen segments wrapped in a dense suit of bristles that discourages grabbing beaks
- Bristles — These setae neither sting nor inject toxin — they prick, insulate, and weave the future cocoon
- Legs — Short true legs up front and fleshy prolegs behind grip stems and shuffle over soil
- Rear end — The tail band starts the famous winter-forecast folklore — which field tests never support
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