Automeris io larvae are pale green with a red and white lateral stripe and are studded with clusters of branched spines called scoli. Each spine is hollow and connected to a venom gland at its base. When a spine tip breaks against skin, venom enters the wound, producing burning pain, reddening and raised welts within minutes. Reactions typically resolve within hours, though sensitive individuals can react more strongly.
The adult carries a different defence. At rest the moth folds its yellow or brown forewings over the hindwings and resembles a dead leaf. Disturbed, it drops the forewings forward to expose two large eyespots ringed in black and white against blue and yellow, an abrupt display that has been shown in related species to startle small birds and interrupt an attack.
Stinging caterpillars are a small minority of North American Lepidoptera, and io larvae are among the few likely to be encountered in gardens. Most caterpillars with conspicuous hairs are harmless, and treating all hairy larvae as dangerous misreads the pattern.