Other hairy caterpillars are not known to cause reactions to persons of average sensitivity. The urticating setae of the io larvae are shown greatly enlarged in the drawing below. Urtication results from touching poison spines or setae, thereby breaking the tip and releasing the poisonous chemicals, or by injecting into the skin via a puncture when more forcefully contacted. Adult io moths do not have any urticating scales. They range west to southern Arizona and south to Central America, at least as far as Costa Rica.
Adult males are mostly yellow, while females have brown forewings.
The larvae have characteristic long yellow or green spines covering most of the body. Only a few other caterpillars may look similar, but these do not have the well-defined red and white lateral line. Earlier instars are yellow overall and the lateral line is reduced. After several weeks of feeding, they make a simple paper-like cocoon away from the host plant. In Florida, there are three to four generations per year.