Young caterpillars are a reddish orange and covered in spikes. The mature caterpillar is bright green and covered with tufts of green spines like those seen on a prickly pear cactus. These are stinging spines that inject small amounts of venom, which causes skin irritation and pain. A red and white stripe run along the bottom of the caterpillar's body. It may form its cocoon under a wrapped leaf or near leaf litter at near a tree when it is ready to pupate. They feed on elm, maple, aspen, alder, hickory and willow trees as well as others.
The name "Io" comes from Greek mythology. Io was one of Zeus' mortal lovers and was his wife's priestess. Zeus turned her into a cow to hide her from his wife.