Adult Io moths commonly rest with wings flattened out to the sides and over their backs. When carefully prodded, they often part the forewings to reveal the hindwings, which each have a single large, prominent, blue and black eyespot. The forewing color is different in males and females: Males are generally yellowish; females are more brown, rusty red, or purplish. Females are generally larger than males.
Mature larvae are yellowish green to bluish green with a red-and-white stripe on the sides. They are thickly covered with bristly, stinging spines that cause severe irritation in some people. When young, larvae are orange and stay in groups; they become more solitary as they mature.
Wingspan: 2–3¼ inches.
Statewide, with larger populations in eastern and southern Missouri.
Habitat and Conservation
Occurs in a variety of habitats, but especially in forests and parklike areas.
Larvae feed on a great variety of plants, including grasses, herbaceous plants, shrubs, deciduous trees, and conifers. They have been recorded on more than 30 species of plants in Missouri. In the Ozarks, Io moth caterpillars are commonly found on sassafras. Adults, like other members of the giant silkmoth family, have reduced mouthparts and do not eat.
Adults fly from early May into September. There are two broods in Missouri. Like most other moths, this species is nocturnal and attracted to lights. Adults seek mates from just after dark to shortly after midnight. Females lay clusters of eggs on host plants. The caterpillars, especially when young, are gregarious (live in groups) and sometimes form lines, “marching” in single file. The papery cocoons are made on the ground in leaf litter or on plants. They are spun of silk and sometimes incorporate leaves.
Scientists in former days were well educated in the Greek and Roman classics, and this is reflected in the names they gave to many natural subjects. Io, the name of this moth (as well as of one of Jupiter’s largest moons), is a character from Greek mythology associated with moons.
The caterpillars are herbivores that graze on vegetation. Although the caterpillars have stinging spines for defense, some predators probably are immune to them. Among the many foods birds, mice, and other animals hunt for in winter are cocoons, egg cases, and other inert insect forms.
The Io moth is a textbook example of “startle” coloration. At rest, the adults are camouflaged, but when disturbed, they quickly draw the forewings apart, suddenly revealing the two bold eyespots, which can scare predators away, or at least startle them long enough for the moth to escape. The coloration and wing-parting reflex apparently evolved because moths with genes for those traits tended to survive longer and have more offspring than those without the traits.