Similar species: American Painted Lady has large eyespots below. West Coast Lady has orange bar across black patch.
Life Cycle: Barrel-shaped pale green eggs are laid singly on thistle (Cirsium), Aster or mallow. Caterpillars range up to 1¼"(35mm). Their color varies from purple with yellow back stripe to chartreuse with black marbled appearance. Chrysalis 1" (25mm) pale green to brown, bumpy, hangs upside down.
Flight: Two or more broods; year-round in south, April-June until frost in north. Habitat: Anywhere; Meadows, fields, open areas.
Range: All of North America, south to Panama. Resident in Hawaii.

Even though the Painted Lady cannot overwinter in any stage above a certain (undetermined) latitude, it is perhaps the most widespread butterfly in the world. It is found throughout Africa, Asia, Europe and North America. Most of North America has no Painted Ladies at all between the first hard frosts of winter and the advent of spring. In February and March, they begin emigrating from their wintering grounds in the Southwest and perhaps other warm regions; by late spring they have repopulated the entire continent. Unlike the Monarch butterfly's round trip migrations, the Painted Ladies only travel one way. The number of butterflies varies greatly from year to year, although no one knows why. Various theories have been advanced: parasite population swings, host plant variations, nectar availability due to abundance of rain or lack thereof. 

September 11, 2003 - The Chicagoland area is experiencing a population explosion of Painted Lady butterflies, just as posited by the above paragraph. They are everywhere, by the hundreds of thousands. I found dozens of them grazing in just one small patch of goldenrod, more butterflies than I'd ever seen in one place at one time.