I figure this butterfly is an Eastern Tailed Blue, common across much of North America. Erika and I found discovered it in the Carleton College Arboretum last week. The wiry tails that should be sticking out from its hind wings are often destroyed, as in this critter. Liking open country, this species benefits from deforestation, and is common in fields, roadsides, and mud puddles. According to Wikipedia, Eastern Tailed-blues secrete a substance that attracts some ants. The ants, in turn, protect the butterfly larva from predators.