The Azures comprise almost a dozen forms that were once considered to be a single, variable, and abundant species across North America. Now several species are recognized, but their taxonomy is confusing, even for experts. Kaufman in his Field Guide to Butterflies of North America suggests that amateurs simply enjoy these bright blue-backed, white-undersided butterflies as Azures. Let the lepidopterists argue about species. In mid-summer in Minnesota, however, the choices are somewhat limited. I suspect the odds are excellent that this photograph is of a Summer Azure.
Many Azure caterpillars have a symbiotic relationship with ants. The caterpillars secrete a sugary substance that feed the ants, while the ants protect the caterpillars from predators. In other species the caterpillars feed the ants while on the caterpillar host plants, but then move into ant nests and begin feeding on ant larvae. Nevertheless, the ants continue to protect the butterfly pupae. The adults break out of the pupae case and must crawl out of the ant nest before the butterfly can take flight (Wikipedia).