This seems to be a 'Chalcedon' Variable Checkerspot butterfly. We spotted many of them in the San Rafael Wilderness on Memorial Day weekend.
The butterflies were gathering in pairs -- or groups of 3 or 4. Pheromones must have been strong in the air. They seemed to particularly enjoy these flowers, which I think are buckwheat. Meanwhile, the butterflies are enjoying a sort of Roman-style orgy while helping the flowers out with pollination.
