These are Purple Emperor and Lesser Emperor butterflies (Apatura genus) getting nutrients from the body of a dead frog. Butterflies that "puddle" at muddy spots or on scat or on carrion (as shown here) are seeking sodium, which is rarely found in plants (potassium is the principal cation in vegetation).
Photo source, via Uncertain Times.
Reposted from 2011 to add this relevant photo of Eyed Browns by Douglas Buege, via Wisconsin Butterflies' Recent Sightings webpage:
