Nettle-tree butterflies gather in thousands to drink from mud

In the mountains surrounding Lake Kerkini in northern Greece, the Nettle-tree butterfly creates a spectacle rarely seen outside the tropics. These insects are the only European members of the Libythea family, a group otherwise concentrated in equatorial regions. During the heat of the summer, they gather by the thousands on damp patches of ground to drink, forming dense clouds that erupt into a blizzard of wings when disturbed.