Mud puddling

Mud-puddling, is a behaviour most conspicuous in butterflies, but occurs in other animals as well, mainly insects; they seek out certain moist substances such as rotting plant matter, mud and carrion and they suck up the fluid. Where the conditions are suitable conspicuous insects such as butterflies commonly form aggregations on wet soil, dung or carrion. From the fluids they obtain nutrients such as salts and amino acids that play various roles in their physiology, ethology and ecology.

Lepidoptera (butterflies and moths) are diverse in their strategies to gather liquid nutrients. Typically, mud-puddling behavior takes place on wet soil. But even sweat on human skin may be attractive to butterflies such as species of Halpe. More unusual sources include blood and tears. Again, similar behaviour is not limited to the Lepidoptera, and for example, the various species of bees commonly called sweat bees are attracted to various kinds of sweat and tears, including that of humans, and other bee species have been recorded as doing so to various degrees.

Source: wiki