African dung beetles rolling a ball away from a pile keep a straight line by reading the pattern of polarised light the Moon makes across the sky.

Moonlight is scattered by the atmosphere into a polarisation pattern centred on the Moon, and the beetles have detectors in the tops of their eyes tuned to it. On moonless nights they switch to the band of the Milky Way instead — demonstrated by putting beetles in a planetarium, which is the sort of experiment that deserves to have worked.