JOHANNESBURG (Reuters) - A species of South African dung beetle has been shown to use the Milky Way to navigate, making it the only known animal that turns to the galactic spray of stars across the night sky for direction.

Researchers have known for several years that the inch-long insects use the sun or moon as fixed points to ensure they keep rolling dung balls in a straight line - the quickest way of getting away from other beetles at the dung heap.

But scientists have puzzled over how the beetles, which perform an orientation dance on top of their dung balls before setting off, achieve a straight line on moonless nights.

To prove the Milky Way theory, scientists at Johannesburg's Wits University took beetles into the university planetarium to see how they fared with a normal night sky, and then one devoid of the Milky Way.

"The dung beetles don't care which direction they're going in. They just need to get away from the bun fight at the poo pile," Wits professor Marcus Byrne said. "But when we turned off the Milky Way, the beetles got lost."

And on cloudy nights without a moon or stars?

"They probably just stay at home," Byrne said.