In caves in Brazil there lives a tiny insect with the most extraordinary story. It feeds on bat droppings and chews on the dead carcasses of fallen bats. When it copulates, it does so slowly — a single sexual act takes up to 70 hours, or three full days. But that's not even the oddest thing about it. In this species the female has developed a penis, and the male a vagina.

Kazunori Yoshizawa is an entomologist at Hokkaido University in Sapporo. In 2011, he was visiting a colleague in Geneva who had collected some of the cave insects from Brazil. Yoshikawa examined preparations of the insects under a microscope and saw the female penis for the first time.

"Quite immediately, I realized that not only the direction of genital insertion, but the direction of the sexual selection is also reversed in this insect," he recalls. "The female penis has a function to hold the male coercively."