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.”

Let’s look at what this means. Sexual selection is a form of natural selection. It’s the competition that takes place between males to win mates, and it also operates when females choose between males. It’s why male deer have evolved antlers (and kabutomushi Japanese stag beetles have incredible horns) — to fight other males. And it’s why males also evolve beautiful displays, such as the opulent peacock’s tail. Females tend to be attracted to males that have the most attractive displays.

That’s what usually happens. Sometimes, the situation is reversed. When males offer an edible gift to the female in return for access to mating, you sometimes get competition between females for the gift. Millions of years ago, this was probably the case in the Brazilian cave insects.

The males of this insect, Neotrogla, produce a large and nutritious ejaculation. Females living on dry scraps of bat droppings are desperate for the extra food. Not only do they compete for access to the males, they have evolved an organ that grips hold of them. The female penis looks like that of many males in the animal kingdom: It swells inside the female and is spiked, both of which wedge the female inside the male. The very long duration of copulation is probably controlled by the females, the authors say, in order to extract maximum resources from the males.

Last month, Yoshizawa and his colleagues won a prestigious Ig Nobel prize for their research. The spoof prizes are awarded at Harvard University each year to celebrate unusual or trivial discoveries in science. Yoshikawa’s team won the biology prize. Was he happy to receive this honor?

“Of course,” he says. “I have been an Ig Nobel watcher for more than 15 years.”

Neotrogla reminds me of another bizarre insect that lives in caves — the bat bug, which is found in caves in Kenya. In this insect, the males stab the female with a knife-like penis and inseminate directly into the bloodstream. The sperm then swim to the ovaries, bypassing the female reproductive tract. To try and limit the damage done by these attacks, females have evolved a counter-measure: defensive structures on their abdomens that soak up the damage. The odd thing — or even odder thing — is that the males are so keen to mate that they also stab other males and try to inseminate them as well. This seems to have had such an impact over the generations that males have evolved similar defensive abdominal structures to the females. Bed bugs — the ubiquitous pests that you find in unclean hotel beds — have a similar method of stabbing sex. Entomologists call it traumatic insemination.

The Ig Nobel prizes are awarded by Marc Abrahams, who runs a magazine called the Annals of Improbable Research. They’re designed to make you laugh and then to think. In their acceptance speech (filmed from a cave in Japan), the scientists said: “Every dictionary around the world defines a penis as a male structure. Our discovery makes billions of dictionaries outdated.”

As a biologist who has written about all kinds of bizarre animals, I wholeheartedly applaud the Ig Nobel prize for Neotrogla.

Natural Selections covers issues related to scientific research in Japan on the fourth Saturday of the month. Rowan Hooper is the news editor of New Scientist magazine. Follow him on Twitter @rowhoop.

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