This picture of two mating insects comes with an unexpected twist. The one on top is a female, and she has a penis. The one on the bottom is a male, and he has the equivalent of a vagina. During sexual bouts that can last for 40 to 70 hours, she penetrates him and uses her genitals not to deliver sperm, but to collect it.

The 3-millimetre-long species is called Neotrogla curvata, and it lives in Brazilian caves. There are four Neotrogla species and the females all have penises. They belong to a group of insects called barkflies or barklice—closely related to lice and true bugs, and only distantly related to actual flies.

Caver Rodrigo Ferreira discovered these insects several