Female insects with "penises" have been discovered in a cave in eastern Brazil.
Scientists think they are the first example of animals with reversed genitalia.
The females of four species of Neotrogla insert their erectile organs into males' vagina-like openings.
The structure, known as a "gynosome", is used to suck out sperm and nutritious seminal fluids, which provide the females with food as well. This may be an important survival strategy as they live in a cave environment where food is scarce.
Copulation lasts an impressive 40-70 hours.
Once inside a male, the female gynosome inflates. It has numerous spines which anchor the two insects very firmly together.
When the researchers attempted to pull a male and female apart, the male's abdomen was pulled from its thorax without breaking the coupling.