Scientists have discovered gender-bending female cave insects with "very impressive" penis-type organs which can have sex for up to 70 hours at a time.

Females of the neotrogla species insert their penis into males during mating, who instead of having a penis have the equivalent of a vagina.

The neotrogla were discovered in a cave in eastern Brazil and are believed to be the first example of an animal with sex-reversed genitalia.

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Researchers at the Hokkaido University and Keio University Japan found that during copulation, which lasts between 40 and 70 hours, female insects insert an elaborate, penis-like organ into males' small, vagina-like opening.

The penis has been named a gynosome by the researchers and once inside grips on tightly, sucking out sperm and nutritious fluids, researchers noted in their study.

To lock on, the membranous part of the female gynosome inflates and numerous spines internally anchor the two insects together.

In one instance, when the researchers attempted to pull a male and female apart, the male's abdomen was ripped from the thorax without breaking the genital coupling.

"Although sex-role reversal has been identified in several different animals, Neotrogla is the only example in which the intromittent organ is also reversed," researcher Kazunori Yoshizawa said.

Scientists believe the insects' sex organs and sex-role reversal may have been driven over evolutionary time by the resource-poor cave environment in which the bugs live. Males provide females with nutritious seminal gifts in addition to sperm, making it advantageous for the females to mate at a higher rate.

"It will be important to unveil why, among many sex-role-reversed animals, only Neotrogla evolved the elaborated female penis," lead researcher Yoshitaka Kamimura from Keio University.

The study Female penis, male vagina, and their correlated evolution in a cave insect is published in the journal Current Biology.