London, April 18 - It may sound obnoxiously bizarre but these females have penis-like organs that leave males gasping for more. They even mate for an impressive 40-70 hours!

In a first such case, researchers have discovered cave insects in Brazil with rather novel sex lives.

The insects belonging to the genus Neotrogla are the first example of an animal with sex-reversed genitals.

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

During copulation, female insects insert an elaborate, penis-like organ into males' much-reduced, vagina-like opening.

The researchers speculate that 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 of the genus provide females with nutritious seminal gifts in addition to sperm, making it advantageous for females to mate at a higher rate,” Yoshizawa added.

To learn more, the researchers observed the mating behaviour to find that the penis-like structure, termed the gynosome, is inserted into males and used to receive generous capsules of nourishment and sperm.

Once inside a male, the membranous part of the female gynosome inflates and numerous spines internally anchor the two insects together.

Neotrogla offers new opportunities to test ideas about sexual selection, conflict between the sexes, and the evolution of novelty, the researchers noted.

The study was published in the journal Current Biology. (IANS)