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Brazilian bugs' bizarre bawdy bits baffle biologists

Four species of a cave-dwelling insect have been found to have sex-reversed genitalia, with the females sporting a penis-like organ and the males lacking one. 

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Current Biology, Yoshizawa et al.
These are Neotrogla curvata in copula.

In certain insects that dwell in Brazilian caves, the sex roles are reversed.

Four species of bugs in the genus Neotrogla are the first example of an animal to have sex-reversed genitalia, with females sporting "a highly elaborate, penis-like structure, the gynosome, while males lack an intromittent organ," noted researchers in a paper titled "Female Penis, Male Vagina, and Their Correlated Evolution in a Cave Insect" in the journal Current Biology on April 17.

Although sex-role reversals have been known to exist in several different animals, "Neotrogla is the only example in which the intromittent organ is also reversed," says Kazunori Yoshizawa from Hokkaido University in Japan and an author on the paper.

The scientists who observed the mating behavior of the insects in the laboratory say that copulation between the male and female can last from 40 to 70 hours, during which "a large and potentially nutritious ejaculate is transferred from the male via the gynosome," according to the paper.

But why did Neotrogla in particular evolve in such a manner?

Researchers do not have an answer yet, but they speculate that the evolution of the sex organs and sex-role reversal had something to do with their habitat. Caves are dry and poor in resources, so males provide female with "nutritious seminal gifts" along with the sperm cells.

"In some other insects, males expend personal resources to create highly sought-after ‘nuptial gifts’ of sperm and nutrients that they bestow upon their mate during copulation. Although it’s not clear whether Neotrogla couples do likewise, the females accept seminal gifts and drain them even when they’re too young to reproduce, Yoshizawa says, so it’s obvious they’re using the sperm capsules for more than mere reproduction," Regina Nuzzo of Nature writes.

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