In This Insect Family, the Females Have Penises and the Males Have Vaginas

In the animal kingdom, penises are almost always a feature that belongs to the male of the species. But some members of a strange tribe of cave-dwelling insects known as Sensitibillini buck this trend by having reversed sex organs: essentially, the females have penis-like appendages while the males have vagina-like pouches.

This evolutionarily intriguing tribe is made up of three related closely genera—or groups of species—known as Afrotrogla, Neotrogla and Sensitibilla.

When researchers first discovered Neotrogla in Brazil four years ago they noticed that the genus had reversed sexual organs, much like Afrotrogla, which was already known to science.

In order to have sex, the females of these insects anchor their penis-like organs into the male pouch for a lengthy amount of time (between 40 and 70 hours for one species of Neotrogla) so that they can receive large volumes of semen from the male. Puzzlingly, the Sensitibilla genus does not have reversed sexual organs.

To understand whether the reversed sexual features evolved just once, before Neotrogla and Afrotrogla diverged, or if they evolved independently, an international team of scientists from Japan, Brazil and Switzerland examined the sex organs of all three genera, reports.

Image showing a male and female Neotrogla copulating. Biology Letters (2018)

The team found that, the reversed sexual organs likely evolved independently because the size and shapes of the penises in Neotrogla and Afrotrogla are significantly different, according to a study published in the journal Biology Letters.

The team then tried to determine why this feature evolved twice in two related genera—a surprising coincidence given its almost total absence in the animal kingdom.

The answer to this riddle came when the researchers analyzed the environments in which the two genera evolved and continue to live today. The caves in which they inhabit do not contain much food, which has resulted in the males evolving to be more interested in finding a meal than a mate.

Over time, this has caused the females to take on the role of being the pursuer and initiator of mating. The lengthy mating ritual can also be explained by this phenomenon. The female "penis" has evolved to be hook-shaped to ensure that the male can't escape before a sufficient amount of sperm has been collected.

Furthermore, the researchers noticed an evolutionary pattern in that the corresponding features within the sexual organs of both genera developed simultaneously, a process which has helped to facilitate the transfer of sperm. This mutually beneficial evolutionary process has been observed in many animals with conventional genitalia.