In desolate caves throughout Brazil live insects that copulate for days, the female’s penetrating erectile organ sticking fast in a reluctant male’s genital chamber until he offers a gift of nutritious semen. Neotrogla seems to be unique among species with reversed sex roles — with choosy males and aggressive, promiscuous females — in also having swapped anatomy, researchers report.

Not all animal species have a male penis, but because the evolution of body parts usually works through slow modification of existing structures, there would need to be a good reason for a female to develop a penetrating organ, says entomologist Kazunori Yoshizawa of Hokkaido University in Japan, a co-author of the study.

Yoshizawa and his colleagues think that they have found that reason in Neotrogla, which was first described in 2012. The insects were originally spotted in Brazilian caves by ecologist Rodrigo Ferreira from the Federal University of Lavras in…