Ecologists spelunking in a Brazilian cave have found a new variety of insect with an unusual sex life. Females of the newly discovered genus Neotrogla, 3-mm-long flylike insects, boast large, penislike structures called gynosomes (inset). Although other animals such as seahorses take on reversed gender roles, species in the new genus are the first to be found with swapped genital structures. During copulation—which lasts 40 to 70 hours—the female mounts the male from behind (as seen in the above photo), thrusting her gynosome into the male’s vaginalike opening. Once inside, the female uses spines on the gynosome to latch on to the potentially reluctant male. The lady’s grasp is so strong that when the researchers tried to separate a fornicating pair, the male’s abdomen was ripped from its body without breaking the genital link. During breeding, the male transfers a large amount of nutritious ejaculate via the gynosome to the female, the researchers report online today in Current Biology. The team suggests that in the resource-scarce cave system, this nutritious seminal gift causes females to compete for sustained breeding. Over time, the scientists believe this female-initiated intercourse could have led the insects to evolve reversed genitals. Still unclear is whether the females worry about the size of their gynosome.

See more ScienceShots.