Sexual reversal

Female Neotrogla possess a penis-like organ, properly termed a "gynosome",^[1] but interchangeably called a penis. They aggressively seek out mates, while males are more selective.^[3] During mating, the female mounts the male and penetrates his small genital opening from behind. Her gynosome swells and tiny barbs on the organ lock the individuals tightly together; when a researcher tried to separate mating individuals the male was torn in two, leaving his reproductive organs attached to the female. During mating, the female uses her gynosome to extract sperm and nutrient-filled seminal fluid from the male. A single mating session can last from 40 to 70 hours.^[2]

The reversal of sex organs may be explained by the lack of nutrients in the cave environment, which makes it evolutionarily useful for the female to extract nutrients from the male.^[4] Female Neotrogla drain males of seminal fluid even when they are too young to reproduce, giving weight to this theory, according to entomologist Kazunori Yoshizawa who co-authored the first study of the insects' mating behavior. If males spend most of their limited resources producing such nutrient-rich fluid, it would also help explain why males are choosy about their partners. Males of other insects are known to produce similar "nuptial gifts" of nutrients to be passed over during mating.^[2] However, the evolutionary origin of the penis-like organ remains a complete mystery. "Usually, a new structure evolves as a modification of a previously existing structure," Yoshizawa explained. Such an adaptation would be "exceptionally difficult" because of the need for male and female genital structures to change at the same time.^[4]

Female penetration of males is known in a few species, such as seahorse, but only Neotrogla females have a well-defined organ that can be described as a penis.^[3] Likewise, reversal of sex roles has been recorded in a few other species of animals. Neotrogla, however, appears to be unique in having both traits.^[2] According to Yoshizawa, the animal offers a singular chance to study the conflict between the sexes and the role of sexual selection in evolution. "It will be important to unveil why, among many sex-role-reversed animals, only Neotrogla evolved the elaborated female penis," he said.^[5] In 2017, Kazunori Yoshizawa, Rodrigo Ferreira, Yoshitaka Kamimura and Charles Lienhard have been awarded an Ig Nobel Prize in biology "for their discovery of a female penis, and a male vagina, in a cave insect".