Researchers have discovered that the Brazilian insects, which represent four distinct but related species in the genus Neotrogla, are the first example of an animal with sex-reversed genitalia.
"Although sex-role reversal has been identified in several different animals, Neotrogla is the only example in which the intromittent organ is also reversed," Kazunori Yoshizawa from Hokkaido University in Japan said.
During copulation, which lasts an impressive 40 to 70 hours, female insects insert an elaborate, penis-like organ into males' much-reduced, vagina-like opening.
The researchers speculate that the insects' sex organs and sex-role reversal may have been driven over evolutionary time by the resource-poor cave environment in which the bugs live.
Males of the genus provide females with nutritious seminal gifts in addition to sperm, making it advantageous for females to mate at a higher rate.
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To learn more, the researchers observed the mating behavior of all four species to find that the penis-like structure, termed the gynosome, is inserted into males and used to receive generous capsules of nourishment and sperm. Once inside a male, the membranous part of the female gynosome inflates and numerous spines internally anchor the two insects together.
In one instance, when the researchers attempted to pull a male and female apart, the male's abdomen was ripped from the thorax without breaking the genital coupling. In other words, it appears that females can hold males very tightly indeed.
The research is published in the Cell Press journal Current Biology.
