Bizarre human sexual behaviors are nothing compared to the extremely weird sex life of the African bat bug. These blood suckers are famous among biologists for a particularly horrible and cruel form of mating and now these insects have also revealed an odd secret to the researchers, "what could be the most extreme form of transexualism yet discovered".

Male bat bugs do not have a "taste" for the vagina, but they pierce the female's abdomen instead and ejaculate directly into the blood stream, where the sperm then moves to the ovaries to fertilize the eggs.

Females seem not to enjoy much being stabbed for sex and they have evolved a defense structure, a paragenital tissue on their abdomen that decreases the damage by heading the male's sharp penile tip into a spongy area crowded with immune cells.

The team led by Klaus Reinhardt of the University of Sheffield, UK, made the research in a cave on Mount Elgon (Kenya) and was shocked to discover that males had been employing their "dagger" penises to hurt other males in an attempt to copulate, and as a result, many males wore scarred abdomens.

This triggered an evolutionary response: many males grow a male type of defensive paragenitalia, functioning like a protective shield against the penis attack of other males. To turn the situation weirder, when the scientists checked 43 preserved female bat bugs, they discovered that 84 % of them carried the male type of genital shield.

It also resulted that the male version of the paragenitalia was more effective, as these females presented less scarring due to penetration than other females.

This is a "a spectacular example of evolution through sexual conflict," said Reinhardt to New Scientist.

"Males started getting nobbled by other males, so they evolved the female defensive genitalia. As this reduced the amount of penis damage they were getting, females evolved the male version of the male genitals." he added.

Bat bugs are parasites feeding on the blood of the bats, and in their absence they can also attack people. A close relative and more known is the infamous bed bug.