Wed Sep 19, 3:03 PM ET

PARIS (AFP) - Sexual relationships between humans may be complicated but they are nothing compared to the bizarre sex life of the African bat bug, the British weekly New Scientist reports in next Saturday's issue.

Renowned among entomologists for a particularly horrible form of reproduction, these insects have now been found to show "what could be the most extreme form of transexualism yet discovered," it says.

Male bat bugs never use the vagina, instead piercing the female's abdomen and inseminating directly into the blood, where the sperm then swim to the ovaries.

In response to this, female bat bugs have evolved defense mechanism -- they grow a paragenital structure on their abdomen that limits the damage by guiding the male's sharp penile prong into a spongey area full of immune cells.

Scientists led by Klaus Reinhardt of the University of Sheffield, northern England, studied bat bugs in a cave on Mount Elgon, Kenya.

They were stunned to find that males had been using their penises to stab other males in an attempt at copulation, and many had scarred abdomens as a result.

In response to this, many males had been growing their own version of the defensive genitalia to protect themselves from other males.

Stranger still, when the team looked at 43 preserved female bat bugs, they found that 84 percent of them had male versions of the defence genitalia.

Females with this male version had less scarring due to penetration than other males.

Reinhardt describes it as "a spectacular example of evolution through sexual conflict," New Scientist says.

"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."

Bat bugs are blood-sucking parasites that feed on bats, but bite humans in the absence of their primary hosts. They are cousins of the bed bug.