A unique organ that could help in the battle against malaria has been discovered on female bedbugs who suffer serious problems from the amorous attentions of the opposite sex.

The organ offers the females a degree of immunity from infections introduced by violent males.

Immune organs are found nowhere else in the animal kingdom and their discovery should eventually help scientists to develop new techniques to prevent mosquitoes passing malaria and other fatal diseases to people.

Professor Mike Siva-Jothy, of the University of Sheffield, was behind the discovery of the organ, which he said formed a reservoir of white blood cells acting as a first line of defence against sexually transmitted infections.

The females evolved immune organs because of the violent mating techniques adopted by the males, which are armed with needle-like penises that they wield like daggers. Instead of availing themselves of the female genitalia the male bedbugs simply stab them in the abdomen and inject semen into the abdominal cavity. The semen migrates through the female's body to fertilise the eggs.

Females suffer a 25 per cent higher mortality rate than males because of infections introduced into the wounds during mating. Females mate only after a meal — they are so engorged, their bodies having swelled by up to 30 per cent, that they cannot escape.

Professor Siva-Jothy, who announced his find at the Royal Entomological Society conference this month, said that without the organ the death rate would be much higher. “This is a bizarre reproductive system. This is so extreme it's only evolved once. Bedbugs live in unsanitary spaces, often crawling through faeces. Males and females are covered in bacteria and fungi and the males introduce these pathogens into the female when they mate. The higher female mortality isn't caused by the wounding, it's the pathogens. Females have responded to this by evolving a whole new immune organ.”

The organ was first spotted as a feature of the female body more than 200 years ago but no one knew what it was until reseachers at Sheffield realised it was an organ in the same way as hearts and kidneys. It developed over the area targeted by the male.

Professor Siva-Jothy said its discovery will allow researchers to study a concentrated form of insect immune systems. In other insects immunity is dissipated throughout the body. “It's important for understanding insect immunity,” he said. “We've now got a way of looking at it. “

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