A beetle that never wants to get pregnant has overturned one of Charles Darwin's theories by engaging in an arms race against the male of the species.

Research suggests that the only way that male Acilius diving beetles can become fathers is to abandon any attempt at courtship and rape any passing females.

The response of the female is to embark on an evolutionary battle of the sexes, in which it develops defences designed to shake off the unwanted attentions of the male.

Darwin believed that variations in the anatomy of the beetle were to assist reproduction. But scientists have discovered that, far from trying to make it easier for males to grab hold of them, the females are developing defences against them.

Research on the 13 species of Acilius diving beetles, two of which are found in Britain, show that every time the female evolves contraceptive features in its anatomy, the male develops a new set of counter-measures.

The female has a variety of furrows, ridges, hairs and pitted dents on its back, developed through time to loosen the male's grip when it tries to grab hold.

Males have suction cups, which work best on smooth surfaces. In order to counter the female defences they have evolved a range of suction sizes.

It is the beetle equivalent of the ever-bigger missiles and antimissile missiles developed during the Cold War and the radar and radar-jamming techniques pioneered by Britain and Germany during the Second World War.

"The females don't want to mate. You could say they always have a headache," said Johannes Bergsten, of the Natural History Museum.

"There's no courtship for these beetles. It's a system of rape. But the females don't take things quietly. They evolve counter-weapons."

Any foreplay between the beetles is limited to the female desperately trying to dislodge the male by swimming fran-tically around the pond.

Males that manage to hang on wait until the female is exhausted by the underwater struggle and then mate.

The male will keep hold of the female, occasionally allowing her to the surface to breathe, for up to six hours to prevent her mating, however unwillingly, with any other beetles.

Systematic rape for reproduction and a co-evolving battle of the sexes have been suspected in a handful of insects but the study of diving beetles is the first to show an evolutionary arms race across an entire genus.

Dr Bergsten, who conducted the research jointly with Kelly Miller, of the Universty of Albuquerque, in the United States, said that it was possible that the battle between the sexes may be the best way to ensure that only the strongest males pass on their genes because only they are capable of overcoming the females' struggles.

By examining the DNA of the insects and the minute differences in body shape between males and females of the Acilius genus, researchers were able to chart the development of the battle of the sexes through the 13 species.

The most recent stage of the conflict is thought to be demonstrated by the separation of a beetle in Japan into two species, A. japonicus and A. kishii, within the past few thousand generations.

Diving beetles are predatory insects found across the northern hemisphere.

The two species living in Britain are A. sulcatusand the rarer A. camalicutus.