The story of John Wayne Bobbitt holds no fear in the earwig world. These well-endowed insects have a standby penis to compensate for accidents in action, Japanese research now shows. 

Male earwigs, like a few other well-prepared animals, carry a spare set of genitals. The earwig's auxillary organ was thought to be impotent, as it points the wrong way and females have only one hole.

To probe the workings of the extra appendage, Yoshitaka Kamimura and Yoh Matsuo of Tokyo Metropolitan University studied the earwig Euborellia plebeja. The male of this species is blessed with a pair of penises that are often longer than its body. Kamimura and Matsuo interrupted the earwigs in the act by pinching them on the behind.

Pulling a male off its mate broke off his penis in its prime. Yet "handicapped males" given another shot with the ladies still performed, the researchers found.

To see if the earwigs naturally suffer similar injuries in the wild, Kamimura and Matsuo collected insects out and about in central Japan. A few females contained leftover penis ends, they found, and the asymmetric genitals of some males revealed signs of damage. The findings suggest that both "paired penes" are working organs, says Kamimura; the second is flexible enough to function despite its misdirection.

"It's an interesting phenomenon," says Mike Siva-Jothy of the University of Sheffield, UK, who studies insects' nether regions. He thinks there must be some evolutionary advantage to the earwig's "unusually long" and fragile organs. Breakage may be part of the insect's strategy to ensure the success of its sperm, Siva-Jothy speculates. "It's hard to imagine why a male would do it without a reason," he says, adding that the end-piece "may act as a mating block."

Many insects have bizarre genitalia - the mouldable chitin that makes up insects' outer body is a versatile building material for imaginative forms. Dragonflies, for example, have structures like brushes, pipe cleaners and inflatable beach balls that scoop out rivals' sperm. "It's stranger than fiction," admits Siva-Jothy.

Paired penes, though common in spiders and crustaceans, are rare in insects, says sexual-selection researcher William Eberhard of the University of Costa Rica. The earwigs' double whammy may be an ancestral throwback - like antennae and legs, penises were originally paired.

Why insects evolved such a diverse repertoire of genitals nevertheless remains a bone of contention. As well as being adapted to ensure that sperm is successful, Eberhardt argues, insects' members may have been influenced by female taste. "Females discriminate amongst males on the basis of their genitalia," he says.