What lousy research.

17:31 13 September 04 A new genetic analysis may finally settle the question, and even help when it comes to getting rid of the little parasites, which are staging a comeback in rich countries.

Linnaeus named the human louse Pediculus humanus in 1758, but later realised there might be two sorts. Debate has gone on ever since. Those who regard body lice as a separate species point out that they are bigger than head lice and live in clothes rather than in head hair.

They can also transmit diseases such typhus and trench fever, something head lice have never been shown to do.

Other experts dismiss these differences and argue that, because head and body lice interbreed if kept together in the lab, they must be the same species. Then again, breeding under artificial conditions is a poor test of a species.

Two big families

To find out if head and body lice interbreed in the "wild", Natalie Leo and Stephen Barker of the University of Queensland in Brisbane, Australia, collected lice from seven boys in Nepal and four girls in Inner Mongolia in China.

A form of DNA fingerprinting of 443 lice showed there were two genetically distinct populations. "The head lice were one big family; the body lice were one big family," says Barker.

Further studies on children who shared sleeping quarters showed that the lice travelled from the body of one child to the body of another, or from head to head, but never between the body and head - evidence that the two populations were not interbreeding and that head and body lice are different species.

The best way to treat head lice is hotly contested. Many health authorities, including the US Centers for Disease Control, advise treating clothes and sheets. Other experts say parents should not waste time boiling clothes.

The latest findings, reported at an entomology conference in Brisbane in August, support the idea that the parents of children with head lice should concentrate on their heads. "For a head louse, shifting to clothes would be like setting out across the desert," says Barker.

Rachel Nowak, Melbourne