Ancient Chinese legend has it that there was once a murderous farmer who was rumbled in an identification parade because of the flies congregating around his blood-stained sickle. Three thousand years on, forensic scientists are still tapping into insects' blood-thirsty tendencies as valuable evidence of where, when and how crimes are committed

Just hours after death, a body - even a well-hidden one - will be mobbed by insects lured by its smell. First on the scene of the crime are usually adult blowflies, followed after a few weeks by waves of other species, including beetles and other types of fly.

Blowflies lay eggs or larvae in every available orifice soon after arriving. So, as Martin Hall from The Natural History Museum, London, UK explains, the age of the maggots or pupae that are found on a body can give a good estimation of the minimum time, at least, that a cadaver has been around. "The humble maggot, generally appreciated only by the angler," says Hall, "can provide real help to solving crime."

But, as he outlined at the British Association Festival of Science in Sheffield, UK, working out the age of a maggot is not quite as easy as it sounds. First, there are a vast number of fly species in the world, within each country and even within each region. Thus each corpse is colonized by a different set of insects, depending on where it has been. Second, the timings for each stage - egg, larva, pupa and adult - of the life cycle are slightly different for each species.

Third, to make things even more difficult, the speeds and sizes of insects' developmental stages can be influenced by temperature, humidity, location and even by the presence of drugs such as cocaine or heroin in the corpse; not to mention the fact that insect material can lose some of its integrity on its way to the lab, thanks to fixing and transporting procedures.

Thus expert entomologists - the scientists who study insects' habits, development and habitats - are vital for sifting this sort of evidence. With the help of 'insect libraries' such as those at the Natural History Museum in the UK, plus genetic profiling and electron microscopy, entomologists can identify insect matter and accurately estimate its age and place of origin: "just as," Hall comments, "a number plate will provide the police with access to all the stored information on a particular vehicle."

Not all forensic entomology is quite so grisly. Cockroaches found in a smuggled drug haul in New Zealand, for example, allowed police to pinpoint its source to within a few miles. On another occasion, beetle larvae nestling in a ski hat spotted at the scene of a crime proved that, contrary to the criminal's claims that he had not worn the hat since winter, the garment had been worn in summer.