Cutting edge: Maggots could trip up a murderer on the fly

In some criminal cases, forensic entomologists can help extract valuable clues from corpses or cannabis, writes Zakaria Erzinclioglu

The idea that insects can be used to solve crimes may surprise many people. The subject of forensic entomology, or the application of insect biology to the investigation of crime, is an obscure subject, even within the forensic science profession and police forces. Happily, it is rapidly becoming better known. It is now acknowledged to be a powerful tool in estimating the time of death in murder cases, even helping to determine the place and manner of death.

The reason why forensic entomology has remained a little-known branch of forensic science is that almost no forensic scientist has ever taken up entomology. It is entomologists who have taken up forensic science. This may be because more than 85 per cent of all animal species are insects - more than enough for an entomologist to tackle. But to a general forensic scientist - dealing with blood, fibres, glass fragments, poisons and what have you - the prospect of facing such a vast world of creepy-crawlies is daunting.

How can entomology help to solve a murder case? The somewhat macabre basis of the subject lies in the fact that, after death, the human body attracts insects to feed and breed. Typically, an exposed corpse on a summer's day will attract bluebottles and greenbottles, which lay eggs on the body. The hatching maggots feed on the body for a number of days, then pupate. A number of days later, the adult flies hatch and the cycle starts all over again. If the age of the maggot can be determined, it should be possible to give a minimum time since death.

The vagueness of the last few sentences is deliberate. A "number of days" can vary from four or five to 40 or 60. This is because the rate of larval development depends on the temperature. At very low temperatures, the maggots will develop very slowly. At high temperatures, growth is quite rapid. (To put it another way - cold lengthens the life of a maggot; warmth shortens it.) So, it is essential to reconstruct the temperatures.

As regards the "minimum time since death", this cannot be overemphasised. The age of the maggot can only give a time after which death could not have occurred, because we do not know the time at which the fly arrived at the body. So, if one can estimate the age of a maggot as, say, ten days, then death could not have occurred later than five days earlier.

But this is not all that can be done. As time passes, different species will be drawn to the body at different times. Anything that changes with time can be used as a clock; and the species present on a body at a given time can provide clues to the time of death. Because many insects are found only in particular geographical ranges and habitats, the corpse fauna can also help reveal whether a body has been moved after the murder. (In the same way, it is possible to ascertain the area in which imported cannabis and other raw drugs that contain many insects have been grown.) Much remains to be done in forensic entomology. It should be possible to determine the age of a maggot to within a quarter of an hour, using molecular techniques. A better understanding of what attracts insects to bodies would be of great benefit. It has been suggested that DNA from a mosquito's blood meal could enable the identification of someone who had been at a crime scene. Insects are useful in forensic investigations because there are so many species. According to ecological theory, no two species will do exactly the same thing. It follows that any particular species found in a locality will tell us something very specific about that locality and what happened in it.

Why did I become a forensic entomologist? As a boy, I was one of those little horrors who kept bringing creepy-crawlies into the house, to the disgust of my elders. I was also a great fan of Sherlock Holmes. The two passions came together in forensic entomology. It is only fair to add that some of my less charitable friends have attributed my strange interest to sheer laziness. I did not have to go out searching for insects like other entomologists. The insects came to my experimental animal carcasses without my having to exert any effort.

Zakaria Erzinclioglu is a researcher affiliated to the Cambridge University Museum of Zoology.