What can a fly tell us about time of death? Mo Costandi looks at the emerging field of forensic entomology.
We still know very little about the insect species that colonise a cadaver. But the latest published study from Sibyl Bucheli’s lab at Sam Houston State University, Texas, suggests they are far more diverse than we had previously imagined.
The study was led by Bucheli’s former PhD student Natalie Lindgren, who placed four cadavers on the Huntsville body farm in 2009 and left them out for a year. During that time, she returned four times a day to collect the insects that she found on the cadavers. The usual suspects were present, but Lindgren also noted four unusual insect–cadaver interactions that had never been documented before – including a scorpionfly feeding on brain fluids through an autopsy wound in the scalp and a worm, previously only known to feed on decaying wood, feeding on the dried skin around where the toenails had been.
Insects colonise a cadaver in successive waves, and each has its own unique life cycle. They can therefore provide information that is useful for estimating time of death and for learning about the circumstances of death. This has led to the emerging field of forensic entomology.
“Flies will arrive at a cadaver almost immediately,” says Bucheli. “We’ll put a body out, and three seconds later there’ll be flies laying eggs in the nose.”
Insects can be useful for estimating time of death for a badly decomposing body. In theory, an entomologist arriving at a crime scene can use their knowledge of insects’ life cycles to estimate the time of death. In addition, because many insect species have a limited geographical distribution, the presence of a given species can link a body to a certain location or show that it’s been moved from one place to another.
In practice, though, using insects to estimate time of death is fraught with difficulties. Time-of-death estimates based on the age of blowfly maggots found on a body are based on the assumption that blowflies colonised the cadaver right after death, but this isn’t always the case: burial can exclude insects, for example, and extreme temperatures inhibit their growth or prevent it altogether. Even if insects appear just after death, they are cold-blooded creatures, so their rate of growth is relative to temperature rather than to the calendar. If you know the temperature and the growth cycle of a fly, you can estimate its age to within an hour or two, says Bucheli – but if not, time-of-death estimates based on insect colonisation can be wildly inaccurate and misleading.
An earlier study led by Lindgren revealed another unusual way in which blowflies might be prevented from laying eggs on a cadaver. “We made a post-mortem wound to the stomach [of a donated body] then partially buried the cadaver in a shallow grave,” says Bucheli, “but fire ants made little sponges out of dirt and used them to fill in the cut and stop up the fluid.” The ants monopolised the wound for more than a week, and then it rained. “This washed the dirt sponges out. The body began to bloat, then blew up, and at that point the flies could colonise it.”
Dr Martin Hall is a biomedical entomologist at the Natural History Museum in London, UK. He assists police forces at the crime during an investigation into a suspicious death. In forensic casework, his role is to establish a minimum post-mortem interval, which helps to indictate when the victim died, from insects found on and around the human remains at the crime scene.
He describes his work in this 7 minute video taken from Wellcome Collection's Forensics: the anatomy of crime exhibition.