Murder, maggots and forensic entomology

Maggots - the larval stage of a fly - might not be everyone's favourite animal, especially when it comes to the kind that are found munching their way through dead bodies. But whether you like them or not, there's no question that these animals play a vital role in our world.

Why are flies attracted to dead bodies?

Necrophages are organisms that feed on the bodies of animals that are already dead for the nutrients they contain.

Necrophagy might sound a bit grisly, but it's an important natural process for breaking down carcasses and corpses. As Dr Erica McAlister, our Senior Curator of Diptera, puts it in her book The Secret Life of Flies, 'if it weren't for these little organic munchers we would be knee-high in dead bodies'.

In fact there's a whole range of insects that exhibit necrophagous behaviour, including some flies and wasps. We even use colonies of flesh-eating beetles to help prepare some of our specimens at the Museum.

When animals, including humans, die, the body starts to decompose from the inside out. The soft tissues begin to break down first through a process called autolysis, and this produces liquids and gases.

The smells emitted at the various stages of decay attract different species of necrophagous insects, which arrive at the body to feed on it or to lay their eggs. The first to appear are usually flies.

The maggots that solved a murder

Necrophagous insects not only play an important natural role in the breakdown of dead bodies, but they can also help us in other ways too.

Take blow fly maggots as an example, doctors have put their taste for dead tissue to good use by using them to munch on dead - necrotic - tissue around wounds. According to Erica, these days, instead of a handful of loose maggots wriggling about on a patient, doctors use larvae contained in mesh bags - that way they can get their tiny 'jaws' through but can't wander off.

Then we have the Ruxton maggots. 'These weren't doctors, they were the detectives,' explained Erica during one of our recent livestreams, and 'were the first maggots used in a forensic entomology case in the UK'.

Dr Buck Ruxton was a GP in Lancaster who in September 1935 murdered his wife Isabella and their housemaid Mary Rogerson. To dispose of their bodies, Buck drove some 100 miles from his home, depositing them in a small ravine in a sparsely populated area in the Scottish Borders.

When the bodies were discovered, several clues led the police to Ruxton, and one of the key pieces of forensic evidence were the insects that were found living on the corpses.

'The detective that came along was very on the ball and he decided that when they took back the bodies, they'd also take back a lot of the maggots around it,' says Erica.

These were identified as bluebottle fly larvae, Calliphora vicina, and by using what were in the 1930s new techniques for the UK, the investigators worked out that the larvae were between 12 and 14 days old. This provided a timeframe in which the bodies must have been disposed of and helped lead police to Ruxton.

'The maggots were used in the court case to help prosecute this man successfully for this diabolical crime. So, these maggots solved a crime - they've done good for society,' exclaims Erica.

In fact, the maggots from the Ruxton murders are now part of our fly collection.

What is forensic entomology?

Forensic entomology covers a variety of practices, from the investigation of pests found in food products to insect infestations in our homes. However, the most famous use of forensic entomology is in criminal investigations, particularly in cases of unexplained deaths.

This field has been around for hundreds of years. In fact, incredibly the first-recorded use of forensic entomology to solve a murder was in China in 1235.

Today, insects are commonly used to work out a minimum post-mortem interval (PMI), more commonly known as the time of death. For forensic entomologists, some of the most important animals for this are flies. The first to arrive at a body are usually blow flies - species in the family Calliphoridae.

Blow flies have a keen sense of smell and this means that some species can detect a dead body from up to 16 kilometres away. They are one of the most important groups for estimating the PMI because of how fast they can arrive at a body. In some cases, they've been known to turn up within just 10 minutes!

Forensic entomology expert Dr Martin Hall has described species in the family Calliphoridae, some of which are commonly seen in the UK, as 'the gold standard' for determining a time of death.

Speaking during one of our live streams, he said, 'These flies are very curious. If you're out in the garden and they're flying about, you can brush them away while you're still alive. But when you die, you can't do that and they will investigate your body. If they're ready to lay their eggs, they will find an ideal spot on you and lay them. That's the beginning of the life cycle.'

Blow flies can lay thousands of eggs on a body, in batches of 50-100 eggs. The maggots hatch and begin to feed, grow and moult through stages called instars. At the end of the third instar, they pupate. This is when the insect forms a hardened case around itself, inside which it metamorphoses into an adult - it's the equivalent to the chrysalis stage of a butterfly's development.

To estimate time since death, forensic entomologists need certain data, including, the species of fly, what stage of their lifecycle they're at and the temperature at which they've been developing, as the warmer it is, the faster they tend to develop.  

Studying human decomposition

Determining a time of death using insects isn't a simple task. Numerous factors can influence the rate at which the flies progress through their lifecycle.

For example, if a body is suspended or enclosed in some way this can prevent maggots being able to attach themselves or even delay the adult flies laying their eggs in the first place. Bodies found indoors might decompose at a different rate to those found outdoors, and the presence of narcotics in a body at the time of death can also change the way maggots develop. 

One of the ways scientists work to understand the impact of these and other factors on human decomposition is through 'body farms' - more formally known as human taphonomy facilities. The first was set up in 1981 in Tennessee, USA.

These are where donated bodies are placed in various settings and what happens to them is studied in detail. They can teach investigators to tell the difference between a natural death and one that had unnatural causes.

Different climates impact how decay progresses too. For instance, results seen at a body farm in Michigan during winter would be entirely different to those seen in a body decomposing at a facility in Australia in summer.

There are no body farms in the UK yet. Scientists here commonly study the decomposition of pigs.

However, not everyone agrees with body farms. These facilities often face obstacles such as regulation and being perceived by some as ghoulish places, with questions over their scientific value.

Forensic entomology at the Museum

As well as having worked alongside police on investigations to determine PMIs, our scientists have carried out and contributed to studies that have helped to advance the field of forensic entomology.

For example, it takes blow flies such as Calliphora vicina about 19 days to develop from an egg to an adult fly, and half of that time can be spent at the puparium stage. Puparia are opaque, so when it comes to those found at crime scenes, it's tricky for investigators to determine exactly how old the insect inside is.

Speaking in 2017, Martin explained, 'You can of course dissect them, but even then the understanding of their age is a bit limited and of course you destroy it by dissecting it - and it's forensic evidence, so it's good to be as non-invasive as possible'.

Instead, our scientists have used x-ray and micro-CT scanning to look inside puparia at regular intervals throughout their development, and so were able to see that for a given temperature, characteristic changes will always occur at an expected time. 

By gaining a deeper understanding of flies and the changes they go through inside the puparium, scientists can use the pupa found at crime scenes to help estimate time of death as accurately as possible to help investigators solve crimes.

You can find out more about fascinating flies in Erica's books The Secret Life of Flies, The Inside Out of Flies and A Bug's World.