Forensics

by Val McDermid

  In 1986 Ken Smith, entomologist at London’s Natural History Museum, wrote A Manual of Forensic Entomology, and dedicated it to Jean Pierre Megnin. His book was a game-changer. Smith pulled together all the available information on carrion-loving insects, particularly flies, and showed more accurately than ever before how to use them to age corpses. The manual was practical, something to be taken to the scene of an investigation. It described the waves of species as they appeared on corpses that were buried, exposed or submerged in water. Smith was an outstanding taxonomist, too, producing identification guides which are still used today. Reading the Manual in conjunction with the guides, it became possible to determine where flies had first found a corpse, even if it had subsequently been moved.

  Ken Smith’s successor at the Natural History Museum is Martin Hall. He’s a tall man who strides through the museum’s galleries giving a cheerful and enthusiastic running commentary. His passion for the 30 million insect specimens he curates is obvious and infectious.

  He juggles his day job at the museum with the role of forensic entomologist. At any time his mobile phone might ping with a police request to drop everything and rush to a crime scene. ‘Collecting insects off a dead body is not a pleasant experience,’ he says, ‘but it is amazing how your professional interest takes over.’

  Martin’s fascination with his subjects was sparked when he was a boy, growing up on the east African island of Zanzibar. There, he realised that the mosquito net that hung above his bed could keep bugs inside his world even better than bar them from it. As he dropped off to sleep every night, stick insects, praying mantises, even the occasional bat crawled, buzzed or flew their way through his semi-consciousness.

  He went to England to study before returning to Africa and spending seven years researching tsetse fly behaviour. One day he saw the massive corpse of an adult elephant on the savannah, its flesh creeping with countless maggots. A week later he came back to find nothing but a giant skeleton, stripped bare. Another week, and blowflies swarmed the site like low rain clouds. ‘It was just extraordinary to watch. Although there were other scavengers, like hyenas and vultures, the maggots were probably responsible for 40 to 50 per cent of the biomass loss.’ An elephant had become a million flies, and a budding entomologist was hooked for life.

  Now, that zest for his work is communicated to everyone Martin meets. When I visited him at the museum, he whisked me behind the scenes and up dozens of stone steps to the very top of one of its high gothic towers with a panoramic view of London. But I wasn’t there for the view. I was there to see the setting for some of the experiments Martin and his team conduct to extend the range of their knowledge. It’s a world where familiar objects have quite different meanings. Carry-on suitcases are home to pigs’ heads, in order to explore which flies will manage to lay their eggs through the gaps in the zips. Dog cages hold rotting piglets. Tupperware sandwich boxes are filled with preserved maggots. It’s all more than a little unsettling. Small wonder I later refused his offer of a sandwich …

  Among the collection of insects in the museum are some that have historic significance. Martin showed me one specimen bottle, then said in hushed tones, ‘These are iconic maggots. They’re from the Buck Ruxton case.’

  The Buck Ruxton case is notorious in British criminal history. It was a landmark case for forensic science in several respects, but forensic entomologists like Martin Hall know it as the first case in the UK in which insects were successfully used to help solve a crime. The case was a sensation, filling column after column of newsprint in the autumn of 1935.

  Buck Ruxton was a doctor of Parsee and French extraction, who had qualified in Bombay and settled in northern England. He lived with a Scottish woman called Isabella, who people knew as ‘Mrs Ruxton’, and their three young children. The doctor was the first non-white medical practitioner in Lancaster, and was very popular, especially with his poorer patients.

  One Sunday morning, Dr Ruxton opened his front door to a scrawny 9-year-old boy. His mother stood behind him expectantly, her arms shielding him from the autumn cold. ‘I’m sorry,’ said the doctor. ‘I can’t perform the operation today, my wife has gone away to Scotland. There is just myself and my little maid here, and we are busy taking the carpets up ready for the decorators in the morning. Look at my hands, how dirty they are.’ The pair turned and walked away disconsolately, the mother wondering to herself why the single hand the doctor had held out looked so clean.

  The Ruxton family had a 19-year-old maid, Mary Rogerson. A few days after the incident on the doctor’s doorstep, her family reported her missing. The police visited Dr Ruxton, who claimed his wife had gone off to Blackpool with her maid and that he suspected Isabella of having a lover. That fitted with the last known sighting of Isabella, who had been seen driving away from Blackpool at 11.30 p.m. after an evening with friends. Her love of having a good time provoked blazing rows between the Ruxtons. Dr Ruxton constantly accused his wife of infidelity and Mary often witnessed his jealous rage.

  After the police visited him a second time, Ruxton claimed that Isabella and Mary had gone to Edinburgh. But he couldn’t stop the tongues wagging in Lancaster. Although Ruxton was a respected member of his community, the gossip spread that his rows with his wife had grown more harsh and bitter over the summer and that something more sinister might be behind the disappearances.

  Then, on 29 September, a woman was walking across a bridge over a ravine near Moffat on the road from Carlisle to Edinburgh when, horrified, she realised she was looking at a human arm sticking up from the bank of the stream below. When the police arrived on the scene they found thirty bloody packages containing body parts wrapped in newspaper. Over the next few days, other body parts were found in the area by police and members of the public. Seventy parts were eventually recovered, from two different corpses. They had almost certainly been butchered to prevent identification – the fingertips had been cut off – and the job had been done by someone who knew about human anatomy.

  Some maggots were found feeding on the decomposing parts, and were sent off to the University of Edinburgh. There the entomologists identified them as a particular kind of blowfly. They narrowed down the time since the body parts had been dumped as between ten and twelve days. And so police linked the parts to the disappearance of Isabella and Mary.

  It was a telling start, but the evidence against Buck Ruxton stretched far beyond maggots. An anatomist and a forensic pathologist from Glasgow and Edinburgh universities painstakingly reconstructed the bodies of the victims. They superimposed photographs of the living Isabella on to photographs of one of the skulls, which matched. Some of the body parts had been wrapped in a special pullout section of the Sunday Graphic newspaper, distributed only in the Lancaster/Morecambe area, on 15 September. Some were wrapped in clothes belonging to the Ruxton children.

  This image, which superimposes a photograph of Isabella Ruxton’s face onto the skull found in the stream, helped seal Buck Ruxton’s conviction

  Ruxton had clearly been less composed than he had hoped. In his hurry to get away from the ravine and back to Lancaster, he had knocked a man off his bicycle in his car. The rider had scribbled down the number plate. It led straight to the car Buck Ruxton owned. The date of the reckless incident dovetailed perfectly with the evidence of the maggots and Sunday Graphic.

  The final piece of the jigsaw came from local knowledge. The stream in the ravine had last flooded on 19 September. The bodies must have been there by that date because some parts, such as the ghoulishly raised arm, had been deposited on the bank at a level where the overflowing stream had reached.

  Buck Ruxton was arrested and found guilty of murder. Nine months after committing his crimes, he was hanged at Strange-ways Prison in Manchester. We will never know the exact circumstances of what became known as the ‘Jigsaw Murders’. But, based on the autopsy evidence, it is most likely that Ruxton strangled his wife with his bare hands. The maid died from having her throat cut, probably to
silence her after she discovered his crime.

  The insect evidence was just one tile in a forensic mosaic that spelled out the murderer’s guilt. But the success of the combination of methods used in the Ruxton case led to increased public and professional trust in the capabilities of forensic science, including the discipline of forensic entomology. People could see that even if Buck Ruxton had wrapped the diced parts of his victims in white paper bags rather than sections of the local newspaper, even if his car hadn’t hit a bicycle, even if the stream hadn’t burst its banks, the maggots would have pointed to him. And so fresh minds were drawn to the discipline.

  Martin Hall has devoted much of his life to blowflies – the family of insects most commonly associated with corpses. There are over a thousand known species in the world. Martin considers blowflies ‘the gold standard indicators’ of the forensic world, for several reasons. Because of their acute sense of smell, which can pinpoint the tiniest drop of blood or the merest whiff of decomposition at over a hundred metres, they colonise dead bodies more quickly than other insect families. Because so much is known and documented about the stages of their growth, they usually provide the best information for gauging minimum time elapsed since death. And because there are so many regional varieties in the UK, they can be used to plot the whereabouts of a murder scene even when the body is found elsewhere.

  Unlike other insect families, which use their sense of smell only until they get close enough to food to switch to eyesight, the blowfly uses its sense of smell right up until it lands on what it has smelt. That makes it very difficult to dispose of a body in such a way that blowflies won’t find it. If, for example, a body is hidden underneath the floorboards, the odours of decomposition will gradually percolate through the airbricks, and the flies will crawl through them to find the body.

  Even if a body can be hermetically sealed off, clues to its location may still be obvious. American police in Indiana searching for a missing person some years ago noticed a cloud of frustrated flies hovering above a covered well. The missing person had been murdered, and the killer had thrown their remains down the well. He had sealed the well up enough to prevent insects getting in, but not the slight odour of decomposition getting out. The flies acted like a swaying gravestone, drawn to a smell beneath that was far beyond the capabilities of the human nose.

  Not long after Barack Obama took office in 2009, a blowfly buzzed around his head during a live interview on CNBC. The fly finally landed on the back of his left hand, and he promptly killed it with a slap from his right. ‘That was pretty impressive, wasn’t it?’ he said. ‘I got the sucker.’ In 2013 another fly landed on the president, this time right between his eyes. It made a good photograph. But when Martin Hall sees something like that, his mind is already racing ahead to what those flies would have done if the president hadn’t been in a position to swat them. ‘They would have gone on to explore his body. If they were females with a pile of eggs ready to be laid, they would look for a suitable site, usually the head orifices, in the nose and the eyes and the mouth. And they’d lay their eggs.’

  And then the banquet would begin. In 1767 Carl Linnaeus, the father of modern taxonomy, observed that ‘three flies consume the corpse of a horse as quickly as a lion does’. This startling observation came to Linnaeus because of the pioneering work of Francesco Redi. In 1668 the Italian had proved with a series of experiments that maggots came from fly eggs. Before Redi, the presence of maggots in corpses was presumed to be the result of spontaneous generation.

  Once a female blowfly has laid her eggs, a biological clock begins to tick. In the height of summer a typical UK blowfly egg will take fifteen days to become a fly. After one day the egg hatches into a maggot, which shreds and rakes in decaying flesh with the two hooks on its mouth. Because its eating and breathing organs are at opposite ends of its body, it can eat and breathe simultaneously twenty-four hours a day. Over the next four days it eats voraciously and grows to ten times its original size, from 2 mm to 2 cm.

  The plump maggot then wriggles away from the body, towards a dark place where it’s less likely to be eaten by a scavenging bird or fox. If its fleshy nursery happens to be outdoors, it will burrow 15 cm down into the soil. If indoors, the underside of a wardrobe or a place between the floorboards does the job. Secure in the darkness, the maggot becomes a pupa, its third and final outer layer of skin hardening into a case. Ten days later, an adult fly breaks out of the case and, if it’s outside, tunnels up to the surface of the soil. This push for freedom is no mean feat. The fly fills a sac on its head with blood, and pulsates its balloon-shaped battering ram inwards and outwards to dislodge the soil. Once it reaches the air, the fly shakes out its crumpled wings, and begins to mate almost immediately. At two days old a female lays her eggs, sometimes on the same corpse that reared her – but because maggots can devour 60 per cent of a human body in under a week, there probably won’t be much left.

  In the woodlands, bedrooms, alleyways and beaches that the police call him to, Martin Hall encounters the strange music of hordes of flies. He sees sights and smells of undeniable variety. ‘Sometimes you hear the description of “the sweet smell of decay”, and it can be sweet at times, but it can also be quite overpowering. I’ve worked on cases where the upper half of the torso is completely skeletonised because it was protruding from a sleeping bag and the lower half doesn’t look that long dead. As we approach the scene it’s not that bad but as soon as the bag’s opened, the waft hits you. The smell is not just of the body, but of the maggots feeding on the body. They produce a lot of ammonia and that can be overpowering, too.’

  Sometimes crime scene investigators collect insect specimens from corpses, and send them to entomologists to inspect. Martin Hall prefers to visit the scene himself. That way he can ensure that the specimens and information gathered are admissible in court, and he has the opportunity to search in places that others might miss or simply not consider. He looks for maggots throughout the corpse and for pupae under the soil. He wants to find the oldest specimens because they reveal when the flies first found the corpse, and so indicate the minimum time elapsed since death. Martin kills some of those maggots in boiling water and stores them in ethanol. He keeps others alive. Maggots grow faster the warmer it is, so Martin puts down a thermometer box which logs the temperature on the hour every hour for the next ten days. He also gets readings for the past couple of weeks from the weather station closest to the scene, so he knows roughly how hot the maggots were as they grew.

  Back at the lab Martin makes the crucial identification of the preserved maggots. ‘Even closely related species develop at different rates, so if you get it wrong you can be giving the police wrong information.’ He incubates the live maggots until they become flies as confirmation of his identification. He examines the anatomy of the preserved maggots carefully to assess their stage of growth. Combining his assessment with the temperature data, he plots a graph leading back in time to the point when the mother blowfly laid her eggs. This is usually a key piece of information, and an entomologist’s most valuable offering to the forensic puzzle.

  But what if the corpse has been there for longer than seven days, which is roughly how long it takes a maggot to become a pupa? Can entomologists look further back in time than a week? As entomologists start to push the boundaries of what insects can tell us, they are discovering how to read the biological clock embedded in pupae, too.

  It takes ten days for a pupa to transform into an adult fly. It’s this process of metamorphosis which is at the heart of what makes insects mysterious, and has been provoking wonder in poets and entomologists alike for centuries. It hasn’t been possible to witness pupae as they change over time, because their case is opaque. With the help of X-rays and miniature CT scanners, though, Martin and his team at the Natural History Museum are changing that. Having helped to plot reliably the growth rates of many species of blowfly maggot, he is now concentrating on the art of aging pupae: ‘At thirty hours I took an X-ray
image of a specimen and it was just the larval [maggoty] tissues. I took an image just three hours later, when I came back after a cup of tea, and that specimen had completely transformed. Instead of this undifferentiated larval tissue you could now see a clear head, thorax, abdomen and the developing legs and wings.’

  It’s tempting to think that, armed with such astonishing insights, forensic entomologists are beginning to deal in the currency of pinpoint certainty. But jurors and entomology students should not to be seduced. In 1994 a cartoon accompanied a BBC programme called The Witness was a Fly, in which a magnifying glass that could have belonged to Sherlock Holmes hovers above a maggot. The maggot holds up a placard saying ‘Murdered, 3 p.m. Friday’. The cartoon is striking but it’s also misleading. Maggots cannot tell us when a murder took place. They can indicate when flies laid eggs on a corpse, and that reveals the point by which the person was definitely dead. In the warm summer months it would be possible to narrow that window to, say, Friday, and possibly, as deductions become increasingly refined, to Friday afternoon. But to expect an entomologist to give a definitive time of death to the hour would be like asking a weather forecaster in November to promise a white Christmas. The range of variables thwarts that level of accuracy.

  One of those variables is based on maggots’ gregariousness. They like to feed in ‘maggot masses’ – a sort of seething mosh pit. They lay down an alkaline residue as they move about, which breaks tissue down into ammoniac goo. Their digestive activities are so intense that they heat carcasses up, sometimes to as high as 50°C. This suits the blowfly family because warm conditions accelerate their growth, but it can be a headache for entomologists trying to map their activity. However, it’s only in the latter stages of development that maggots generate significant heat. So the earlier the entomologist gets to the maggots, the lesser the effect of the maggot mass.