Flies that herald death: Scientists refine the use of insects to solve complex crimes

You’re dying. You don’t know it yet… but the Calliphoridae do. That iridescent green fly that just entered your room belongs to this family of insects. And it smelled you from miles away. It instantly detected the cellular inactivity of your agony (which is imperceptible to humans). The tiny hairs that cover the fly’s entire body are acute chemosensory receptors, capable of detecting the first signs of death even before it occurs. This bug is the true grim reaper.

The fly places its gloomy beauty at the entrance to one of your nostrils, where it’s warm and humid — an ideal place for its offspring to thrive. Anyone would feel a tickle and easily shake it off… but remember, you’re dying and you can’t move. So, the fly lays hundreds of eggs and flies away peacefully, leaving its offspring inside a good place: you. In a few days, they hatch and the larvae thrive, devouring your flesh. When they’re satisfied, they’ll crawl away to become pupae. For weeks, they’ll remain fragile maggots, encased in a brown capsule, until they complete their metamorphosis into parodies of butterflies. Then, they’ll leave in search of a new corpse to repeat the cycle.

The process sounds grim. However, the usefulness of this life cycle inspires respect and compassion. Flies can cooperate with the justice system when a person is found deceased under unknown circumstances. In the event of this, a couple of scientists are enough to make the flies talk.

Moira Battán and Ana Pereira, from the National University of Córdoba and the National University of Comahue respectively, are Argentine research entomologists at the National Scientific and Technical Research Council (CONICET). They’re also forensic experts. And, through their work, they help determine the time of death by analyzing cadaveric fauna. They’re experts on necrophagous flies… the kind that are so annoying during barbecues.

The first thing these insects do, Pereira explains, is detect the place where they believe their offspring will be most protected. “If you see them, they’re sort of palpating, looking, touching. When they sense that something’s [habitable], they start laying their eggs. One after the other.” Flies from the Calliphoridae family are capable of detecting a body from the moment it dies, or even a few minutes before: as soon as cell reproduction stops. They arrive almost immediately and colonize the dead tissues, thus establishing the post-mortem interval (PMI). Depending on the stage of maturation of the offspring, scientists can estimate — with relative precision — how long a body has been dead.

The adult insect lays eggs and leaves. The hatching of its offspring occurs in decay and solitude. As soon as they emerge, the larvae must eat or die. So, they voraciously undertake the first option. “These necrophagous flies have three larval stages that differ in size. Obviously, they eat more and more until they reach a point where they complete their diet and, generally, migrate. They move away from the corpse to [undergo their transformation into pupae],” Battán explains. During this migration, the flies seek a safe haven to face the next stage of life, in which they’ll be more vulnerable and unable to move.

Once they’re pupae, they’ll enter a dormant state — like a butterfly’s chrysalis — for one or two weeks, or even until the next season. They emerge when the perfect moment arrives for their final metamorphosis. “If the body is outdoors, on the ground, [the fly] will try to move away and bury itself. If it’s in an apartment, [the fly] will seek protection under a bed, a rug, a sofa, or in the junction between the floor and the wall. There’s one species in particular — frequently seen in my province — that tends to hide under the corpse, or within its clothing,” Pereira explains.

If the forensic police were to immediately summon these experts to the place where the body was found, the puparium stage could also be part of the analysis… but the entomologists’ intervention is usually requested at the morgue. “Very rarely do I have material from the discovery site. Generally, I have it from the time of the autopsy. So, if there was a first generation of larvae that grew on the corpse and migrated to pupate, I [don’t get to see] it. I only see what was on top. Unless they belong to that particular species (the Calliphoridae family),” Battán laments.

Flies that eat their live hosts

Some species don’t wait. A small fragment of dead tissue is enough for some flies to colonize a wound. This was the case with the body of a tourist who drowned in a riverbed — about a foot deep — in the Argentine province of Córdoba in 2020. For unknown reasons, the still-living woman was unable to get rid of the flies that infested her wounds before she died.

In animals, the presence of larvae of this type of fly often indicates a state of abandonment or neglect. Cochliomyia hominivorax — known as the screwworm fly — is one of the culprits. At the edge of a wound, it can lay up to 500 eggs that hatch within 24 hours. The creatures emerge hungry, ready to eat nonstop for a week: they dig tunnels in bite-sized chunks that aggravate injuries and attract other voracious hosts. This is a very painful process, hence it’s assumed that the presence of the screwworm fly in humans could be an indication of a state of defenselessness.

“There are species that are biontophagous. That is, they feed on flesh, tissue, or wounds, [but they feed] on living organisms. In dogs, this [infestation] commonly [manifests itself] as dipteran diseases. Such an infection is called myiasis,” the researchers explain. Taking the species of flies that cause this into consideration during forensic studies helps avoid misinterpretations when placing a date on death.

In the case of the drowning victim in Córdoba, forensic entomology solved two problems. Firstly, determining when the woman was alive based on the period of insect activity. And secondly, determining the time of death based on the biology of the necrophagous insects (reproduction and survival after immersion). Battán and three other researchers came to these conclusions in the scientific article that analyzed the case of the deceased tourist. It was published in the Journal of Forensic and Legal Medicine.

Insects that reveal the time of year

Insects can provide information not only about the time interval, but also about the season in which a person died. Pereira, an entomologist, demonstrated this in a case in which the body of a man was found in the trunk of a buried vehicle in the Argentine province of Río Negro. The advanced state of decomposition of the body — as well as the conditions in which it was found — made it difficult to estimate the date of death by solely using the usual medicolegal criteria, such as body temperature, color and rigor mortis, among others. Thus, police investigators had to turn to science.

Knowledge of the cadaveric fauna and the surrounding ecosystem helped elucidate much more than the time of death. “[The] Calliphora vicina [fly] was one of the first to arrive, but since there were few pupae and they were dead inside the puparium, what we were able to reconstruct was that — as the killer dug the hole — the corpse was exposed and the flies laid their eggs. He then buried [the body] and the pupae died from lack of oxygen. But some time later, the soil must have weighed down an area of ​​the windshield, where a crack opened up, allowing another species of fly to enter and complete its life cycle. Later, the beetles entered.”

“This allowed us to verify the time that passed between the burial and the entry of other insects,” Pereira recalls. Thus, to place the time of death, the researchers studied beetle larvae. And, to approximate the time of year, they analyzed blow fly larvae.

The particular way in which the seasons develop in the extreme south of the Americas was also taken into account. The scientific article based on the case clarifies that, although Calliphora vicina is a winter species, the beginning of spring in Argentine Patagonia is still cool, with minimum temperatures close to 32 °F (zero degrees Celcius). Hence, the death must have occurred during that period.

The careful analysis of the life cycle of each species that makes up the cadaveric fauna, the evaluation of the climate, as well as the entire ecosystem in which a lifeless body is found, are all factors that provide specific and valuable information. For this reason, the researchers emphasize the need to facilitate interdisciplinary work and coordination between the judicial and scientific systems.

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