The flies that announce death: Science refines the use of insects to solve more complex crimes

You are dying. You don't know it yet, but the Calliphoridae do. That iridescent green fly that just entered your room is one of them, and it smelled you from miles away. It instantly detected the cellular inactivity of your death throes, imperceptible to humans. The tiny hairs covering your entire body are acute chemosensory receptors capable of detecting the first signs of death even before it occurs. It is the true grim reaper.

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

The process sounds grim, but the usefulness of their life cycle inspires respect and compassion. Flies can help justice when a person is found dead under unknown circumstances. In that case, a couple of scientists will be enough to make the flies talk . Like Argentines Moira Battán, from the National University of Córdoba, and Ana Pereira, from the National University of Comahue, who are research entomologists at the National Scientific and Technical Research Council (CONICET) and also forensic experts. With their work, they contribute to determining the time of death based on the analysis of cadaveric fauna. They are experts on necrophagous flies, the kind that cause such a nuisance during an outdoor barbecue.

The first thing these insects do , explains Pereira, is detect the place where they believe their offspring will be most protected. "If you see them, they're like feeling, seeing, touching. When they sense that everything's okay, they start laying their eggs . One after the other." This family of calliphorids is capable of detecting a body 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 postmortem interval (PMI). Based on the maturation stage 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 young occurs in decay and solitude. As soon as they emerge, the larvae must eat or die. So they undertake the former with voracity. “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 produce the puparium structure,” Battán explains. During this migration, they seek a safe refuge to face the next stage, in which they will be more vulnerable and unable to move.

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

If the forensic police were to summon them immediately from 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 material 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 miss it. I only see what was on top. Unless they're of that particular species," Battán laments.

Those that eat their host alive

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, a few centimeters deep, in the Argentine province of Córdoba in 2020. For unknown reasons, the still-living woman was unable to rid herself of the flies that infested her wounds before she died. The presence of larvae of this type of fly often indicates a state of abandonment or neglect in animals. Cochliomyia hominivorax is one of them. At the edge of a wound, it can lay up to 500 eggs that hatch within 24 hours, hungry and ready to eat nonstop for a week, digging tunnels in bite-sized chunks that aggravate the injuries and attract other voracious hosts. Its action is very painful, so it is assumed that its presence in humans could be an indication of a state of helplessness.

"There are species that are biontophagous. That is, they feed on flesh, tissue, or wounds that are necrophagous, but on living organisms . This is what is commonly seen in dogs as dipteran diseases. This infection is called myiasis," explains the researcher. Considering these species in forensic studies helps avoid misinterpretations when dating death.

In this case, forensic entomology solved two problems: first, determining when the woman was alive based on the period of insect activity, and second, determining the time of death based on the biology of the necrophagous insects (succession and survival after immersion),” concludes Battán, along with three other researchers, in the scientific article analyzing the case of the tourist who drowned in Córdoba. It was published in the Journal of Forensic and Legal Medicine , published by Elsevier.

Those that reveal the time of year

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

Knowledge of the cadaveric fauna and the surrounding ecosystem allowed us to elucidate much more than the time of death. “C alliphora vicina 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 while the killer dug the hole, the corpse was exposed and the flies laid their eggs. Then he buried it, 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, allowing another species of fly to enter and complete its life cycle. Later, the beetles entered. This allowed us to determine the time that passed between the burial and the entry of other insects ,” explains Pereira. Thus, to date the death, they studied the beetle larvae, and to approximate the time of year, they analyzed the blowfly larvae.

The unique way in which the seasons develop in the southernmost tip of the Americas was also taken into account. The scientific article based on the case clarifies that although Calliphora vicina is a wintering species, early spring in Argentine Patagonia is still cool, with minimum temperatures close to 0°C, so the death must have occurred during that period.

A careful analysis of the life cycle of each species comprising the cadaveric fauna, an assessment of the climate and the entire ecosystem in which a body is found, provide specific and valuable information. For this reason, the researchers emphasize the need to facilitate interdisciplinary work and coordination between the justice system and the scientific system. So that, should you end up as a hidden or forgotten corpse, science and the flies will preserve your memory.