Forensic Fly Moves North

An Oriental latrine fly feeds on the body of a dead alligator.

Who, aside from global warming activists, cares about a tiny blowfly making its way north with the changing climate? Forensic scientists do. Such insects, when discovered on corpses, can offer vital clues about when a victim died. But when a new species starts showing up on bodies outside its known range, it can throw off the time-of-death clock.

Christine Picard, an assistant professor of biology and forensic biology researcher at Indiana University-Purdue University Indianapolis, was collecting fly samples last fall when she discovered an individual Chrysomya megacephala, a blowfly commonly known as the Oriental latrine fly. The species is native to the southern United States and hadn't previously been reported north of New Mexico—certainly never in Indiana. Picard says the newcomer could become helpful to forensic teams in her home state, but for now it is likely to confuse matters instead.

Time-Telling Flies

Various flies and certain beetles are typical visitors to dead bodies, but the blowfly in its larval stages is particularly useful in death investigations. The fly is extremely sensitive to odors associated with decomposition. Some biologists estimate that within 15 minutes of a person's death, the insect can detect the corpse—which serves as a potential incubator, hiding place, and feeding station all in one. For insects that eat flesh, or whose larvae do, what better place to lay your eggs than in a rotting human body?

With that in mind, what comes next is a pretty gruesome process. Once the fly finds a body, it will settle down and lay eggs in an opening—often in the face or genitals, or inside a wound. The time of hatching depends on the species and the temperature, and the first larval stage is that wriggling white maggot that makes us cringe.

Soon the maggot molts into a second larval form, and then a third. Entomologists know the timing of each life cycle stage and, combining that with ambient temperature data (warmer temperatures can speed up these cycles), can work backward and assess the age of the insect. This provides a pretty good idea of the person's time of death, or at least the "postmortem interval," the minimum amount of time he or she has been dead.

Maggots Pointed to Murder

The first time insect data were used in a murder trial, in 1935, blowfly larvae were front and center. Human remains, complete with maggots, had been discovered in a ravine in Dumfriesshire, Scotland. The bodies turned out to belong to the wife and the maid of a Dr. Buck Ruxton, who was later found guilty of both women's murders and hanged for his crimes. The age of blowfly maggots on the remains had been a key clue to the timing of the murders, information that helped convict the killer.

Today's forensic investigations also focus on the progression of species that land on a body. "There's a succession, an order by which various insects arrive," Picard explains. "Certain flies come early, but as the remains dry out they are replaced by other flies and beetles. All of these can give us a rough postmortem interval."

But even though the order of arrival is clear, exact timing can be hard to pin down, making for an imprecise science. "It can be hard to estimate exactly when a fly came in to lay her eggs," says Picard. "Say a body is concealed, or if it's in a house with all the windows closed, it might take longer for the process to begin."

Plus, samples may be overlooked or ignored completely. It's rare for an insect expert to have access to a dead body during initial processing, and other investigators at the scene may refuse to pluck clue-rich maggots from decomposing flesh.

In Texas just this year, the Harris County Institute of Forensic Sciences became the first and only U.S. coroner's office to employ a full-time forensic entomologist. Michelle Sanford has that job, and she says that before she was hired, "we simply did not have regular access to death investigations or the opportunity to work so closely with crime scene investigators, death investigators, and forensic pathologists."

Often, then, searching for bugs has to wait until the autopsy, far from ideal timing. "Movement and placement of the body in a body bag and morgue cooler disturbs the insects and causes them to migrate deep into the body cavities and into the body bag," Sanford says.

And then, insect clues may be low on the examiners' list of priorities. "One medical examiner told me the first thing she does when a body arrives at the morgue is wash all the maggots off," says Picard. "It's unfortunate." She says with better education, attitudes are changing about the importance of such clues, "but slowly. We are still fighting for them."

In the cases when insects are collected, they can tell investigators more than just time of death, Sanford says. "They can be used to include or exclude suspects, to assist in developing timelines of events, and to aid in the identification of wounds on decomposing bodies." They also have the potential to provide DNA data (a fly's mouthparts, packed with their fleshy meal, might aid in identifying a victim) and toxicological evidence, she says.

Fly in the Ointment

So what happens when a new fly species, like the blowfly invading Indiana, shows up on the scene? "First, even if that species is useful elsewhere in forensics, we don't know that it's relevant here," says Picard. "Theoretically, the population could be different genetically and developmentally," which means scientists can't apply the same assumptions to it as they do to native populations.

Second, and just as important, "sometimes new insects affect the native species directly, throwing off their development by delaying or speeding it up. We need to know how they interact," she says, to determine if previous data about native insects' life histories become invalid. In the case of the blowfly, it could be years before its effects on native species become clear.

In the future, entomologists expect there will be more exotic insects arriving in more northern locations. "That's why we keep collecting specimens," Picard says, "because we know the species composition is changing over time." And, notes Sanford, "we are only just beginning to see how these interactions [between species] change development."

And that means when a dead body is found, the expert with bug know-how should be given an early, thorough look at what is living within.

Jennifer S. Holland
for National Geographic
Published July 10, 2013