Forensic investigators typically use the well-known stages of blowfly development on cadavers as a biological clock to determine the time since death, or the postmortem interval. That clock can be misleading, however, if the temperatures around a body are outside the moderate range, causing the time since death to seem longer or shorter than it actually is.
In research supported by the National Institute of Justice, researchers from Texas A&M University noted that although blowfly development in cadavers “seems accurate and precise within moderate temperature ranges” it remains “largely untested against more extreme temperatures observed on decomposing carcasses.” Principle investigator Aaron Tarone, an entomologist, said in a report that his team’s research provides “information as to how suboptimal environmental conditions, such as heat waves or cold snaps, affect adult colonization and larval development.” The research, he wrote, “attempts to better understand basic aspects of blowfly thermal biology in order to improve our use of blowflies as evidence in death investigations.”
A core assumption for estimating the postmortem interval, Tarone noted, is that blowflies immediately colonize a body after death. That isn’t always the case, he said, citing the case of a body found in July on a hot concrete sidewalk in Las Vegas. Investigators assumed the body hadn’t been there long enough for blowflies to locate and colonize it. An alternative explanation, Tarone said, is that the “temperature of the concrete surrounding the body, or the temperature of the body itself, was above the thermal tolerance of blowflies.” It was simply too warm for the flies to be active on or around the body, he said.
The research used two blowfly species and measured several aspects of their behavior at different temperatures to determine “when and where blowflies can be active based on environmental temperature,” Tarone noted. Such measurements “could provide evidence that a body has been transported after death and could provide an alternative explanation as to why some dead bodies are found uncolonized.”
After testing flies during their four larval stages and as adults, the researchers established a range of high and low temperatures for each group in which they experienced “knockdown,” defined as the inability to walk, or locomote. Their key finding, for both immature and adult blow flies, was that exposure to temperatures outside of the moderate range affected the flies’ ability to function and the longer the exposure, the greater the effect. The findings varied depending on the sex and life stage of the fly.
Tarone concluded that the research data can aid forensic investigations in several ways. “If we know the thermal limits of blowflies, we can potentially explain why some cases lack blowfly colonization,” he wrote. “In cases where a body is colonized by larvae in suboptimal temperature conditions, the presence of blowflies provides evidence that the body was moved after death.”
And given the effects of warming and cooling on blowflies, he said, investigators should be aware that if a body is colonized in the morning and temperatures increase during the day, it is possible to overestimate the time of colonization as the warming could accelerate blowfly development. Conversely, he noted, investigators could underestimate the time of colonization if the colonization event occurred in the evening when temperatures are typically cooler.
Republished courtesy of NIJ. Photo: Close-up of the head of Calliphora vomitoria. Credit: JJ Harrison.