When a human body calls it quits, it can take as few as 30 seconds for blowflies to begin feasting on it. For the next several hours to days, a carnival of blowflies, other flies and beetles make the departed their personal bed-and-breakfasts. A determination of that succession of insects is one of the tools that crime-scene investigators (CSIs) use to estimate the postmortem interval (PMI), or the time elapsed since death.
Since the dawn of modern forensic science in the 19th century, the general belief has been that the order in which carrion-attending arthropods descend starts with blowflies, proceeds to maggots (from the fly eggs) and then moves on to beetles and other predators. Now a 27-year-old graduate student has determined that this is not always the case. Her research could represent a fly in the ointment (we could not resist) for the still burgeoning field of forensic entomology, in which scientists use bug evidence to help them solve crimes.
Entomologist Amanda Fujikawa of the University of Nebraska–Lincoln made her discovery while analyzing how the decomposition of mammal carcasses affects nearby ecosystems in Valentine National Wildlife Refuge in the Sandhills region in Nebraska, a unique prairie environment with grass-anchored sand dunes. She chose the area for its limited access. With only a few roads leading into the area, she was confident her experiments would not be interrupted. In late spring and midsummer, she placed dead rabbits and roadkill in various areas around the refuge and then set up traps to collect carrion insects as they were attracted to the carcasses. She soon noticed that beetles led the parade, not flies.
"It's too early to say if it's a game changer, as it could just be a geographic anomaly,” Fujikawa says, but "the fact we saw it and don't know why is most important." Fujikawa’s optimism that the research is noteworthy is shared by biologist Jeffrey D. Wells of Florida International University, who was not involved in the research. "It's quite a surprise from the standpoint of carrion ecology," he says. "This is the first example I've seen of carrion beetles behaving according to a different physical scale compared to the typical pattern. Carrion beetles, burying beetles in particular, often win the race to find and monopolize a very small mammal carcass such as a mouse. I've never seen this happen with a rabbit or larger dead mammal. For the forensic entomologist, this raises the possibility that beetles can delay or even eliminate the typical pattern of fly maggots on a corpse. One danger is that if this goes unrecognized, the entomologist might significantly underestimate the time since death."
Fujikawa's advisor Leon Higley, an insect ecologist, echoes this sentiment. "Amanda's work shows there's a lot more variability in insect succession issues" than previously thought, he says. "She is shaping our perceptions of how events actually occur with regard to decomposition."
The evidence also suggests that some postmortem interval estimates may be wrong. There is still much to learn and confirm in the discipline, Fujikawa says, adding: "We still don’t have a basic understanding of the biology of maggot development on a carcass: how they respire, how their role in decomposition affects the ecosystem and what they are doing when they are not feeding."
Fujikawa is starting to fill in some of the field's blanks. Her master's thesis demonstrated that adult blowflies can alter the morphology of blood stains at crime scenes. When flies feed on blood, they change the dispersal pattern of the original pool. In addition, they may regurgitate the blood and defecate in a different location. If crime-scene investigators do not know what those little critters are up to, they might interpret a stain as part of the blood spatter when, in fact, it is insect regurgitation and defecation. This could lead them to make false assumptions relating to the scene and crime itself, such as angles and trajectories associated with the death.