Why Forensic Entomologists Say Crime-Scene Investigators Should Carry Cans of Tuna
By Ed Ricciuti
The sight of blow fly maggots chomping away with hooked mouthparts at a decomposing corpse may churn most stomachs, but to the eye of a forensic entomologist it is a thing of beauty. From larval to adult stages, blow flies (family Calliphoridae) can reveal to CSI-types a host of clues into the circumstances of a death, including how, where, and when it occurred.
Even a maggot’s exoskeleton—shed twice as it passes through three larval stages—may offer clues, such as traces of drugs or poison ingested from the corpse. Most evidence, however, comes from live specimens, taken from the body at the scene by the evidence collection team and returned to the forensics laboratory for analysis.
It’s not difficult to find blow fly maggots on a dead body, especially around orifices, where females lay the majority of their eggs. Some species show up within minutes of death. The trick is to keep the voracious little critters alive after collection by feeding them a substitute for the body of the deceased, especially if some time elapses before a forensic entomologist can address them.
Typically, laboratory staff have to store and thaw out frozen beef, pork, or chicken liver or buy it fresh, but now a team of researchers suggest feeding blow fly maggots could be as easy as opening a can or packet of tuna. It is cheap enough, easily stored and transported, and fills the nutritional need of the flies. The researchers’ findings are reported in a new paper, “Analysis of Alternative Food Sources for Rearing Entomological Evidence,” published last week in the Journal of Medical Entomology.
“Based on our findings, we would recommend that crime scene investigators add a few cans or packs of tuna to their evidence collection kit, eliminating the need to thaw frozen (or buying fresh) beef liver any time crime scene investigations have an insect infestation,” write the scientists, led by Lauren Weidner, Ph.D., of Arizona State University. “Once the blow flies arrive at the crime lab, the entomologist can transfer them to their preferred food source to rear them.” Conceivably, if a few cans of food are packed along with DNA collection kits, crime scene tape, and the rest of the investigators’ gear, maggots can start chowing down on the way to the lab.
“Our aim was to find a food source that could be easily implemented in crime laboratories to increase the frequency of entomological evidence collection, as well as to improve the chances of insects surviving for long enough to reach a forensic entomologist,” the researchers write. Canned tuna, it turned out, is almost as healthy a diet as the standby beef liver while being as cost effective and easy to keep and use. “I have had law enforcement use the packets and the cans [of tuna] and both situations got the larvae to me alive,” says Weidner.
“This research is significant to forensic investigations,” she adds, ” because the aim is to provide an easy and cost-efficient method for an important type of evidence that is not routinely collected. Having a quick and easy method of collection will hopefully increase the frequency that entomological evidence is collected. This is important because, for us to do our job well, it starts with them, the people on scene collecting the evidence.
The reseachers compared how two blow fly species, the black blow fly (Phormia regina) and the secondary screwworm (Cochliomyia macellaria), fared on diets of canned tuna, a private-label wet cat food from a supermarket chain, and beef liver. Although beef liver seemed the best choice, all three foods worked satisfactorily on C. macellaria. The rub was that almost 90 percent of the P. regina fed cat food died. This species is found virtually throughout the United States and is critically important for forensic analysis. The researchers recommend against feeding blow flies the brand of ground-chicken cat food used in the experiment but noted that other types, perhaps made of organ meats, might work.
Attracted by the gasses arising from decomposition, metallic-colored adult blow flies swarm to unattended corpses and almost immediately lay their eggs. The warmer the weather, the faster the eggs hatch, sometimes within a day. Scientists have meticulously charted the life cycle of blow flies, including how ambient temperature, cloud cover, the chemical composition of nearby soil, and a host of other factors impact how fast the larvae grow. Typically, evidence collectors preserve half the maggots as a record of their exact age when discovered while the other half that are kept alive, hopefully to adulthood.
A mind-boggling number of clues can be deduced from studying blow flies sampled from a corpse. An abnormal concentration of maggots, for instance, can indicate the location of wounds. More maggots than typical on the arms and hands suggest defensive wounds, caused by fighting off an attack with a knife or other weapon. If maggots from a species that prefer shade are found on a corpse in the sun, chances are the body was moved after death.
Despite their importance, the care and feeding of blow flies in the laboratory leaves much to be desired, according to the researchers. “Very few crime laboratories across the country have collection and rearing protocols for these forensically important insects,” their report states. “A lack of knowledge in collection techniques and limited access to an appropriate food source are the main reasons for absence in adequate collection and rearing protocols. Thus, when crime scene investigators or pathologists collect insects, they are often mishandled (e.g., placed into containers with no air holes, no food, or a food source that is not sustainable for their development.)”
Despite the good results with tuna, say the researchers, there could be a rub. It might not work for other species of blowflies, according to the paper. More research, therefore, is needed, especially on other food sources that could be cost effective and easy to store.
Journal of Medical Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.