Regular watchers of CSI and other forensic shows might be surprised to learn that forensic entomology is actually a branch of applied ecology. Decomposition is a critical ecosystem service that humans get for free and often take for granted.

Necrophagous animals ("death-eaters;" with apologies to Harry Potter) are critical to the Earth’s healthy functioning. Because of necrophages' hard work, we aren't clambering over dead dinosaurs and spelunking past deceased relatives in a world covered with layer after layer of corpses. A carcass is an empty bit of habitat waiting to be colonized.

Adult blowflies have an astonishing sense of smell for putrescine and cadaverine, molecules¹ that signal a delicious bucket-kicking has occurred. A dead body is conveniently pre-packaged baby food for fly eggs. Flies arrive in large numbers within minutes after someone begins their dirt nap. It's rather like when a pizza delivery is made to a dorm; undergraduates magically appear out of thin air, summoned by the yummy aroma.

Blowflies’ ability to be first on the scene is why forensic scientists use maggot age to infer how long ago someone died (Post-Mortem Interval, or PMI). It’s rare for blowflies and carrion feeders to lay their eggs in living tissue, so the age of maggots on a body is used as a starting point for estimating when the victim ceased to be.

Greenberg's University of Illinois-Chicago Maggot Raceway; 1990.

There's just one problem with using maggots as evidence: insects can crawl. In fact, during certain stages of their life cycle, maggots are driven to migrate away from the corpse that has been their happy (if goopy and smelly) nursery. It’s normal for the first wave of maggots to eat and then remove about 90% of the body weight of a corpse in what is called a “maggot exodus.” Typically, the maggots burrow down into the soil, encase themselves in a special kind of fly cocoon called a puparium, and begin to turn into a fly.

To estimate a post-mortem interval, you have to find where maggots have gotten to during their pre-pupation walkabout. “Where did the maggots go?” is a question that has occupied forensic entomologists for several decades.

In the late 1980s Dr. Bernie Greenberg, a forensic entomologist from Chicago, built what was essentially a maggot racetrack in his university building. (Let it never be said that entomologists are boring officemates!) He found that blowfly maggots could travel 8.1 meters (almost 9 yards); they might have gone farther but Greenberg ran out of hallway.

More recently, Dr. Eric Benbow at the University of Dayton has been trying to quantify how far a maggot will travel in more natural settings. Using insects to estimate PMI is inferential in nature–it’s rare that a crime occurs in exactly the way in which a lab study is conducted. This is why much of forensic entomology is applied ecology. You put dead things outside and try to quantify what happens.

Pigs are usually used as stand-ins for dead humans in forensic field studies, since they are considerably easier to obtain than humans that have shed this mortal coil. Benbow put multiple ex-pigs outside in a park in spring, summer, winter, and fall. Part of the purpose of this work was to expand our knowledge of how bodies decompose under different conditions. A dedicated crew of research assistants collected extremely putrid samples daily and weekly until the pigs were skeletonized.

And so it came to pass that there were a bunch of dead pigs in the woods in Ohio, and there were some EPIC maggot migrations. These maggots actually traveled up to 27 yards (25 meters) or more from a body in a forest. Check out this video—it’s like a tidal wave of maggots on the move:

Why would maggots travel in a group? They might be playing a numbers game. Maggots that stick together actually reduce the probability of being eaten by a predator. Think of a maggot as a tasty tortilla chip. One chip in a bowl alone is sure to be captured and eaten. But if 300 tortilla chips are in the bowl, the odds of any individual chip being eaten are greatly reduced.

The other thing that this maggot mass might accomplish is making a lot of small organisms appear as one larger organism. A giant writhing, stinking mass might disorient potential predators. It certainly would startle me if I encountered it!

Benbow and Lewis' study raises some serious questions about just how far afield crime investigators need to search for maggots. Searching and collecting in a 25 yard radius around a body is going to be labor and time-intensive. As a crime investigator, it’s important to find the oldest maggots, since that will make your estimated time of death more accurate. Benbow says:

"In standard evidence collection protocols, about a 10 meter radius is searched for the oldest larvae. Our research suggests maggot migration distance can be highly variable. Environmental conditions where human remains are found clearly have an effect on how maggots scatter. We're working out how this might impact insect evidence collection at crime scenes right now."

In Dayton? Have a strong stomach? There may be some part time jobs available in the future for maggot herders, as Benbow and his crew work out new guidelines for forensic investigators.

Full Citations:

Benbow M.E., Lewis A.J., Tomberlin J.K., Pechal J.L. 2013. Seasonal necrophagous insect community assembly during vertebrate carrion decomposition. Journal of Medical Entomology 50(2):440-50.

A. J. Lewis and M. E. Benbow. 2011. When Entomological Evidence Crawls Away: Phormia regina en masse Larval Dispersal. Journal of Medical Entomology 48(6):1112-1119.

B. Greenberg. 1990. Behavior of Postfeeding Larvae of Some Calliphoridae and a Muscid (Diptera). Annals of the Entomological Society of America, Volume 83(6):1210-1214.

¹FYI: Putrescine and cadaverine are not actually lesser known X-men related to Wolverine