Flies are an expected and very useful presence when a human corpse is discovered. Knowing the biology of flesh-feeding flies can give an investigator all kinds of important information, but not recognizing fly specks for what they are (basically fly vomit) can jeopardize an investigation. To the crime scene investigator, the “specks” left by flies resting on a wall can look very much like impact blood spatter. Investigators measure the “tails” on blood spatter to calculate angle and location of a blow. Flies leave nearly identical tails on the droplets that they regurgitate. Spatter spray around a room can indicate a violent struggle…or a fly infestation.

Loyola University Maryland biology professor David Rivers has been working to isolate an enzyme found in a fly’s gut with the hopes that a test can be developed to distinguish between fly specks and dried human blood. Rivers’ prime area of study is flies that feed on corpses, perhaps the most important aspect of the specialty known as forensic entomology (see Forensic Entomology is Not for the Faint-Hearted!).

Fly Maggots Have Something to Say About That Death

Blow flies, especially, but also flesh flies and even house flies can be found on a dead body, even in a sealed building. Since the fly larvae feed on dead or rotting flesh, the adult female fly is attracted to decay odors as a place to lay her eggs. And it doesn’t take flies long to detect a carcass, whether human or a lesser animal such as a raccoon or rat. A blow fly can zero in on a body less than an hour after death and from a mile away (see Blow Flies & Carcasses).

With blow flies, it’s all about fast in and out. After all, a decaying carcass doesn’t last forever as a food source; the flesh dries out soon enough. Eggs are laid on the corpse within hours of death, the fly larvae or maggots hatch out only hours later and begin feeding. Depending on temperature, humidity, and the size of the carcass, maggots can feed for a week before pupating and turning into adult flies. It’s that temperature-dependent development, among other things, that allows forensic entomologists to work backwards and determine the time, and sometimes even the location, of the victim’s death.

Blow Flies Will Find Carcasses in Homes, Too

We frequently get calls from homeowners about large, bumbling flies in their homes, even in winter (see Why Are There Blow Flies in My House?!). Sometimes these are blow flies that are just spending the winter in hiding, usually in attics. At other times of the year, blow flies inside a structure can be an indication that there is a dead animal such as a squirrel or raccoon in an attic, crawlspace, chimney, or wall void.

When large flies are found indoors in large numbers, you need the pest control version of a Crime Scene Investigation. A professional inspection by an exterminator can find the source of the flies, whether animal carcass, rotting garbage, or simply flies overwintering in an attic. When it comes to household flies, we investigate the scene and we take no prisoners. Elimination guaranteed!

Photo Credit : By Rhitamvar ray – Own work, CC BY-SA 3.0, Link