Ask a Scientist: What is a coffin fly?

Dr. Mohamed Noor is a professor and chair of the biology department at Duke University.
Dr. Mohamed Noor is a professor and chair of the biology department at Duke University. DUKE UNIVERSITY

Dr. Mohamed Noor is a professor and chairman of the biology department at Duke University. Here he describes a creepy creature known to burrow and breed 6 feet under. Questions and answers have been edited.

Q. What is a coffin fly?

A. “Coffin fly” is a generic name for several related fly species that feed and lay eggs on decaying matter such as excrement or dead animals. Despite the spooky name, you’ve probably seen some of these flies in your home or office and assumed they were fruit flies – look for ones that have a humped-back and run in jerky starts and stops when on the bathroom mirror or other surface. You’re especially likely to see them near toilets and urinals, unemptied trash bins or food compost.

Q. Where did the coffin fly get its name? Is it true that they breed in human corpses?

A. Coffin flies have that name because they are particularly talented at getting into sealed places holding decaying matter, including coffins. Given the opportunity, they will indeed lay their eggs on corpses, thus providing food for their offspring as they develop into maggots and ultimately adult flies.

But these flies aren't picky: They’ll breed on decaying vertebrates, decaying insects, decaying plants, decaying mushrooms, laboratory bacterial culture media, curdled milk, fruits, feces from various animals, shoe polish, and even paint.

Q. Do coffin flies pose any danger to humans?

A. Thankfully, these flies do not pose much danger here in the United States, at least no more than any other flies do by spreading germs. However, there are a few confirmed cases of these flies breeding in open wounds of humans. But don't panic – such cases are rare and seem most prevalent when there are hygiene issues, such long-term exposure of the open wound in an unclean environment. There are also a few reports of these flies developing while in human intestines, possibly after the person accidentally swallowed a fertilized female, but this, too, appears rare at best.

Q. I heard that coffin flies are important in forensic entomology. What does that mean?

A. Forensic entomologists determine how long a corpse has been dead by looking at what insect species have invaded it, and what stage of development the insects have reached. As an example, imagine a body is found in an apartment that has been kept at 70 degrees. In this body, you find large coffin fly maggots that you estimate are 15 days old. The prime suspect was in the local area 10 days ago. As such, the victim was likely dead before when the suspect was in the area, and the suspect is vindicated.