Have you ever noticed how there are these little tiny flies that run, stop, and run in a seemingly random fashion around your kitchen or bathroom? Sorry, I sound like Andy Rooney. Well, they are not schizophrenic “fruit flies,” but a different kind of fly altogether. Scuttle flies belong to the family Phoridae, and are also known as humpbacked flies or coffin flies.
The most common species of phorid by far, at least in the urban environment, is Megaselia scalaris. Males are roughly two millimeters long, females about three, so if they didn’t move you might not even notice them. They are also very easily confused with vinegar flies (aka pomace flies) in the family Drosophilidae, which we call “fruit flies.” I’ll point out the differences shortly.
I habitually find Megaselia scalaris around the garbage disposal in the kitchen sink. This should not be surprising. The larval flies feed in a variety of decaying organic matter. These little maggots might drown in the drain trap, but they can swallow air to make themselves buoyant and avoid that fate.
Dr. Brian V. Brown, an entomology curator at the Natural History Museum of Los Angeles County, is one of the leading authorities on the family Phoridae. While 90% of the specimens he is asked to identify turn out to be Megaselia scalaris, there is an enormous number of undescribed species, even within the genus Megaselia, which accounts for about half the species in the family.
It is important to note that even the cleanest homes and businesses are bound to have scuttle flies at some point, and they rarely reach population levels that make them any more than a nuisance. They present no health threat to the average person, but they have been recorded as infesting wounds in hospital patients. This phenomenon is known as “myiasis,” and is exceedingly rare. Check out This abstract if you want the gory details of an example from overseas.
The vinegar flies in the Drosophilidae family are probably a little more common than phorids in the average house, but with a little practice you can easily tell the two apart. Above is an image of a Drosophila. Note the large head in proportion to the body, with bright red eyes. Scuttle flies, like the magnified specimen shown below, have smaller heads with dark eyes. The “thighs” on the hind legs are heavier than those of vinegar flies, too.
Still, behavior is the biggest clue. Scuttle flies run in a darting fashion, only flying if dodging you on foot doesn’t work. Vinegar flies perch, and while they may walk slowly, never seem to run. Their instinct is to fly to avoid a potential threat.
Sources: You will find this link from Bugguide.net to be helpful in learning more about these common yet fascinating flies. The chapter on Phoridae in the Manual of Nearctic Diptera is in volume 2, now available as a free PDF download. The Manual is the reference to North American flies, even though it is already somewhat out of date.