Megascilia sclaris adult larva pupa
Phorid flies, family Phoridae
Megaselia scalaris, the scuttle fly
The phorids are a large family of small flies (0.4 - 6 mm) with more than 3500 species worldwide. Other common names for flies in this family include: humpbacked, coffin, and scuttle flies. As a group, they have a wide diversity of shape and form, and their larvae gain sustenance from a variety of sources. The smallest fly in the world, Euryplatea nanaknihali, is a phorid fly originally described from Thailand. This species attacks very small ants in the genus Crematogaster - the acrobat ants - developing within the ants' heads. Other phorid species are known to attack the imported red fire ant, various wasps, bumble bees and honey bees. Most of the species, however, deposit their eggs in decaying animal or plant material and it is in these instances that they come into conflict with man. Two of the species most frequently found in homes are a humpbacked fly, Dohrniphora cornuta and the scuttle fly, Megaselia scalaris. The later is discussed here.
M. scalaris eggs are somewhat "boat-shaped" with an average length of 0.5 mm, similar in size to the small fruit fly Drosophila melanogaster. Egg developmental rates are temperature dependent with optimal time of 9 hours at 93oF.
Larvae are cylindrical, tapering toward the head and cream in color. They average about 3 mm in length, pupating in 55 - 65 hours at 93oF.
Pupae are elongate oval, 4 mm long and complete development in approximately 170 hours.
Total developmental time from egg through pupal stages is approximately 10 days at 93oF. and 17 days at 73oF.
Adults average 3 mm in length, vary in color from a light brown to yellow and have the characteristic thickened and shorted costal wing vein of other phorids. Depending on temperature, they can survive for 1 - 2 months.
Habitats and Behavior
The scuttle fly has a propensity to run erratically across surfaces rather than fly. Its common name is derived from that behavior and can be helpful in identification for the layperson.
These flies are found feeding in many types of substances. Homeowners should first search for rotting bags of potatoes, onions or other stored tubers. When no apparent food source can be located consideration should be given to the possibility of broken septic lines or cracked septic tanks. Consult with a reputable plumbing firm that can provide examination of the lines using a flexible scope and/or injection of smoke to determine if lines are fractured. Scoping doesn't always reveal cracks but smoke will follow the path used by adult flies when entering buildings. Commonly, smoke will escape along foundation expansion cracks or similar openings. M. scalaris will also breed within dead animals and cadavers. The flies are most disconcerting when seen flying about mausoleums or in churches that have crypts. In either of these cases it is important to contact a professional in the management of structural pests for advice.
As with any pest management strategy, accurate identification is the first and arguably the most important step of the process. Very few people are capable of identifying phorid flies and most homeowners believe these are "fruit flies", not recognizing the potential threat to their health. Our advice is: any small fly in your home that does not have red eyes should be collected for identification and given to a professional entomologist.
Authored by: Steve Jacobs, Sr. Extension Associate