Pest Control

How to Identify Drain Flies

As I filled my tea kettle from the water fountain at Saint Michael’s College, something jumped along the wall and caught my eye. The motion, a short crawl followed by two jumps in a row suggested a jumping spider; a pair of wings suggested otherwise. The insect was a drain fly (Diptera; Psychodidae), innocuous little insects that often spend their larval stages in sink drains, toilet water, or inaccessible moist crevices in bathrooms.

Drain fly wingspans max out at about ¼” and may well go unnoticed in your home or mine.

I departed with the kettle and returned with a petri dish to collect the fly. I had seen these flies before, but the slack time between semesters provided a good opportunity to really see this one up close. After some brief flailing about in my dish, the fly landed upside down kicking its legs in the air affording a quick look before it righted itself. The fly was far too active to examine for long under the microscope, so I popped the dish in the refrigerator and made my tea.  

Fifteen minutes of cooling its flight muscles and I could examine my catch of the day at leisure. I was struck immediately by the hairiness of the fly’s body and wings, a characteristic that inspires an additional moniker: moth fly. The wings were divided up by nearly parallel veins running all the way to the tips. On closer examination, each wing vein looked like a microscopic bottle brush with stiff hairs pointing out in all directions above and below the wing forming a dense whorl.

Insects use antennae to both feel and smell their environment. Drain fly antennae are on the hairier side of average.

My immediate impression was that this was the opposite of aerodynamic and I wondered how the thing could ever live up to its name and actually fly. I took the lid off for a closer look. Hairs were everywhere and even lengthened the wing by growing long off the trailing edge. Any doubts I had about flight were quickly dissuaded. In my distraction I forgot that my specimen’s muscles were rapidly warming to room temperature; it had had enough of my observations before I did and took off for a darker location and left me with my tea.

But before it left, I managed to capture several photographs for a later examination. The antennae were reminiscent of pine trees in miniature. Divided into regular segments, again with hairs, this time growing out in circular patterns from the intersection of each segment like the branches of a very slender Christmas tree.

I was curious about the reason for all these hairs. In common with the terrestrial adults of many insects with aquatic larvae, these flies need ways to emerge successfully from drains; least a watery habitat becomes a watery grave. For drain flies, the solution is water-repellant hairs. These hairs make them impervious to drowning, even in warm to hot and even sometimes soapy water found in sink drains where their larvae thrive on sludge attached to the interior surfaces.

Insects may have earned the right to be hairy, but drain flies seem to abuse the privilege.

Their sludge-eating habit is one reason why drain flies commonly emerge from old-style trickling-filter sewage plants. As sewage is sprayed on the gravel bed of these facilities, layers of biofilm build up on the rocky surfaces. The larvae feed on the bacterial film before pupating and emerging as adults. This habitat has yielded yet another common name: sewage flies.

The name “sewage fly” is an unfortunate one in my mind because it suggests that the presence of these flies in your house may indicate a sewage issue. In reality the flies are more likely to have come from a floor drain or shower stall. If they occur in large numbers, simply taping clear plastic cups over all the drains in a building reveals the source which can then be cleaned to eliminate the larvae.

Or you could choose to live and let live. Moth flies have not been implicated in disease transmission and they don’t bite. Perhaps shoo these cute little jumping fliers outside to find greener pastures?

Declan McCabe is a professor of biology at Saint Michael’s College. His forthcoming book, Turning Stones: Discovering the Life Of Water is available for pre order through Down East Books at or where ever you find your books.

Images provided by Wikimedia Commons

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