I HAVE IDENTIFIED THESE FLIES FROM THE VEIN PATTERNS ON THE WINGS AND GENERAL DESCRIPTION ON MANY INTERNET SITES. WE HAVE BEEN TREATING ALL OF THE DRAINS IN THE HOUSE WITH NO LUCK. WE HAVE A NO PEST STRIP IN ONE BATHROOM AND ARE FINDING MANY DEAD FLIES BUT THEY KEEP COMING. CAN'T SEEM TO GET ANYONE TO COME OUT AND LOOK.
These flies develop in all sorts of decaying vegetable/plant material including the "gunk" that forms inside drains. In that sense they are similar to drain flies (see http://www.livingwithbugs.com/drain_fly.html for control of drain flies). In your case the flies could be coming from another source perhaps even outside the house. Check around for water sources (such as leaky pipes) that might be supporting algae (green, slimy growth) as these can be sources as well. In short anything that is wet and rotting. Also check any compost piles, even outside sources. Once the source is found sanitation is the best control.
I can answer questions about the control of pest insects, spiders, mites and related arthropods. These household pests include termites, carpenter ants, powderpost beetles, nuisance ants, bed bugs, cockroaches, fleas, wasps, and many others. I can also answer questions about using pesticides and other pest control tools such as baits and traps.
I am a retired university extension entomologist. I've taught and conducted research in urban and agricultural entomology. I've published over 70 extension publications, 20 research publications and several books about insects.