So last night, over beers, I told this story to Dr. Claude Desplan, my fellow professor of biology at NYU and an expert on fly vision, about this. He had never heard of this phenomenon and had no explanation, but he thought it may be an interesting experiment. I guess if there’s a slow day in his lab, he’ll go ahead and it and see what happens……

When I was in Japan last year, I saw water-filled plastic soda bottles placed outside Japanese homes. I thought it was some superstitious thing but I was told it’s a silly solution to keep stray cats away.

Apparently the water bags do drive houseflies away. Not mosquitoes, not no-see-ums, not spiders, not roaches, not yellowjacket wasps, just houseflies. Evidently, houseflies, being highly edible and defenseless, are nervous types, and don’t like to sit still when they see something moving nearby, because it could be a predator. The water bag acts a bit like a lens–try it some time–in which the movements of people in the area are reflected. Even if the fly is too far from the action to see it directly, it can see a shifting of light and dark in the water bag, which it interprets as nearby movement, and it will fly away from the bag. The reason it doesn’t work on any other insects is that the other insects listed don’t have eyesight worth a plugged nickel.