What’s the purpose of bags of water hanging in restaurants?

Dear Straight Dope: What is the deal with the bags of water hanging in restaurants? In the past month I’ve been enjoying beers on the outdoor patio of various restaurants and noticed large ziplock bags filled with water hanging from the ceiling. A friend told me it is supposed to keep away flies. I really couldn’t imagine what difference bags of water would make to a fly but I’ve seen this enough to make me wonder. Is there some truth to this or is a myth perpetuated by the sandwich bag industry? Jason

Doug replies:

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.

Doug

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What’s the purpose of bags of water hanging in restaurants?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD