Why does a plastic bag, filled with water, hung in a doorway, prevent flies from entering that door?

Question

Have seen this done at flea markets and have tried it in my own home, because we have no back screen door but I like to have the door open for the breeze. The trick seems to work, I just wondered if anyone out there knew why it works. And, more importantly, how on earth did the flea market people discover it?

Answer 1

The water bags do drive houseflies away. Houseflies, being highly 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 like a lens 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.