Fly Control Plastic Bags
I’ve seen the use of water in a plastic bag as a fly repellent for over a year and it seems to be growing in use. What an “expert” from my area claimed was that the fly mistook the bag as a large spider web. Something about the way the water bulging in a clear plastic bag causes a prism effect and confuses the fly. Who knows what a fly thinks. Maybe they just don’t feel comfortable flying around a place that hangs up bags of water. The rounder you can get the bag, the better fly control. Do not let the bag get dusty or dirty.
This technique must really work. I’ve received dozens of similar reports on hanging clear plastic bags of water and completely eliminating fly problems.
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. Basically it distorts what they are seeing and confuses them.