Does anybody know how clear plastic bags full of water (hung up) repel flies?

I'm starting to notice more and more at bars and restaurants clear (ziploc) bags hanging from the rafters, filled with sugar water and a penny. We were told that the bags were used to repel flies. Does anybody know how that works?Does anybody know how clear plastic bags full of water (hung up) repel flies?
actually you are supposed to poke a few holes in the bags above the water line... the flies smell the water and think its going to rain and leave if you'll notice there are always more flies right before it rains and none duringDoes anybody know how clear plastic bags full of water (hung up) repel flies?
supposably, the flies get disoriented by the reflection of the penny in the water - makes it look like a spider web to them, and scares them away....
One theory is the flies might mistake the bag of water for a wasp nest. Wasps hunt other insects for food, and so it makes sense that a fly would want to avoid a wasp nest.