Apparently this water bag keeps the mosquitoes and flies away. #clever #guatemala (Taken with instagram)
Not mosquitoes, only flies. From The Straight Dope:
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.