The water bag method of fly repellant has many supporters, from restaurant owners to backyard grill-masters. Many success stories ranging from the mild to the miraculous litter the Internet.
So how does the method drive flies away? Some insist the flies perceive the clear liquid as the surface of a body of water. Others claim the insect flies away at the sight of its own magnified reflection. But the most popular reasoning that pops up among entomologists and patent-filing entrepreneurs is simple light refraction.
Refraction takes place when a clear or opaque object, such as a piece of glass or a bag of water, alters the course and velocity of light. The rays of light, which normally travel in a straight line, bend. This effect is responsible for a number of optical illusions, such as mirages, that occasionally baffle humans as well. For more information on refraction, read How Light Works.
In theory, refraction can be just as confusing for some species of insect, especially the housefly. It boasts a highly sensitive array of eyes which allow it to see in multiple directions at once.
The insect's head mostly consists of a pair of large complex eyes, each of which is composed of 3,000 to 6,000 simple eyes. These eyes can't move or focus on objects like human eyes, but they provide the fly with a mosaic view of the world around them. Each simple eye provides one small piece of the puzzle, much like the way a screen's pixel delivers one detail of the larger picture.
A housefly bases its sense of direction on the direction sunlight comes from. Some entomologists believe that when these complex, sensitive eyes experience refracted light, the insect becomes confused and flies away.
While some supporters claim water bags keep all kinds of flying insects away, most report success with complex-eyed insects, like houseflies.
Can a Bag of Water Keep Flies Away?
I've heard rumors that if you hang up a clear bag of water, it will repel flies. It sounded like a bunch of crap to me. But in digging a little deeper, I have heard a lot of people swear up and down it's true. Then I came across this article at HowStuffWorks.com: