To ask the question in the title assumes that a clear bag of water actually does repel flies. There are plenty of people who swear by this method, and they use it for picnics, yard sales, around the kitchen and in restaurants. Since no one has yet conducted a thorough scientific assessment of whether and under what conditions this trick works, people can only speculate -- if it does work at all -- about why.
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The basic technique for the Ziploc-water-fly-repellent system is to fill the bag half full with plain water. Some people also add pennies to the bag; but the reason why this is believed to improve the efficacy of the rig is also a mystery. The bag is then hung, with a clip and string, a clothes hanger and clothes pins, duct tape or even a fishhook and line. Some people say it has to be hung where it catches light from outdoors.
One theory about why this method works is that flies are skittish by nature, and that a fly's compound eyes, containing thousands of facets, are exquisitely sensitive to motion and light. The bag of water acts as a great lens, refracting light and magnifying images that it then projects in several directions. This confuses the fly into believing that there is an imminent threat. Anyone who has fought back aggressive flies at a crowded family picnic, with all its activity, will have some reason for scepticism about this theory.
The Eye of Sauron Theory
A theory that is popular in the Southern United States, where this practice is most prevalent, is that flies are sensitive to unnatural changes in light-dark contrasts, and that the bags of water magnify light and dark objects, confusing the fly. A further elaboration of this theory is that this odd refraction is suggesting to the fly that it is the eye of a gigantic predator. Many people from the same areas that hold to these beliefs also place empty soda bottles on the tips of tree limbs to ward off spirits; but this geographic correlation may or may not be significant.
The "MythBusters" Test
The Discovery Channel program "MythBusters" conducted an experiment on this belief, in an enclosed space, where 55 grams of flies were placed in a chamber. How they weighed the flies was not explained. Two adjacent chambers contained equal portions of rotten meat. When the flies were released, they entered both chambers to feast on the meat. The chambers were sealed after the release, and the chamber with a water bag contained 35 grams of flies, while the chamber without contained 20 grams of flies, suggesting the water bag actually attracted more flies. This experiment was conducted with artificial light, however, so some may claim the results are invalid.
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