Flies, gnats, maggots, midges, mosquitoes; keds (louse flies), bots, etc., present a diversity of names and documents the importance of Diptera to man and reflects the range of organisms in the order. The order is one of the four largest groups of living organisms and can be found at: http://www.sel.barc.usda.gov/diptera/safari.htm.
Several years ago I read a “heard around town comment” about a French bistro in North London that removed one of the items from its menu after it received several complaints from customers, who variably called it "disgusting." Fly In Soup, a rare Gallic delicacy, has always been a favorite amongst French expatriate diners at the unnamed restaurant, but British patrons, who are apparently much more discerning in their tastes than the French expatriates, have boycotted the soup and the owner, a Frenchman, who has emerged from a culture and people who routinely scoff flies along with frogs, snails, slugs, dogs and even their own species.
Prompted by an anonymous complaint, North London Environmental Agency officials raided the bistro and, to their horror, found several mesh cages full of swarming flies, which, the owner told them would have been used in the preparation of his “Soup of the Day” that day. With the flies confiscated, and an admonition to cease and desist having any flies in the restaurant, the owner told patrons: “As of maintenant (now), soup’s off.”
To comply with the North London Environmental Agency demand, my suggestion would be for the owner to visit (http://www.madsci.org/posts/archives/feb2002/1012966124.En.r.html) for an inexpensive fix: Water-filled bags as fly-repellants. Possibly it’s an “urban legend” or just a story; however, a lot of people, my sister included, believe that Ziploc bags, filled with a few pennies and water, works but there is no scientific evidence. John Carlson, MadSci Network Entomologist, suggests a test.
If bags of water do scare flies away, then there could be many reasons for it. The most common guess that I’ve read is that the flies might mistake the bag of water for a wasp nest.
This point of view is stated by an unnamed Alabama pest control technician. Wasps hunt other insects for food, and so it makes sense that a fly would want to avoid a wasp nest. I have not seen any evidence that flies really do know to avoid things that might be wasp nests, but it could be true. In the end, this is only guessing because no one has done the experiments to make sure that that is the case. No mention of phototropic phenomena.
Personally, I think the millions of molecules of water present a prism effect and, given that flies have a lot of eyes, to them it's like a zillion disco balls reflecting light, colors and movement in a dizzying manner. When you figure that flies are prey for many other bugs, animals, birds, etc., they simply won't take the risk of being around that much perceived action—a theory called adaption and suggested by the “known predator” theory folks associate with the loss of small game where nonnative species like pythons lurk.
July in Florida is a good month to test the Ziploc bags filled with a few pennies and water fly theory.