• Flies are repelled by hanging bags of water."Female Tabanus Horse Fly" is Copyrighted by Flickr user: Opo Terser (Thomas Shahan) under the Creative Commons Attribution license.
  
  

Function

• Fill a sandwich bag half full with water and attach it above the entrance to your home. Flies will not fly under the bag and so your home remains fly free. It is an inexpensive form of pest control. It is also humane; you do not kill the flies, only turn them away.
  
  

Geography

• This method of controlling fly infestation is not limited to a specific geographic area. Richard Fagerlund, a columnist for the San Francisco Chronicle Online, wrote about plastic bags filled with water for fly control in his column. He states, "I have gotten a ton of mail from all over the world when this column originally ran and the mail ran 9 to 1 saying this method worked."
  
  On the forum at Ask Meta Filter.com, posters from Pennsylvania to Chile to Spain responded to a query as to the science behind this method. It is, it appears, a global phenomenon.
  
  

Effects

• As to how a plastic bag filled with water repels flies, there is only speculation. One theory put forth claims the multiple eyes on flies causes them to see the bags as a confusion of light and darkness. They become disoriented and fly off in the other direction.
  
  Another popular theory is the bag, with its ability to cause visual distortions, is interpreted as a predator. Since a large number of bugs like to eat flies, the flies fear for their lives and leave the vicinity.
  
  

Features

• The bags, to be effective, must be clear and strong enough to hold the water without leaking. Freezer bags seem to be popular, as they tend to be a bit thicker. You can then seal the bag and hang it over your doorway using tape or nails or any other device that allows you to attach the bag securely.
  
  It's a cheap enough device that you can replace it, should a windstorm destroy it or the neighborhood kids swipe it as a prank.
  
  

Misconceptions

• The plastic bag filled with water does not repel any other type of bug. Bees, wasps and mosquitoes will still fly in. Creepy crawlies such as spiders and roaches are likewise not intimidated by hanging bags of water. It appears this simple home remedy repels only flies.