The amazing fly repellent plastic bag of water
Fly eye photo from Cosmos Magazine.
Butch has noticed that the restaurants in Pangasinan on the way to Baguio have clear plastic bags of water hanging from their windows like Xmas decorations. There are three or four bags of water for each window. So he asked the waiters what those plastic bags are for, and they said it’s to keep the flies out. Amazingly, there are no flies in those restaurants. How does it work? Butch figures it has to do with compound vision in flies.
I put the question to Michael and Din. Din’s theory: “Regarding the flies and plastic bags of water: flies have highly evolved and specialized vision; lacking any defense against larger predators, flies rely on their vision to avoid being killed or eaten by larger predators. As I’m sure you know, all insects have compound eyes. Light is detected by structures called ommatdia. I’m not sure, but I recall that among insects, flies have the most ommatidia in each eye, and each ommatidia (ommatidium?) has eight photoreceptor cells (specific for a particular wavelength range).
“Consider now a restaurant with three or four plastic bags containing water, hanging in each window, gently rocked by a passing breeze. The plastic bag with water reflects and refracts any ambient light, casting all manner of light and shadow, constantly changing. It’s even possible that the water will polarize the ambient light. The flies’ eyes will be telling them that this is a hotbed of predator activity, much too risky to hang around, so they scoot and don’t return.
“Whew! Thanks for setting me to thinking! I was getting bored watching Craig Ferguson.”
Flight control in a flapping-wing fruit fly simulator
Nerds can analyze a problem and deduce an explanation, but it takes real inductive genius to come up with that fly-repellent plastic bag of water. Who thought up those plastic water bags? Must investigate. Meanwhile we will consult a leading expert on fly vision.
Answers to questions you might be asking, unless you wandered onto this site purely by accident >>>
February 5th, 2010 at 13:41
The Straight Dope offers a similar answer. Link.
Apparently the water bags do drive houseflies away. Not mosquitoes, not no-see-ums, not spiders, not roaches, not yellowjacket wasps, just houseflies. 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.
February 5th, 2010 at 18:51
We hang old CDs in the garden, but this is to scare off the birds. Maybe CDs could also be used in place of the plastic bags of water, for the purpose of reflecting light and movement?
February 5th, 2010 at 19:33
I’ve read this technique before in Lifehacker. http://tinyurl.com/lz2mzd
The readers made similar suggestions on how to keep flies away.
When I was in Japan last year, I saw water-filled plastic soda bottles placed outside Japanese homes. I thought it was some superstitious thing but I was told it’s a silly solution to keep stray cats away.
February 5th, 2010 at 23:09
Good Lord, I just realized I am so old that the first free-association thought I had of “dope” was “a narcotic used to stimulate racehorses!”
February 6th, 2010 at 13:05
@deckshoes. CDs won’t have the ‘wide-angle lens’ effect I think. You know those little peepholes on doors where you could see a fish-eye view of whoever’s knocking on your door? The plastic bags of water would have that same effect, that is, capture movement from a wider area.
February 7th, 2010 at 01:19
Hi!
So last night, over beers, I told this story to Dr. Claude Desplan, my fellow professor of biology at NYU and an expert on fly vision, about this. He had never heard of this phenomenon and had no explanation, but he thought it may be an interesting experiment. I guess if there’s a slow day in his lab, he’ll go ahead and it and see what happens……
February 8th, 2010 at 00:22
it reminded me of the ex-philosophy-professor-turned-busboy from “Atlas Shrugged”.