Okay, so houseflies are the objective of waterbags according to this mailbag item.
I would like to know why cars traveling through the desert have those heavy canvas water bags hanging on the front bumper.
To the moderator: If I should have posted this in General Questions or MPSIMS, please transfer it…Thanks 
The canvas bags permit evaporation, enough to cool the water somewhat (the phase change requires heat). I’ve never heard of putting them on car bumpers, but it sounds plausible.
Yep, that’s right. The water bags on the car bumpers are a sort of extra cooling system for extreme hot-weather travel. I’m not sure just how effective this is, but you used to see it a lot on older-model cars and trucks.
Meanwhile, back to the OP…
The Staff Report answer sure lacks any references. A Google search of “water bag”+houseflies turns up only two URLs, and one is the SD article. The other is somewhat skeptical and doesn’t cite any study that could remotely be called scientific.
With all due respect, Staff Reporter Doug, it sure sounds like an urban legend to me.
Musicat, our Staffer Doug is a practicing, professional Ph D entomologist. He don’t need no steenkin’ web sites.
Now, at a patio restaurant in Boston, I was told that the water bags were hung there to drive away birds. It all sounds like nonsense to me, and it sounds like Doug needs to be a bit more skeptical.
I can defend myself on this one. This topic has come up repeatedly on entomology mailing lists, and twice now people have come up with citations to controlled studies showing that the number of flies in a given area is reduced statistically when there are water bags hung there. The mechanism is admittedly speculation, but reasonable and well-founded speculation, especially since it’s just about the only thing that makes any sense at all.
Maybe HE don’t, but I do. Even The Great God Cecil posts references on occasion.
[supplicant mode][sup][sub]Please, I beg of you, just one cite…[/sub][/sup][/supplicant mode]
What, no PhD entomologist has ever been wrong?
OK, so here’s another speculation. Flies don’t like the smell of wet plastic. Do those studies allow the “lens” theory but not this one?
Oh, wait – flies project an energy field that bounces off baggies, and, uh…I’ll work on this one.
Musicat said:
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How well do flies smell? I suspect they don’t smell well - but I’m not the entomologist.
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The water is in Ziploc bags, or other zippered sealable bags. I’m wondering how the wet smell gets out, since these are promoted as being air-tight.
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If it is the smell of wet plastic, then would not a bucket of water on the table work just as effectively? When’s the last time you were at a picnic and your plastic cup of beverage kept the flies away?
So I would say “Yes, the studies do support the ‘lens’ theory over the wet plastic theory.”
And you have read these studies? I haven’t; I’m still waiting for a cite.
My “wet bag” theory wasn’t meant to be taken seriously; neither was my “energy field” postulation, but while we’re on the subject, it is my understanding that many household plastics, being membranes, are not vapor-tight, even if sealed.
I find it slightly odd that, in a message board with a high percentage of skeptics (and I mean that in a good way), the idea that an insect, and only one kind of insect, is strongly repelled by random images transmitted thru a small, crude lens in one part of a room is not challenged. No proof was offered to begin with, and I don’t believe this is an established fact taught in bug class, if I understand Doug’s comments.
And don’t we all know how people, even scientists, can be fooled? Sometimes the weakness of a study is obvious, sometimes less so. Example: I was reading the September 21,2001 issue of a newsmagazine “The Week”, where, in the Science & Technology section, pages 18-19, are some brief news summaries. (FYI: This magazine is presented as a TIME clone, and I wouldn’t put it in the class of the Weekly World News.) Here are some of the statements made, each in a different article:
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English-speaking children take more than twice as long to learn their native language as those in Italy, France. or Spain.
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When you put on sunscreen or wear clothes, even if sunlight hits only your eyes, you will get dark.
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Men see women with C-cup-sized breasts as more professional than women with smaller or larger breasts.
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Dieting can make you live longer.
All of these may be true; some may, or none (I give #4 the best chance). They were obviously written by a journalist, not a scientist, and suffered from a plethora of defects, such as:
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Extremely small sample sizes (Ex:15) or none given
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Tests were done on mice, not humans, but the conclusion was extrapolated to humans
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How the results were judged & tabulated was not given – were these double-blind?
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Possible biases by the researchers were not named, source of funding was not specified
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None of the tests were replicated
So for now, I can easily dismiss the conclusions the journalists made until more evidence is in. These are not fantastic claims, but they require more scrutiny before they are accepted as fact.
And so does the waterbag as fly-repellent theory.
I didn’t state it as fact. The mailbag item quite explicitly states “Apparently…” and “…evidently…”
One thing you learn when you publish research is that nothing is 100% certain, and you get used to giving statistics on the degree of uncertainty. But even in more casual communication, you always let people know when something is uncertain, even without resorting to statistics. I consider the bag-as-lens hypothesis to be the only one yet offered that explains the phenomenon, and that’s enough for me, for now.
Ok, Musicat, I have not read the studies in question, so I guess you’ve got me on that. As for your wet plastic comment not being meant seriously, perhaps not but it was still a flawed suggestion that perhaps you didn’t consider close enough, or you wouldn’t have used it as an example - because that theory is frequently demonstrated as ineffective by common experience.
Do studies need to be considered carefully? Certainly. Hey, maybe we should be more skeptical that water bags are really keeping the flies away. But you didn’t question whether the effect was occurring, you questioned whether the lens theory was correct, and proposed two not-so-serious alternatives. I merely responded that your alternatives were in fact ruled out.