There are many solutions that claim to repel flying insects; some even work.
For instance, there are chemicals – stuff you put on your skin, stuff you burn like incense – at least this makes sense: we can see how some smelly material may offend the insect’s finer sensibilities. Then there are hi-tech solutions, like the electronic gadgets that supposedly emit a hypersonic sound that mosquitoes can’t stand. We once tried this out – my son sneaked up on a mosquito standing on a wall and turned the gizmo on right next to the critter’s ear (or whatever they have). The insect didn’t even deign to move an antenna; maybe it was deaf (or maybe we were sold the electronic equivalent of snake oil).
But the most amazing device is one I snapped in a restaurant today: a nylon bag filled with water, hanging from the rafters (I’ve also seen them hanging from tree branches occasionally). And this traditional system is in use not only in Israel – as I discovered in Google, the same method is used in the US, Mexico and elsewhere: so it’s really globally known.
I asked the waiter why a water bag will drive flies away, and he said with great certainty that the fly sees its own enlarged reflection in the water and it scares it away. I heard this explanation before; presumably the fly thinks it just met a bigger, meaner fly, and decides that retreat would be prudent.
I have no idea whether anyone confirmed this theory (any input on this is welcome), but whether the explanation is correct or not, you have to admit this is one solution that is perfectly acceptable from all angles: environmentally friendly, harmless to humans, humane even to the flies, and as cheap as it gets.