[rozab]

Fun idea, but presumably there is a big bias in that some flies will be easier to catch for various reasons. I suppose lots of population studies on birds, cetaceans or whatever have this bias, since you can't just slap a quadrat down and catch everything. In fact with these more advanced animals, perhaps they're harder to observe a second time since they've learned to be wary of humans.

[m12k]

Right, if you're measuring long enough that predators eating some of the flies could be an issue, then you'd want to mark them with something only visible under a blacklight, or similarly "neutral", so it doesn't introduce a bias.

[marcosdumay]

If predators or age (or moving out or into the measured area) are a factor, they are already biased and keeping your method neutral will still give you very wrong results.

[Woberto]

Unless the predator can see in the UV spectrum, like bats