[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