Marking flies

Hi Melanie, if it is not too late to reply, the following might be useful. 

For most flies, brightly coloured nail polish will work. A very small brush and a dab on the fly is sufficient. The nail polish will usually dry before the fly cleans it off (again depends on which group). You might have to experiment to find an appropriate place (top of the thorax is often best).

For delicate hairy flies fluorescent powder works well - the flies will glow in ultra violet light, useful for tracking flies at night or in the dark (we used them for tiny phlebotomine sandflies in the desert).

good luck

Richard