Allotments 4 All

My first season and I am planting a number of cabbages.  I recently planted out some cabbages. Before heading home I made certain that all plants were covered with a finegreen mesh covering.   When I returned a couple of days later I noticed that there were some tiny black flies clutching to the underside of the netting.   I don't know how they got there.  I am concerned. Are these likely to be the dreaded cabbage fly?   It's not as if the plants had been in the ground long.  I have made some cardboard disc that I will place around the base of the plant.

Please advise.  :-\                           

Flea beetles or aphids?.... First one's a PITA but rarely a crop-killer, second one.... catch and release some ladybirds under there.....