The insect is not a fly.  It is a type of hymenopteran.  This insect group is the ants, wasps, and bees.  The picture is a good picture, but for identification purposes as to which it actually is; then, it is not as focused on the necessary characters.  I think this is an ant.  I am not sure of which species because of some coloration, antennal characters, and the petiole.  If you wish clarification you can collect specimens and send them into the cooperative extension office nearest to you.