|MadSci Network: Zoology|
It's hard to tell what they could be, based on your description. If they're coming from a mound, they're probably Hymoptera (wasps) or Isoptera (termites). The way to tell termites from wasps is to look at their "waist"- where the abdomen joins the thorax (the thorax is where the legs are attached). If the "waist" is very thin, it's a wasp, if the waist is thicker it's a termite. The way to tell flies from wasps is to look at the wings- wasps and termites have four wings and flies have two (actually flies do have four wings, but one pair is very reduced). You can see a good introduction to the insects here.
Hope that helps!
Rob Campbell, MAD Scientist
Try the links in the MadSci Library for more information on Zoology.