Why do my dogs chase flies

I think it's just instinctive for dogs to chase things. It doesn't matter whether it's a stick, ball, frisbee or a flying insect. It's in their genes to chase. I haven't seen Gigi try to catch a fly in the house yet, but I have no doubt that she would try it one got in. My cats used to do this at our old house. It was fun to watch them jump up in the air to try to catch them. Of course, them rarely ever caught one.