Ensign Flies, or Black Scavenger Flies (Sepsidae) are very small - you can see its size compared to the wasp in the second photo. They look like ants. Flies have their hind wings reduced in size and these act as gyroscopes when they fly - you can see these pale blobs at the base of the wings. They have a dark patch on their wings, which may be significant in their curious habit of 'wing waving'.