Flies with mantis-like forelegs prowl the shore

Shore Flies with raptorial forelegs (Ochthera sp.) prowl the sandy shore of the the main retention pond at Arthur Storey Park, in Houston, TX, USA. I'm not sure exactly what they're doing, but it looks like maybe there's some mix of hunting behavior and egg-laying in this video. The adult flies are supposed to hunt other adult flies, and the larvae are supposed to hunt other fly larvae.