We call them black flies in New England, other places call them gnats or buffalo gnats. They're miserable, bitey pests that drive the horses nuts. A fly mask with ears will go a long way towards making them more comfortable. One of my mares has to have a full fly sheet with neck as she gets horrible hives from their bites. Where I live, the old adage is that they come out on Mother's Day and are gone by Father's Day, which is holding true so far this year!