The eyes are bright green or gold with zigzag patterns or other patterns on them. The hind legs have tiny spurs that help the fly stay in place on its victim or landing place. The eggs of the deer fly are shiny black. They are stuck by the female to the leaves of emergent plants just above the water level. They then bite quickly and leave.
Some deer flies carry bacteria that cause a disease called tularemia in rabbits and hares.
Rarely ever does this disease effects people. The American horse fly has a large, broad body that is 20 mm to 28 mm long. It is found in all parts of southern Canada east of the Rockies. The head is ash-grey to tan with large green eyes. Some people call these horse flies 'Green Heads'.