When you and your horse are attacked by a horse fly, it is generally a single lumbering horse fly that lands and can easily be swatted if you do not mind the mess from such a big bug. It is an entirely different story with a deer fly attack. They are smaller and more maneuverable. They come in fast and are elusive. Horse flies are like four-engine bombers while deer flies more like fighter planes. They are usually in groups and are very persistent. They favor the horse’s manes and ears but occasionally land on the neck or shoulder. It is only the female of the species that bites animals and once landed they take a blood meal, inflicting a painful bite. They use their knife-like mandibles and maxillae to make a incision and then lap up the blood. It is rare for a horse fly to land on a person but deer flies will bite people all the time. I never particularly was one to wear a cap or hat while working outside but I have learned that in the summer it keeps the deer flies out of your hair. The flies are most active on a hot calm day and seem to like areas where there is a transition between light and dark. They also favor woodland and usually will not trouble the horse while on open land.
Deer flies do not lay their eggs in animal manures. They lay them on vegetation in moist areas near streams. The life cycle can be a year before an adult fly emerges to wreak havoc. The experts concede that they are hard to control. Fly repellents do not seem to faze them. There are traps but you cannot take them riding with you. Many outdoorsman put double stick tape on their hats but I suspect that is a drop in the bucket. I find that in deer fly season it is best just to avoid them. Ride in the cool of the morning and on breezy days when they are less active.