Horse flies and deer flies are bloodsucking insects that are known to bit cattle, horses, and humans. Deer flies are the smaller of the two and commonly bite humans. These large flies can make outdoor activities miserable, as they can be extremely persistent.
Their bite is painful, and they are often attracted to the shiny surfaces of swimming pools. The females are bloodsucking, while the males often feed off nectar. They are also known to feed on other insects and larvae.