Deer flies range in length from 7 to 10 mm while horse flies are from 10 to 25 mm. The deer flies are yellow to black, have stripes on the abdomen, and possess mottled wings with dark patches. Yellow flies are yellowish with the same body shape of deer flies, but have dark purple to black eyes marked with florescent green lines. Horse flies are black to dark brown with green or black eyes. Adult deer flies have apical spurs on the hind tibiae that are not present in horse flies.
Sight is the main host finding mechanism, but carbon dioxide and odor also play a role.
Moving objects, especially if dark colored, are most prone to attack. Attacks occur during daylight hours with a peak beginning at sunrise and lasting three hours. A second peak is two hours before sunset and commences shortly after. Attack frequency is low on overcast days or at temperatures below 22 and above 32 degrees Celsius. On livestock, biting occurs on the abdomen, legs, and neck.