Deer flies and horse flies are in the family Tabanidae. They are both biting flies. Scientists have classified most of the horse flies in the genus Tabanus and most of the deer flies in the genus Chrysops.
Deer flies are about the same size as houseflies – about ¼” to 0.5″ in length. They may be black or brownish in color. There are several physical differences between deer flies and horse flies, but most of them are hard to see without a magnifying glass.
The most obvious differences are in the wings and the eyes. Deer flies have dark spots on their wings while horse flies have clear wings. Many deer flies also have spots on their compound eyes.
Several species of deer flies are common in the United States. Chrysops callidus (Osten Sacken) and C. carbonarius (Walker) are widespread throughout the United States. C. discalis (Williston) is chiefly found in the western U.S, while C. flavidus (Weidemann) is more common in the east.
Adult deer flies feed on nectar from flowers and honeydew. Females also seek a blood meal in order to produce eggs. They deposit their eggs in water or damp soil.
The larvae of most species are scavengers. They feed on decaying organic material. They molt several times as they grow. When they are full grown, the larvae make a case where they change into an adult fly.
Deer flies are good fliers. They feed during the daytime and they can fly a long distance from the breeding site in search of a meal. They are attracted to movement and dark colors.
Unlike mosquitoes, deer flies do not pierce the skin of the host. The female deer fly uses her mouthparts to tear the skin of the host. She drinks the blood as it flows from the wound.
The female deer fly injects some saliva that prevents the blood from clotting during feeding. Many people have an allergic reaction to the saliva. People have experienced lesions and high fever after deer fly bites. Many people have increasing allergic reactions after repeated bites. In some cases, people experienced swelling of the arms or legs and red streaks in the affected area.
One species of deer fly, C. discalis, has been known to transmit tularemia from infected rabbits and rodents to humans.
Deer flies occasionally enter buildings, but after they get inside, they fly toward the windows and keep flying there until they die. Experts recommend securing the screens on windows and doors to prevent deer flies from entering.
It is probably not practical to try to eliminate deer flies outdoors. However, experts recommend wearing light-colored clothing and using insect repellants during outdoor activities. The local pharmacy may be able to recommend a repellant that works well in the local area.