Horseflies are some of the largest flies in North America, with adults of some species growing to over an inch long with a 2½- inch wingspan.
Horseflies are heavy bodied with the abdomen tapering to a narrow end. Colors range from black to brown, sometimes with stripes or spots.
Horse flies are extremely annoying, have painful bites, and can be serious threats to the health of livestock and horses when they feed in large numbers. Painful bites from large populations of these flies can reduce milk production from dairy and beef cattle and interfere with grazing of cattle and horses because animals under attack will bunch together. Animals may even injure themselves as they run to escape these flies.
Horsefly females are blood feeders, while the males feed on plant juices. The mouthparts of the female are like scissors. They use them to slash open the skin, cause the blood to flow with their saliva, and lap up the blood.
Females lay batches of 25 to 1,000 eggs on vegetation that stand over water or wet sites. The larvae that hatch from these eggs fall to the ground and feed upon decaying organic matter or small organisms in the soil or water. The larvae stage usually lasts from one to three years, depending on the species. Mature larvae crawl to drier areas to pupate and ultimately emerge as adults.
Livestock blood loss can be significant where horse flies are numerous. Horse flies consume about 1 cc of blood at one meal. If there were 20 to 30 flies feeding for 6 hours they would consume 20 teaspoons, which amounts to one quart of blood in just ten days.