Horn flies measure 3 to 4 mm in length. Although they may be confused with house flies, their bodies are generally more slender than the bodies of house flies. Horn fly coloration ranges from grey-brown to brown-yellow. Adults feed on blood and choose animals as hosts, particularly cattle and horses. As such, they are most commonly found on farms and ranches.
Horn flies leave their hosts at nightfall. Females may also take leave of their hosts in order to lay eggs. Horn flies favor fresh manure as egg-laying sites. Eggs typically hatch in spring and summer, although their mating season lasts throughout the fall.
In large numbers, horn flies can be particularly damaging to livestock. In severe cases, more than 500 specimens may plague a single animal. When this occurs, the animal’s health will suffer: affected animals lose weight and cease to produce milk. When horn fly populations are observed, it is advised that professional help be sought.