They are dark grey in color with two transparent wings folded flat over the back, often in a delta wing position. Eggs are reddish-brown in color and about 1 mm long. Larvae are white and resemble house fly larvae or maggots, but measure only 8 mm long when full grown. Larvae pupate within reddish-brown puparia that are about half the size of a wheat kernel. Horn flies can be distinguished by their habit of congregating and resting head downwards on the back and shoulders of their host and by their delta-wing appearance.
Horn flies tend to rest quietly until disturbed whereas other flies always seem to be on the move.
Horn flies rarely leave their host except to lay eggs, to move to other cattle in the herd, or when cattle enter buildings. Infestations spread by the active dispersal of newly emerged adults to herds up to 8 km away in one season. Infestations are also spread by the movement of infested cattle, either on foot or by truck. Horn flies feed by piercing the hide and sucking the blood. Most feeding occurs along the underline of the animal and results in scabby, often bleeding, sores.