Head of a horsefly. Macrophotograph of the head and biting mouthparts of the horsefly, Chrysozona pluvialis. The large compound eyes occupy much of the head; the vivid colouring of each eye is produced by light interference patterns on their surface. Horseflies feed on the blood of warm- blooded animals, relying on sight and powerful mouthparts for this activity. Black piercing mandibles are seen on the head between the eyes; the horsefly also secretes a saliva that prevents blood from clotting. Although humans can be bitten painfully by some species of horsefly, it is cattle and horses that suffer most.