According to PawNation, female horseflies feed off of the blood from livestock, such as cattle, sheep, goats and horses. Male horseflies only consume plant nectar and do not consume blood.
According to Purdue University, female horseflies commonly feed on stationary hosts during the day. Female horseflies are typically aggressive and are known to continuously bite until they have consumed enough blood. These bites cause pain and discomfort for victims. Horsefly larvae live near ponds or muddier areas and feed off of small organisms, such as insects, earthworms and snails, according to Eco Spark. Horsefly larvae also consume decaying materials from plants and animals.