Horsefly is the common name for many of the stout flies in the family Tabanidae. The bite of adult female horseflies is an annoyance to humans, but an even greater menace to livestock. Adult males survive solely on pollen and nectar, but the larvae of the species are fierce predators that sometimes display cannibalistic behavior.
During its lifecycle, a single horsefly will go through several stages. Eggs are laid in large masses on rocks or vegetation that overhang water, hatching after approximately four days. As larvae, horseflies are fairly mobile and usually pass through six to nine instars, or intermediate phases, before pupation. The process often takes several months and allows overwintering of the species. Horsefly larvae feed on other larvae, crustaceans, snails and earthworms, capturing prey with their sharp mandibles and injecting them with venom. When the larvae are ready to pupate they migrate towards water and the top few inches of soil where they remain for one to three weeks. Emerging from their pupal cases, the primary concern of adult horseflies is feeding, closely followed by the need to mate.
Adult females are serious pests. They affect milk and meat production because the cattle they attack are heavily occupied in fending them off. A cow may lose more than a quarter of a liter of blood per day from horsefly bites. The flies, which are easily interrupted while feeding, may make numerous bites in order to finish a single meal. Horseflies readily move from one animal to another and are capable of transmitting diseases, such as anthrax and tularemia. Although the diseases they may carry primarily afflict animals, they can also occur in humans through contact with infected animals.