There are many types of flies, the most infamous of which is the head fly (Hydrotaea irritans). These flies spread bacteria such as summer mastitis in cattle.
Horse-flies are large but agile flies with large eyes. Female horse-flies are particularly blood-thirsty and suck the blood of large mammals and even humans, as soon as they land. Horse-fly bites are painful, very itchy and can even transmit bacterial infections. Male horse-flies are much less aggressive; larvae grow in semiaquatic habitats and feed on rotting plants. Horse-flies are predominant in coastal areas; their ability to travel long distances, allows them to “chase” horses and other cattle, which can cause panic.
Contrary to horse-flies, horse botflies do not bite, although they do spread parasites. Horse botflies lay small yellow eggs on the horse’s coat, most commonly in their neck and legs, which the horse attempts to lick away, but instead become ingested. The eggs can attach themselves to the stomach wall, grow and hatch larvae which are subsequently expelled with the manure where they develop into mature horse botflies. Horses become restless when horse botflies (try to) lay eggs on them and infected horses might suffer from abdominal pains.
The presence of insects depends greatly on the type of ground, vegetation and the presence of large bodies of water. There are numerous sprays and other products that can be used to control the presence of, and protect your horse from, insects such as horse-flies, tabanid flies, horse-flies and mosquitoes.