A horsefly is a large, aggressive fly that bites animals and people to feed on their blood. Unlike mosquitoes that pierce skin with a needle-like mouth, horseflies have scissor-like mouthparts that actually cut into skin, which makes their bites particularly painful. The name comes from the fact that these flies commonly attack horses, cattle, and other large animals, though they'll bite humans too if given the chance.

Female horseflies need blood to produce eggs, so they're the ones doing the biting. Males stick to feeding on flower nectar. These flies are surprisingly fast and persistent: swatting at one rarely discourages it for long. They're most active on hot, sunny days near water, where they lay their eggs.

If you've ever been outside on a summer afternoon and felt a sudden sharp sting, only to see a chunky fly buzzing away, you've probably met a horsefly. The bite often swells up and itches for days. Farmers and ranchers consider horseflies serious pests because they stress livestock and can spread diseases between animals. People who work with animals outdoors have battled these determined insects for as long as animals have been raised that way.