For most sensible people, of course, horseflies are a real nuisance. It's actually female horseflies that bite as they need to feed on mammal blood in order to reproduce (the males don't have the necessary mouthparts) and seem particularly keen on human blood due to our relatively soft skin. Unlike other species of fly, such as mosquitos, the horsefly leaves you in no doubt that you've been bitten. It cuts into the skin with its mandibles, sometimes removing a small piece of skin in the process.
The images below show the female horsefly's cutting equipment - no wonder it hurts when they bite...!
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