This is the compound eye of a Deer Fly. A common pest around these parts, this fly has razor-sharp “teeth?” and will cut an incision into your skin and drink the blood that spills out. If I had a choice I’d rather feed a mosquito – at least there’s no mess! As with mosquitoes, it is an instinctual reaction to swat them when they land on you. Unfortunately they are far more difficult to swat and far more annoying.
This one I managed to hit, so yes, this fly was intentionally injured by me (he had it coming!). I stunned it and broke a wing so it couldn’t fly away, and the I got the crazy idea to photograph it’s eye. This was the result.