These are the pests that fly circles around your head on hot muggy mornings and take chunks of flesh out of the back of your neck. Nasty creatures.
And the roughly equivalent horse-fly.
With that being said, the worst biting insects I've ever had to deal with were the sand flies along the coast line of Honduras. While you can barely see them, their bites itch like heck and last for up to a month. On each of my vacations to Honduras, my legs and arms would be covered by bites once I got home. I'd rather get stung by a bee than bit by those things.....