This whopper of a fly is a large, very distinctive looking member of the Tabanidae, the horse flies (which also includes deer flies). FWG visitor, Lisa, found this sitting on a milkweed near the centre. This fly could inflict a noticeable bite, but in fact, these gals (and it is only the females who bite) prefer to bite cattle not people. So this one may have wandered over from the farm where the cows are.