There are quite a few flies in this family and they all have painful bites. Most females must have a meal of blood before they can reproduce, so they are determined to bite either livestock or people to get that meal. Some do carry parasites or diseases, but the most miserable thing bout them is the bite.
That being said, they can be important pollinators in some areas. Nothing is without some important aspect in the biodiversity of any given locale.
This fly was photographed September 4, 2012, in Green Lane, Pennsylvania, USA. It was on my car – did you notice how clean I keep my vehicle?
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I used a SONY DSC-H9 camera.