This female deer fly was biting my dog (and I think many of her sisters had been biting me!) so, after dispatching her on my dog's head, I took this photo. I identified it as belonging to the species Chrysops geminatus based on the great on line photo-based key at http://www.biology.ualberta.ca/bsc/ejournal/tm_08/chrysops89.htm produced by the Biological Survey of Canada. This species occurs throughout the eastern US, Ontario and Quebec. Notable but a little difficult to see in the photo are two brown dots in the center of the animal's back, surrounded by yellow.

Female deer flies feed on blood from vertebrates in order to lay eggs. Adult males feed on pollen. The juveniles remain as larvae for up to a few years and are aquatic.