The Small-headed Fly belongs to a family of flies called Acroceridae. They are all small to medium in size with a very tiny head relative to the rest of their body, which has a somewhat humpbacked appearance. The head is comprised almost entirely of their two eyes (their eyes are holoptic, meaning they meet at the top of the head).
These flies can buzz like a bee when picked up, which can be quite startling and unnerving. Some members of the family have a long, slender proboscis and feed on flowers. Others lack a proboscis entirely and so apparently do not feed at all in the adult stage. Interestingly, the larvae of these flies are internal parasites of spiders! The flies lay their eggs in large numbers on vegetation and they hatch into tiny flattened larvae called “planidia.” The planidia wait for a passing spider to attach to, and subsequently enter into the spider’s body. The larvae pupate outside the body of the spider, often in the spider’s web (if it’s a web-spinning species of spider).
You can learn even more about them, and find many more photos, at BugGuide.net: Family Acroceridae