Adult deer flies (Chrysops spp.) are 6 to 12 mm long, with striped or banded wings and striped or solid-colored body. These deer flies are distinguished from clegs, which have banded eyes and spotted wings. Larvae are usually found in the wet soil along the shores of ponds and marshy areas. They are cylindrical and pale yellow to white; there are longitudinal ridges along the body. There are swellings on segments 4-10, and the posterior spiracles are on an extended spine. North American species include: C. callidus, C. ater, C. flavidus, and C. vittatus. Their abundance depends on weather conditions and the availability of prey.

Here's a good look at that nasty proboscis and those wild eyes

Chrysops calvus is about 8 mm long and has a dark-grey to black abdomen. The wings have smoky-gray markings. These flies are active from spring to fall. (1) Their breeding sites in marshy areas bring them into contact with humans, and their habit of hovering around the head and shoulders and biting people can really piss one off. They are known for following moving animals and repeatedly buzzing the head; I wear a hat when walking in the sunshine when these flies are active. They can really take a swat, too, and get up and fly off - a disconcerting occurrence when you think you've dealt a death-blow to an annoying bloodsucker!