Horse and deer flies belong to the same family, and there are about 350 species of them in North America north of Mexico. Like other true flies, they have only one pair of wings, short antennae, and large compound eyes. Distinguishing them from other kinds of flies, horse and deer flies are stout and usually medium to large. Overall, most are drab browns, grays, and blacks, but many species have bright, iridescent, or rainbow-colored eyes, sometimes with spots or stripes. Horse and deer flies are notorious for drinking blood from cuts they make into their host’s skin.
Deer flies are usually smaller than horse flies, and they often have spotted eyes and a dark-spotted pattern on the wings. Horse flies are larger and usually have solid or striped eyes. In both, there is a space between the eyes in the females (when viewed from above), while the eyes of males nearly touch.
The larvae are fairly straight, segmented, wormlike maggots that are tan, whitish, or brownish. They are aquatic or live in mud, and most are predaceous.