Deer fly; Deer flies are about the same size as houseflies, and they are yellow or black in color, with patterned wings. They are mostly active on warm, sunny days with little wind and they thrive in areas such as beaches, lakes, or woods near damp areas. Bites are usually painful, though most of the time not severe. In rare cases, the flies can transmit the Tularemia bacterium, which requires medical attention. Prevent deerfly bites by wearing protective clothing and using insect repellent.