DEET is only marginally effective as a repellent. Most common species are gray or light brown, sometimes with patterned bodies and wings, and have large colored eyes. Deer flies are day biters, produce a painful bite, and frequently draw blood in the process. They lay eggs on grasses and other aquatic vegetation around the edge of small ponds and other permanent standing water. The larvae develop within the mud and plant matter around the edge of the pond, feeding on decaying organic matter and small invertebrates.
Deer flies have a one year life cycle.
Adults are present for two to three weeks. They rest on shrubbery or tall grass when not mating and feeding. Horseflies are closely related to deer flies. They are somewhat larger and generally have similar habits, although larvae are thought to be primarily predators of Insects developing in mud around ponds. Horse flies found in Colorado almost never bite people but can be occasional nuisance pests of livestock.