Deer flies are a biting fly species very similar in appearance to horse flies, although they are slightly smaller. In both species, only females feed on blood, while males feed on pollen. Females use scissor-like mandibles to slice the flesh of bite victims.
Deer flies are known carriers of the bacterium that causes tularemia. Deer flies are also commonly found on farms, where they prey on cattle and horses. These feeding behaviors expose them to more animal-related, blood-borne pathogens.
Deer flies lay their eggs in moist vegetation such as old leaves and rotten logs. Deer flies may also lay their eggs in running water. Eggs hatch within five to seven days, from which larvae emerge and begin to feed on available organic material. Some species of deer fly larvae are also capable of feeding on other insect eggs and larvae. Depending on the species, adult deer flies may live three to four weeks.