Deer flies are bloodsucking insects considered pests to humans and cattle. They are large flies with large brightly-coloured compound eyes, and large clear wings with dark bands. They are larger than the common housefly and smaller than the horse-fly. There are 250 species of deer fly in the genus Chrysops. Their distribution is worldwide, though they have not been reported in Iceland, Greenland, and Hawaii.
One hundred to 800 eggs are laid in batches on vegetation near water or dampness. During the larval stage, which lasts one to three years, they feed on small creatures or rotting organic matter near or in the water. After a pupal stage, then emerge as adults in late spring and summer. While male deer flies collect pollen, female deer flies feed on blood, which they require to produce eggs. Females feed primarily on mammals. They do not enter buildings. They are attracted to prey by sight, smell, or the detection of carbon dioxide. Other attractacts are body heat, movement, dark colours, and lights in the night. They are active under direct sunshine, hours when the temperature is above 22 degrees. When feeding, they females use scissor-like mandibles and maxillae to make a cross-shaped incision and then lap up the blood. Their bite can be painful. Anti-coagulants in the saliva of the fly prevents blood clotting and may cause severe allergic reactions. Parasites and diseases transmitted by the deer fly include Lyme disease, tularemia, anthrax, anaplasmosis, equine infectious anemia, hog cholera, and filiariasis. DEET is not an effective repellent.
Predators of the deer fly (and other Tabanidae) include nest-building wasps and hornets, dragonflies, and some birds including the killdeer. They cannot be controlled by humans because insecticides cannot be applied in the sensitive wetlands where the larvae typically develop. Also the problem adults may have developed a long way from where the eggs were laid. Trapping devices and protective clothing can help humans to avoid the annoyance of deer flies attempting to take blood.
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