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Scientific Name: Chrysops sp.
Common Name: Deer Fly

(Information for this species page was gathered in part by Mr. Christopher Hone for an assignment in Biology 220M (Spring semester, 2007)

“Deer fly” is a common name for any of 250 species of the genus Chrysops. Deer flies are found all over the world, and 45 deer fly species are found in Pennsylvania. Most Chrysops species prefer habitats that are fully to partly sunny with a sufficient sources of water and prey species to support their reproduction. Deeply shaded areas (like dense woodlands and inside barns and stables) are typically avoided by deer flies.

Deer flies are between 5 and 12 mm long (larger than house flies but smaller than horse flies). They are black, yellow and black, or, sometimes, gray and often have “bee-like” abdominal stripes. Their wings are cross banded with black stripes, and their eyes are brightly colored (gold, green, orange, or blue). The genus name “Chrysops,” in fact, is derived from the Greek words “chrysos” (gold) and “ops” (eyes).

Diet, Prey Species and Diseases
Female deer flies require a blood meal from a vertebrate prey species in order to make viable eggs. Reproduction is not possible without this blood meal. Typically, females wait in shaded microhabitats for passing prey (which includes, but is not limited to, deer, cattle, horses, and people). Visual perception and/or carbon dioxide or body odor detection attracts the female fly to its prey. Moving objects (often darkly colored (blue seems to be the color that most strongly attracts deer flies)), lights in the night, body heat, and, in the case of one species, wood smoke all serve as attractants for deer flies. Most often the head and neck of the prey species is attacked. In general the hours around dawn and dusk are the times of maximum deer fly activity. Temperatures below 22 degrees C (72 degrees F) or above 32 degrees C (90 degrees F) and overcast skies prevent or inhibit deer fly activity.


Once a female deer fly has located its prey individual, it uses its mouthparts (mandibles and maxillae) like a scissor to cut open a wound. Anti-coagulants in the fly’s saliva prevent the blood from clotting (and may also serve as an antigen for a sometimes severe allergic reaction), and the fly laps up the flowing blood with its sponge-like labrum. The wound may continue to bleed after the fly has departed and may also form a large, painful welt. A number of diseases and parasites can be transmitted by the various species of deer fly. These include anthrax, tularemia, equine infectious anemia, anaplasmosis, hog cholera, filiariasis (including the Loa Loa worm), and Lyme disease. Deer flies can reduce milk production in dairy cows by 20 to 30% and can cause up to 100 pounds in weight loss in cattle. They can also have severe impacts on tourism!

Life Cycle
Throughout the summer, eggs are laid by gravid females on clusters of vegetation (often cattails or sedges) in marshy or wet soil areas. Following a 5 to 12 day incubation period, emerging larvae drop into the aquatic or semi-aquatic habitat, burrow into the sediment or wet soil, and begin to broadly feed on organic materials, insects, annelids, and crustaceans. The larvae remain somewhat active through the winter and then pupate in the drier conditions of the late spring. Pupation lasts 6 to 12 days after which the adult flies emerge. Males emerge a few days before the females. Mating occurs in early summer after which the females begin their hunt for blood meals.

Natural Controls
The natural controls on deer fly populations are extensive, but their overall impacts are limited. Deer fly eggs are parasitized by several species of wasps. Nematodes, fungi, and bacterial parasites and pathogens can quickly kill larvae. Many birds (especially species of swallow and flycatchers) eat adult deer flies, but none seem to specialize on them as prey. Dragonflies, robber flies, spiders, and numerous hymenopterans of genera Vespula, Crabo, and Bombix also prey on adult deer flies. Years ago, I observed my horse standing motionless in one area of his pasture with swarms of deer fly hunting white-faced hornets around his legs and neck. The horse would regularly return to this “cleaning station” in an attempt to get relief from the high level of deer fly activity.

The deer fly is a very visible member of the seasonal fauna of the Penn State New Kensington Nature Trail. It is a sometimes painful, annoying pest for hikers and ecologists alike, but we must remember that it is an important link in many long and vital food chains. Besides, we can always fight them with DEET, long pants, and long sleeves, and by remembering not to wear blue shirts in the summer!

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