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Chrysops excitans

What do they look like?

Chrysops excitans is a medium sized deer fly (9 to 12 mm) with a mostly black body. Males have eyes that touch, while females have eyes that are separated. The wings are clear with black markings in the middle. They have large mouthparts, and the body has yellow or orange triangles on the back.

The eggs are about 2 mm long. Larvae are thinner at both ends and have 11 body segments. The first seven body segments have several tiny pairs of legs on them. The pupae are covered in a case, though you can still tell which parts are the head, thorax, abdomen, and appendages. (Bland and Jacques, 1978; McAlpine, et al., 1981; Teskey, 1969; Teskey, 1990; Thomas and Marshall, 2009)

  • Sexual Dimorphism
  • sexes shaped differently
  • Range length
    9 to 12 mm
    0.35 to 0.47 in

Where do they live?

Chrysops excitans is a common deer fly found throughout much of North America. It lives across Canada and Alaska, south to California in the west, and New Jersey and West Virginia in the east. (Teskey, 1990; Thomas and Marshall, 2009)

What kind of habitat do they need?

Chrysops excitans larvae are aquatic and live close to shore in both slow-moving and fast-moving waters, such as lakes, ponds, swamps, marine beaches, and rivers. Adults are found in the forests, grasslands, taiga, and mountains near the bodies of water where they lived as larvae. (Merrit, et al., 2008)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

How do they grow?

Deer flies go through complete metamorphosis, with life stages of egg, larva, pupa, and adult. Females are parasitic and must feed on the blood of a mammal before they can produce eggs and mate. Eggs are laid attached to each other and to a surface over or near water in groups of less than 100 eggs. Eggs hatch after about 5 days, after getting warmed in the sun. Larvae drop into the water below, and they develop through 6 stages called instars. It takes about 9 to 10 months for larvae to finish developing, and then they crawl into dirt or soil at the edge of the water and become pupae. After 2 weeks, pupation ends and adults emerge in the morning. After a little wait, their wings expand and harden, making them able to fly away. (Lake and Burger, 1980; Magnarelli and Anderson, 1977; Magnarelli and Anderson, 1981; Magnarelli, 1976; Roberts, 1980; Teskey, 1990)

How do they reproduce?

Males perch on plants and fly around looking for mates, and chasing after any flies that fly past. Most species mate in the morning. Females likely mate at least two times during their adult lives. (Anderson, 1971; Catts and Olkowski, 1972; Leprince, et al., 1983; Troubridge and Davies, 1975)

Females have to feed on blood from a mammal about 4 to 8 days before they have eggs ready. When the eggs are ready and fertilized, they are laid as a group on the stems or underside of leaves over water. They are usually laid on warm, sunny days in the morning. (Anderson, 1971; Catts and Olkowski, 1972; Leprince, et al., 1983; Magnarelli and Anderson, 1977; Teskey, 1990)

  • How often does reproduction occur?
    Females seem to mate periodically over the course of the summer, with a blood meal between each mating.
  • Breeding season
    Breeding takes place from early June to the end of August.

Females provide yolk and other nutrients in eggs. They also lay the eggs over water where the larvae can drop into after hatching and live. Otherwise, these deer flies do provide any parental care. (Magnarelli and Anderson, 1977; Magnarelli and Anderson, 1981)

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • female

How long do they live?

Lifespan for these deer flies is about a year, at most. Adults likely live for several weeks after emerging from pupation. (Teskey, 1990)

  • Typical lifespan
    Status: wild
    1 (high) years

How do they behave?

Deer flies are active from late May to mid-September. They are the most active on sunny days with little or no wind. Females bite and feed on blood from mammals. Their bites are very painful, which causes the animal being bit to try to get rid of the flies before they can finish their meal. To complete a meal, females must feed on several different animals. (Burnett and Hays, 1974; Krinsky, 1976)

Home Range

Deer flies travel about 1 to 2 km from where they were born.

How do they communicate with each other?

Deer flies use vision and detect chemicals to communicate and gather information about their environment. Males use vision to find mates, and females use vision to find animals to feed on, as well as places to lay their eggs. Chemicals are also used when attracting mates. Females are also attracted to heat sources, which is how they find a warm mammal to feed on. (Anderson, 1971; Catts and Olkowski, 1972; Leprince, et al., 1983; Mihok, et al., 2007; Teskey, 1990)

What do they eat?

The larvae apparently feed on organic matter in moist soil. Adult males feed on nectar and pollen. Females feed on nectar, but also eat honeydew produced by Hemiptera and rotting fruit. Adult females feed on blood to produce yolk for their eggs. They feed on blood from deer, humans, cattle, sheep, hogs, horses and other domestic animals. (Lewis and Leprince, 1981; Magnarelli and Anderson, 1977; Magnarelli and Anderson, 1980; Magnarelli and Anderson, 1981; McAlpine, et al., 1981; Teskey, 1990)

  • Animal Foods
  • blood
  • Plant Foods
  • fruit
  • nectar
  • pollen

What eats them and how do they avoid being eaten?

Chrysops have been recorded as prey for birds, amphibians, dragonflies, robber flies, and wasps (including Vespula, Crabro, and Bembix). (Teskey, 1990)

What roles do they have in the ecosystem?

Males may pollinate the flowers on which they feed. They are also preyed upon by other animals. Females are parasites and must feed on the blood of an animal before they can lay any eggs. They feed on deer, humans, cattle, sheep, hogs, horses and other domestic animals. As blood feeders, they can also transmit disease to the animals they bite. These deer flies are also attacked by many different parasites. (Krinsky, 1976)

Species (or larger taxonomic groups) used as hosts by this species
Commensal or parasitic species (or larger taxonomic groups) that use this species as a host

Do they cause problems?

Deer flies are known as pests because the female will feed on the blood of certain mammals, especially humans. Many deer fly species can transmit diseases to the people and animals they bite. This especially causes problems with livestock such as cows and horses. In North America, they do not cause much disease in humans, but their bites are painful and annoying. (Lewis and Leprince, 1981; Luger, 1990; Lyon, 2013; Magnarelli and Anderson, 1980)

How do they interact with us?

Males may pollinate the flowers that they get nectar from. This would help the flowers to reproduce.

Are they endangered?

Chrysops excitans is not an endangered species.


Brian Steel (author), University of Michigan Biological Station, Brian Scholtens (editor), University of Michigan Biological Station, Angela Miner (editor), Animal Diversity Web Staff.


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University of Michigan Museum of ZoologyNational Science Foundation

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Steel, B. 2014. "Chrysops excitans" (On-line), Animal Diversity Web. Accessed March 01, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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