Wild species 2010: chapter 17

Insects: Horse flies

Tabanidae - Family of insects in the Diptera order. This family includes any of the swift, usually large dipterous flies with bloodsucking females.

Photo of a Bothersome Deer Fly
Photo: Bothersome Deer Fly, Chrysops excitans © Stephen P. L. Luk

Quick facts

  • There are over 3700 species of horse flies worldwide, of which 144 have been found in Canada.
  • When excluding species ranked as Extinct, Extirpated, Undetermined, Not Assessed, Exotic or Accidental, the majority (85%) of horse flies in Canada have Canada General Status Ranks (Canada ranks) of Secure, while 9% have Canada ranks of May Be At Risk and 6% have Canada ranks of Sensitive.
  • Horse flies are found almost everywhere in Canada, except for the arctic islands.
  • Small horse flies with banded wings (Chrysops species) are called “deer flies.”
  • Most female horse flies need a blood meal in order to develop some of their eggs, but many do not need one to mature their first batch of eggs. Only the females are biting.
  • Males have enormous eyes that cover most of their head, and they can often be found hovering over hilltops, waiting for females to visit.
  • Larvae are predators of other invertebrates.


Horse flies are well known to most Canadians who venture out of urban areas in the summer. Even the thought of one of these large flies landing and biting can cause great anguish. In addition to causing physical and mental torment in humans and other animals, horse fly attacks have caused significant economic losses in the livestock industry.

Flies in this family are commonly known as horse flies or deer flies; but are also given other names such as clegs and stouts. The name deer fly specifically refers to species in the genus Chrysops; these flies are generally smaller than other tabanids, have banded wings, and golden or orange eyes with distinct black spots and blobs. Other horse flies, notably those in the genera Hybomitra and Tabanus, have eyes that are strikingly banded with iridescent greens and other colours. Interesting fact, the horse fly species Hybomitra hinei is the fastest known flying insect, having been clocked at 145 km/h for a brief instant as it took flight.

Only female horse flies bite. They feed on blood by using their knife-like mouthparts to slash skin, and then lap up the blood. Most females need a blood meal in order to develop at least some of their eggs, but many do not need one to mature their first batch of eggs. In these cases, enough nutrients are carried over as fat from the larval stage to complete the first egg-laying cycle. But blood meals are valuable because they allow females to lay many more eggs in subsequent cycles. However, based on studies elsewhere, some Canadian species (especially some Atylotus and Apatolestes) may never seek a blood meal. Many species have distinct preferences for certain hosts and for certain feeding locations on their hosts. Although female horse flies can home in on hosts using chemical cues (e.g. the carbon dioxide we breathe out), they rely primarily on their huge eyes to find appropriate targets.

Biting horse flies have been implicated as vectors or potential vectors of a number of diseases, including tularemia, equine infectious anemia, vesicular stomatitis, hog cholera, encephalitis, anaplasmosis, trypanosomiasis, and filarial dermatosis of sheep.

All adult horse flies need to feed to sustain energy and reproduce, and the primary energy source for both sexes is sugar from flower nectar or from insect (mostly aphid) honeydew. Most species are active only on warm, sunny days when there is little wind. Some species (e.g. Tabanus sackeni and Tabanus catenatus), however, are normally crepuscular or nocturnal, and others have been observed coming to lights during the night.

Because male horse flies don’t seek out animals for blood meals, people don’t often encounter them. They can be found on warm summer mornings at their special mating sites, where they hover either singly or in small groups, or wait on vegetation or rocks for females to pass by. Many species meet on hilltops in order to find each other. On one small hilltop in southern Quebec, more precisely at the Rigaud hill, 17 species have been observed over a season! The various species share these sites by hovering at different heights above the ground, or by gathering in forest openings of different shapes and sizes. The eyes of males are even larger than those of females – they almost cover the entire head – and the males use them to detect and capture fast-flying females as they buzz by.

Females lay eggs in compact masses, usually on plant stems or on the undersurfaces of leaves overhanging water or wet soil, where the larvae of most species live. Up to 800 eggs are laid in a mass. Eggs hatch almost simultaneously and the larvae drop to the substrate below. Larvae are predators on insect larvae and worms; however, the food habits of Chrysops species are unknown. In Canada, larvae overwinter, and when full-grown, migrate to an appropriate pupation site, where they remain for 1-3 weeks before emerging as adults. In the far north, some larvae may take 2-3 years to mature.

Parasitoid wasps (Chalcididae, Proctotrupidae) that attack tabanid egg masses are the most important control agents; most egg masses are attacked, and often half of the eggs are consumed. Larvae and pupae are also parasitized by various wasps and flies, but extensive studies in Ontario showed that these parasites account for only 2% mortality. Adult horse flies are eaten by birds, dragonflies, robber flies, and wasps.

Close-up photo of a horse fly Close-up photo of a horse fly showing the iridescent banded eyes of Tabanus similis
Photo: Tabanus similis © Tom Murray

Status of knowledge

Because of their biting habits, horse flies have had a certain amount of attention paid to them in entomological research, but most people would be surprised to find out how little we really know of the life history and ecology of most species in Canada. The broad picture of distribution is known for most species, but there are many regional gaps in basic distribution information. These gaps are reflected in the large number of Undetermined ranks at the provincial and territorial levels in this report.

The state of knowledge in Canada as of the 1980s was admirably summarized by Teskey (1990). This book includes identification keys, distribution maps, species accounts, and photographs of many species. For many species, the book remains the best account of the family, twenty years later. However, Thomas and Marshall (2009) have recently published an excellent online identification guide to the subfamilies Chrysopsinae and Pangoniinae east of the Rocky Mountains, which offers pictorial keys to adult females and up to date distribution maps for eastern species.

Older, regional identification manuals have been published for Ontario (Pechuman et al., 1961), Quebec (Chagnon and Fournier, 1943), the Maritimes (Lewis and Bennett, 1977; Thomas, 1978), and for the Chrysops of Alberta (Thomas, 1973). A manual of Illinois Tabanidae (Pechuman et al., 1983) is also noteworthy, since it provides, among other features, North American range maps and illustrations of and keys to larvae.

Richness and diversity in Canada

A total of 144 species have been found in Canada; species richness is highest in south-central Canada, peaking in Ontario (100 species, figure 17) and Quebec (75 species). Another five species are known to range close to the southern border and may eventually be found in Canada. The genus Tabanus is dominant in the southern half of North America and 30 species enter Canada, but these are generally restricted to the country’s southern latitudes. Like Tabanus, species in the genus Atylotus are generally midcontinental in range; 12 species extend into Canada. The genus Hybomitra, on the other hand, dominates the north and is represented by 46 species in Canada. Three species of Haematopota are found in Canada, including the wide-ranging Haematopota americanas and two much rarer or more reclusive species. The largely tropical and subtropical subfamily Pangoniinae is represented by only six species in the genera Apatolestes, Stonemyia, and Goniops; all of these species are confined to Canada’s deep south. Deer flies (genus Chrysops) are more evenly distributed, and are represented by 45 species. Other members of the deer fly subfamily are Merycomyia whitneyi, a rare species known in Canada only from three localities in southern Ontario and Nova Scotia; and Silvius gigantulus, restricted to southern British Columbia.

Species spotlight - Merycomyia whitneyi

Merycomyia whitneyi is one of Canada’s least-known tabanids. It is a large horse fly, 19-23 mm long, but is rarely seen, especially as an adult. This species is known from two sites in southern Ontario (Gilmour and Hamilton), but hasn’t been seen in the province since 1947. This population is apparently quite separate (disjunct) from the species’ primary range along the Atlantic lowlands. It was recently discovered in Nova Scotia, when three specimens were collected during studies of bogs in the southern part of the province. Normally, the larvae live in thick, flocculent silt deposits in stream backwaters. Because we know so little of this apparently shy species, it has been assessed for the time being as Undetermined in Canada.

Species spotlight - Chrysops excitans

This species is exciting in the sense that it is the commonest, most widespread, and generally most bothersome deer fly in Canada. Larvae live among moss and other moist vegetation at the edges of marshy lakes, peatland pools, and woodland swamps. It is found throughout Canada south of treeline, with the possible exception of the plains of southern Saskatchewan. Unfortunately for humans, its flight period is also extensive, from early June to late August.

Species spotlight - American Horse Fly

The American Horse Fly (Tabanus americanus) is the world’s largest (22-27 mm long) horse fly, but it is also one of Canada’s rarest. Its massive size, reddish legs, and white hair tufts make it easy to identify. Although its primary range is in the southeastern United States, there is an apparently small, isolated population in the Carolinian region of Ontario. It is ranked as May Be At Risk in Canada. Eggs are laid on foliage overhanging swamps, marshes and ponds. The larvae feed on aquatic insects.

Results of general status assessment

The majority (71%) of horse fly species have a Canada general status rank of Secure; 16% are not well known enough to rank (Undetermined), and 13% are considered May Be At Risk or Sensitive (figure 17 and table 24). Because of a general lack of information in many regions, the proportion of Undetermined species rises to over 26% when provincial and territorial ranks are examined. Those species considered Secure stay at much the same proportion as the national percentage (70%), whereas those ranked May Be At Risk or Sensitive drop to 4%.

Figure 17. Results of the general status assessments for horse fly species in Canada in the Wild Species 2010 report.
bar graph (see long description below)
Long description for Figure 17

Figure 17 shows results of the general status assessments for horse fly species in Canada in the Wild Species 2010 report. The bar graph shows the number of horse fly species ranked Extinct, Extirpated, At Risk, May Be At Risk, Sensitive, Secure, Undetermined, Not assessed, Exotic, and Accidental in Canada, each province and territory and the 4 oceanic regions. Of the 144 species occurring in Canada, 11 were ranked May Be at Risk, 7 Sensitive, 103 Secure, and 23 Undetermined. Of the 29 species occurring in the Yukon, 20 were ranked Secure, and 9 Undetermined. Of the 25 species occurring in the Northwest Territories, 21 were ranked Secure, and 4 Undetermined. Of the 25 species occurring in Nunavut, 2 were ranked Secure and 23 Undetermined. Of the 62 species occurring in British Columbia, 1 was ranked May Be at Risk, 5 Sensitive, 46 Secure and 10 Undetermined. Of the 50 species occurring in Alberta, 36 were ranked Secure and 14 Undetermined. Of the 37 species occurring in Saskatchewan, 31 were ranked Secure and 6 Undetermined. Of the 52 species occurring in Manitoba, 29 were ranked Secure and 23 Undetermined. Of the 100 species occurring in Ontario, 13 were ranked May Be at Risk, 1 Sensitive, 69 Secure and 17 Undetermined. Of the 75 species occurring in Quebec, 61 were ranked Secure and 14 Undetermined. Of the 58 species occurring in New Brunswick, 2 were ranked Sensitive, 44 Secure and 12 Undetermined. Of the 55 species occurring in Nova Scotia, 1 was ranked May Be at Risk, 35 Secure and 19 Undetermined. Of the 16 species occurring in Prince Edward Island, 6 were ranked Secure and 10 Undetermined. Of the 31 species occurring in Newfoundland and Labrador, 15 were ranked Secure and 16 Undetermined. There were no species listed as occurring in the oceanic regions.


Table 24. Canada ranks for horse fly species as determined by the National General Status Working Group.
Canada rank Number and percentage
of species in each rank category
0.2 Extinct 0 (0%)
0.1 Extirpated 0 (0%)
1 At Risk 0 (0%)
2 May Be At Risk 11 (8%)
3 Sensitive 7 (5%)
4 Secure 103 (71%)
5 Undetermined 23 (16%)
6 Not Assessed 0 (0%)
7 Exotic 0 (0%)
8 Accidental 0 (0%)
Total 144 (100%)

Threats to Canadian horse flies

Species considered to have some levels of risk of extinction are concentrated in the extreme south of the country, especially in southern Ontario and British Columbia. As they are for other inhabitants of wetlands, the major threats to horse flies of conservation concern are probably the loss and degradation of wetland habitat. These threats include the draining and/or filling of wetlands, and degradation through pollution (including agricultural pesticides and fertilizers) and siltation.


While many horse fly species are abundant, widespread, and often painful nuisances, some species are rare and live in habitats that are declining or even endangered. Eighteen Canadian species (13% of the fauna) are ranked as May Be At Risk or Sensitive. Most of these 18 species are confined to southern Ontario or the lowlands of southern British Columbia, regions where wetlands have declined and continue to decline.

Although there is some good published information on tabanids in Canada, much of the country is poorly known with regards to these flies. Future surveys in the north will fill in many knowledge gaps, and intensive surveys and research in the south will reveal much more about the species of concern.

Further information

Burger J. F. 1995. Catalog of Tabanidae (Diptera) of North America north of Mexico. Contributions On Entomology International 1: 1-100.

Burnett, A. M. and Hays, K. L. 1974. Some influence of meteorological factors on flight activity of female horse flies (Diptera: Tabanidae). Environmental Entomology 3: 515-521.

Butt, C., Hicks, B. and Campbell, C. 2008. The diversity and abundance of Tabanidae in black spruce forests and sphagnum bogs in Gros Morne National Park, Newfoundland, Canada. Journal of the Acadian Entomological Society 4 :7-13.

McElligott, P. E. K. and Lewis, D. J. 1996. Distribution and abundance of immature Tabanidae (Diptera) in a subarctic Labrador peatland. Canadian Journal of Zoology 74: 1364-1369.

Turner, W. J. 1985. Checklist of Pacific Northwest Tabanidae with new state records and a pictorial key to common species (Diptera: Tabanidae). Pan-Pacific Entomologist 61: 79-90.

University of Florida. Book of insect records. (Accessed August 10, 2010).

Wilkerson, R. C., Butler, J. F. and Pechuman, L. L. 1985. Swarming, hovering and mating behavior of male horse flies and deer flies (Diptera: Tabanidae). Myia 3: 515-546.


Chagnon, G. and Fournier, O. 1943. Les Tabanides du Québec. Naturaliste canadien 70: 49-84.

Lewis, D. J. and Bennett, G. F. 1977. Biting flies of the eastern Maritime Provinces of Canada. I. Tabanidae. Canadian Journal of Zoology 55: 1493-1503.

Pechuman, L. L., Webb, D. and Teskey, H. J. 1983. The Diptera or true flies of Illinois. 1. Tabanidae. Bulletin of the Illinois Natural History Survey 33: 1-121.

Pechuman, L. L., Teskey, H. J. and Davies, D. M. 1961. The Tabanidae (Diptera) of Ontario. Proceedings of the Entomological Society of Ontario 91: 77-121.

Teskey, H. J. 1969. Larvae and pupae of some eastern North American Tabanidae (Diptera). Memoirs of the Entomological Society of Canada 63: 147 pp.

Teskey, H. J. 1983. A revision of eastern North American species of Atylotus (Diptera: Tabanidae) with keys to adult and immature stages. Proceedings of the Entomological Society of Ontario 114: 21-43.

Teskey, H. J. 1990. The horse flies and deer flies of Canada and Alaska (Diptera: Tabanidae). Part 16. The insects and arachnids of Canada. Publication 1838, Agriculture Canada, Ottawa: 381 pp.

Thomas, A. W. 1973. The deer flies (Diptera: Tabanidae, Chrysops) of Alberta. Quaestiones entomologicae 9: 161-171.

Thomas, A. W. 1978. Records of horse flies and deer flies (Diptera: Tabanidae) in New Brunswick. Canadian Journal of Zoology 56: 1546-1549.

Thomas, A. W. and Marshall, S. A. 2009. Tabanidae of Canada, east of the Rocky Mountains 1: A photographic key to the species of Chrysopsinae and Pangoniinae (Diptera: Tabanidae). Canadian Journal of Arthropod Identification, No. 8. (Accessed April 14, 2010).

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