The Tabanidae family, commonly known as horse flies, clegs, and deer flies, comprises stout, medium-to-large sized insects that play a significant role in ecosystems and can pose a nuisance to livestock and humans. This guide delves into the identification, characteristics, and ecological importance of various genera and species within this family.
Understanding Tabanidae Anatomy and Characteristics
Tabanid flies are characterized by their very large eyes, which often display bands or patterns when the insects are alive. In males, these eyes meet or nearly meet at the top of the head, while in females, they are well separated. The female’s proboscis is adapted for piercing, enabling blood-feeding, whereas both sexes utilize their mouthparts for nectar feeding. Their antennae have a third segment that is ringed but lacks a style. Wing venation is distinctive, with veins R4 and R5 splayed widely towards the wingtip, and R5 converging with vein M1. A well-developed squama forms a large flap adjacent to the thorax. Globally, there are approximately 4,500 species of Tabanidae.
The Impact of Tabanidae on Livestock and Disease Transmission
Female tabanids are significant biting pests of vertebrates, particularly livestock. Their painful bites cause distress to grazing animals, leading to reduced weight gain, decreased milk yield, and lower feed utilization efficiency. In certain regions, these flies act as disease vectors. They can transmit equine infectious anemia virus to horses, trypanosomosis (Trypanosoma evansi) to camels, and anaplasmosis (Anaplasma marginale) to cattle. Deer flies (Chrysops) are also implicated in the cyclical transmission of the nematode Loa loa to humans. Despite their pestilent nature, both male and female tabanids feed on flower nectar, contributing to the pollination of some plant species.
Subfamily Chrysopinae: Deer Flies
Genus Chrysops (Deer flies)
Deer flies are recognized by their long antennae and wings that often exhibit dark bands or are largely blackish. The second abdominal tergite is frequently yellow with a black marking, or the abdomen is predominantly black. Their eyes are notably bright or bronze-green with dark spots.
Chrysops caecutiens (Splayed deerfly)
This medium-sized deerfly, measuring 9-10 mm in body length, is identifiable by the female’s first two abdominal tergites, which are yellow with a distinctive “splayed” black marking resembling two crossed straps. Variations exist, with reduced markings or an almost entirely black second tergite. A clear patch is present near the anal margin of the wing. Ventrally, the first two abdominal segments feature a black median stripe. The middle tibiae are typically black, along with at least the distal half. Dried specimens may show less distinct eye patterns. Male Chrysops caecutiens have darkened wings and black abdominal tergites, with narrowly orange sides on tergites 1 and 2.
Chrysops caecutiens inhabits wet woods and shaded areas of bogs and marshes, with a wide distribution across western, central, and northern Europe. In Britain, it is more common in the south. Its flight period spans from mid-May to early September, peaking in late June and July.
Chrysops relictus (Twin-lobed deerfly)
Chrysops relictus is another medium-sized deer fly (8-10.5 mm). Females possess a pair of diverging black lobes on the second abdominal tergite, joined at the base by a yellow inverted “V” mark. This feature distinguishes it from Chrysops viduatus. The clear area near the wing’s anal margin is suffused with black, creating a distinct spot. All tibiae are reddish-yellow, differentiating it from Chrysops caecutiens, which has black middle tibiae. Males also exhibit diverging lobes on the second tergite, though less pronounced. The wing beyond the median band has a narrow clear band or white spots, darkening towards the margin.
This species is found on damp moors and heathland throughout western and northern Europe. In Britain, it is frequent in the Midlands and southern England, extending into the Scottish Highlands. Its flight season is from mid-May to mid-September, with a peak in late June and July.
Chrysops viduatus (Square-spot deerfly)
The female Chrysops viduatus (8.5-10 mm) is characterized by a small, quadrate black spot at the base of the second abdominal tergites, which may be slightly bilobed, triangular, or heart-shaped and typically does not reach the tergite’s hind margin. The hind tibiae are entirely reddish-yellow, and the wing is clear behind the sub-apical spot. Males are similar to male Chrysops relictus but lack grey-brown shading near the wing’s hind margin. The middle tibiae are reddish-yellow, while the fore and hind tibiae are generally dark.
Chrysops viduatus inhabits wet meadows, mires, fens, and wet woodlands across Europe into central Asia. It is on the wing from late May to September, with its peak activity in July.
Subfamily Tabaninae: Horseflies
Genus Hybomitra
These medium-to-large horseflies lack wing markings. Their eyes are typically hairy, and they possess a raised ocellar tubercle. Both sexes often display three stripes on each eye.
Hybomitra bimaculata (Hairy-legged horsefly)
This medium-large horsefly (13-16.5 mm) exhibits various color forms in females. The commonest form has a dark grey abdomen with distinct whitish triangles along the dorsal midline (form bisignata). Other forms include lateral reddish spots extending to the third segment (form collini) or stopping at the second segment (form bimaculata). Distinguishing features include orange upper tibiae and unusually long hairs on the mid tibiae. It is active from May to August in woodland edge habitats and sheltered fens and marshes.
Hybomitra ciurea (Yellow-horned levels horsefly)
Hybomitra ciurea (14.5 mm) females have extensive orange side markings on the abdomen from tergite 1 to 4. The bases of the antennae are orange, hence its common name. This feature helps distinguish it from Hybomitra distinguenda, which has grey-black antennal bases. A more reliable separation involves examining the hairs on the lateral thirds of the second tergite: Hybomitra ciuraei has distinct areas of black hairs on the orange ground color, while Hybomitra distinguenda has only orange hairs. Males have a reddish-brown first antennal segment and upper eye facets significantly larger than the lower ones. This species is common across Europe and Russia but rare in Britain, restricted to grazing marsh in southeastern coastal areas. Its flight period is mid-June to mid-August.
Hybomitra distinguenda (Bright horsefly)
Hybomitra distinguenda (15-18 mm) females also display extensive orange side markings on the abdomen from tergite 1 to 4. The key differentiator from Hybomitra ciuraei lies in the hairs on the lateral thirds of the second tergite; in Hybomitra distinguenda, these are entirely pale-haired over the orange ground color, giving it a bright appearance. Males have a narrow black stripe on the abdomen and grey-black first antennal segments. This species is common throughout Europe and inhabits diverse biotypes, including wet heath, bog, and wet meadows. Its flight period is early June to late August, peaking in early to mid-July.
Genus Atylotus
These small to medium-sized horseflies have clear wings and yellowish, pale green, or grey eyes with narrow bands. The antennae are entirely orange, and femora are often orange. Males have hairy eyes, while females’ eyes are weakly haired or bare.
Atylotus rusticus (Four-lined horsefly)
Atylotus rusticus (12 mm) is ash-grey with inconspicuous pale yellow thoracic hairs, distinguishing it from Atylotus fulvus. The abdomen features indistinct longitudinal dark stripes formed by light and dark hairs. The femora are black except for pale tips, differentiating it from Atylotus fulvus and Atylotus latistriatus. The eyes are pale green with a narrow reddish band. Historically common in Cambridgeshire, it is now rare in Britain, found in coastal grazing marshes and other specific locations. Its flight period is in June and July.
Genus Tabanus
This large and diverse genus is characterized by completely bare eyes and the absence of a raised ocellar tubercle.
Tabanus bromius (Band-eyed brown horsefly)
Tabanus bromius (13-15 mm) has a dark, hairy abdomen with three longitudinal rows of yellowish or greyish-brown spots. The front corners of tergite 2 are pale yellow with yellow hairs. The mesonotum is grey with five indistinct longitudinal lines. A key feature is the bare, greenish eyes with a single, straight violet-red eye band. Males lack a distinct zone of small facets along the hind edge of the upper eye. It flies in July-August, commonly feeds on cattle and ponies, and is widespread in northern Europe. In Britain, it is relatively common in southern England.
Tabanus sudeticus (Dark giant horsefly)
This very large species (around 25 mm) has small, equilateral pale median abdominal triangles that do not reach the preceding tergites. The abdomen typically shows little to no lateral reddish coloration. This distinguishes it from Tabanus bovinus. Tergites have black or dark brown bands. The third antennal segment is reddish-brown basally and blackish apically. In life, the eyes are blackish-brown with a coppery sheen, and the parafacials have abundant black hairs. Males have extensively yellow-orange abdomens, with significantly larger upper eye facets. It is distributed widely in northern Europe and Britain, primarily found in boggy areas in the north and west, and also common in the New Forest.
Genus Haematopota
Haematopota species hold their wings roof-like over the abdomen at rest, and their wings display characteristic “rosette” patterns.
Haematopota pluvialis (Common or Notch-horned Cleg)
This species has pale brown wings, antennae that are partly reddish-yellow, and an apical notch on the first antennal segment. Females have distinctively patterned eyes. Their flight and feeding activity are dependent on humidity and temperature. Bites can be painful, and the flies are resilient. Haematopota pluvialis is found in a wide range of habitats from May to October across Europe, Russia, and China, and is the most common tabanid species in the UK.
Identification and Acknowledgements
The identifications provided are based on the best available knowledge, utilizing resources such as “Stubbs & Drake (2014)” and “Oldroyd (1969)” for taxonomic information. Distribution and biology data are supplemented by “Drake (1991).” Any errors are unintentional and corrections are welcomed.
