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Identification & Distribution:


Chrysops caecutiens is a medium-sized deerfly with a body length of 9-10 mm. The female has the first two abdominal tergites yellow, with a 'splayed' black marking usually in the form of two crossed straps (see first picture below). These black markings are sometimes much reduced or may expand to make tergite 2 mainly black. There is an extensive clear patch near the anal margin of the wing. Ventrally the first two segments of Chrysops caecutienshave a black median stripe (see second picture below).


At least the distal half and usually all of the middle tibiae are usually black (see both pictures above), although the first tarsal segment is reddish brown. The first picture below shows the red and green reflections of the eyes with a distinctive pattern of spots. As with all tabanids, the pattern is much less evident in dried specimens.


Picture of male by Pristurus (Own work), CC-BY-SA-3.0 via Wikimedia Commons

The male Chrysops caecutiens has extensively darkened wings. It also has black abdominal tergites, although the sides of tergites 1 and 2 may be narrowly orange. The face has large bare spots almost meeting in the middle line and a distinctive pattern of pale-dusted patches occupying about one-third of the area.

Distribution & Seasonal Occurrence

The splayed deerfly is found in wet woods and the shaded parts of bogs and marshes. In Britain Chrysops caecutiens is most frequent in the south, but becomes scarce in northern England and is rare in Scotland. It is widely distributed over western, central and northern Europe. The flight period is from mid May to early September, peaking from late June to late July.


Biology & Ecology:

Blood feeding

As with Chrysops relictus, there is very little detailed information on the host range of Chrysops caecutiens. It is known to feed on large mammals including cattle, horses and deer, but is also very ready to bite man if available.


Females attack in ones and twos, and almost invariably encircle the top and back of the head several times before alighting on hair and moving to a place where they might bite. They may also alight on the forehead (Grayson, 1997 ).

Nectar feeding & puddling

Kniepert (1980) found that only 3.8% of Chrysops caecutiens (N=156) had either fresh or digested blood in the midgut. 80.0% of flies tested positive for fructose (N=95) indicating recent nectar feeding. Compared to other tabanids, this was a low proportion with blood and a high proportion with evidence of nectar feeding.

Picture of male by Pristurus (Own work), CC-BY-SA-3.0 via Wikimedia Commons

Breeding sites

Eggs have been found on reeds over water. Larvae feed on organic matter in mud and silt at the edge of streams and rivers.

Trapping & odour attractants

We have regularly caught various Chrysops species using cow urine/acetone - baited NG2F traps in several habitat types in southern England (Brightwell & Dransfield, 2014). However, catches of Chrysops caecutiens were very low in all the habitat types sampled.


Disease transmission

There is some evidence that this (and other) tabanid species may transmit Lyme disease (Borrelia sp.) to man (Stanek et al., 1987; Luger, 1990), although the main vector for Lyme disease is Ixodes ticks.


  • Brightwell, R. & Dransfield, R.D. (2014). Survey of Tabanidae (horseflies) in southern England 2014. A preliminary survey of tabanids using odour-baited NG2F traps. 14 pp. Full text

  • Grayson, A. (1997). Personal notes on attacks by female tabanids. Larger Brachycera Recording Scheme Newsletter 15Full text

  • Kniepert, F.W. (1980). Blood-feeding and nectar-feeding in adult Tabanidae (Diptera). Oecologia 46, 125-129. Abstract

  •  Luger, S.W. (1990). Lyme disease transmitted by a biting fly. New England Journal of Medicine A 322 (24), 1752 (Jun 14). Full text

  •  Stanek, G. et al. (1987). Epidemiology of borrelia infections in Austria. Zentralbl Bakteriol Mikrobiol Hyg A. 263(3), 442-449. Abstract