Tabanidae – horseflies | Curator of Diptera

Once more I am writing in defense of some very attractive but much-maligned creatures that, due to the maternal habits of the females, are universally disliked. People scream, run, swat them with wild abandonment to stop these ladies from providing essential resources to enable them to produce their next generation – it’s not very nice of us to let them get food for their offspring!

Macrophotograph of the head of the fly showing the dramatic colouration of its eyes

The Tabanidae have it… the dramatic banding of the eyes of Haematopota pluvialis

Yes, Tabanidae, or horseflies to give one of their common names, are some of the most painful biters of all flies but, like their also very much-maligned cousins, the mosquitoes, the males are vegetarian and can also be very important pollinators (e.g. Philoliche species found in South Africa – see the reference in one of my older blogs).

Horseflies have various common names including gadflies (after gad – meaning a spike) which has been now become common vernacular for an annoying person who is persistent in their criticism. These flies are not liked by many and there are species in Central and South America that further damage their reputation by transporting bot fly larvae around on them – you can be sliced by the adult horsefly and then chewed upon by the bot fly maggot! Horseflies are great vectors for bot flies as they are silent fliers and so are able to sneak up on us.

Biology

Female horseflies have some of the most impressive mouth parts having modified theirs into hardened stabbing devices with sharpened blades along the edges (also check out Anthony Thomas’ blog on mouth parts). Not all females are blood suckers and will often supplement their diet with some nectar.

If blood feeding, the females slice through the skin and then sponge up the pooling blood, and these flies are referred to as telmophages – pool feeders. They have anticoagulants which prevent mammal’s plasma from clotting and this results in often painful, itchy, swollen bites which on the rarest of rare occasions has caused an anaphylactic reaction which has sadly resulted in death.

These are the largest of the blood sucking flies and the UK species Tabanus sudeticus is one of our most impressive insects in terms of size (and also hardness ;)) (Fig ?) You can often hold the females and she won’t bite as she only feeds after mating. The larger species can remove up to 200 ml of blood… yes, that’s nearly half a pint but these are generally the feeders on large bovines.

The smaller species luckily only take between 20-30 ml, but that’s still 5-6 tsp! Don’t panic too much – the larger species generally go for a direct attack and so you are more likely to see them coming (the smaller ones are the sneaky ones…). And if you think that slicing and biting is the most unpleasant part she may also secrete a colourless fluid from her derriere whilst feeding to get rid of the excess fluid from the blood (termed malphigian secretion).

Photo showing the finished drawing of the dark giant horsefly, with the artist's hand holding a brush at the edge of the photo

Tabanus sudeticus, the ‘dark giant horsefly’, wonderfully drawn by Carim Nahaboo – check out his fly artwork

The males in comparison all have weak mouthparts as they are, as with most of the blood sucking species, vegetarians and as such don’t need such formidable mouthparts.

Photo of the horsefly mouthparts obtained via light microscope

Mouthparts of a horsefly with the sharp cutting stylets on the right and the spongy mouthparts on the left (photo by Alan R Walker, via Wikipedia)

Males and females can readily be told apart in this family as males always have holoptic eyes – that is their eyeballs join at the top of their heads, whilst the females always have an eye bridge or gap. Males spend more of their time seeking out the ladies and so need to have bigger eyes to do so!

They are often spotted on vegetation, waiting and watching. In fact many of the males have two different sized ommatidia (the clusters of photoreceptor cells that form the complex eyes of insects) – the larger ommatidia are located at the top of the eye and are more sensitive to UV, allowing him to detect in the sky the faster flying females, whilst the lower, smaller ommatidia enable them to resolve the visual details and hone in.

Macrophotograph focussed on the large eyes of the fly

The eyes of a male striped horsefly (Tabanus lineola) showing the larger upper ommatidia and the smaller lower ommatidia (photo by Thomas Shahan, via Wikipedia, CC BY 2.0)

And their eyes are stunning but sadly only when they are alive! Many of them have iridescent bands, squares and zig-zags, but these fade immediately upon death due to the eyeballs contracting and the reflective properties of them altering due to a change in the structure. The colours originate from the cornea colour filters and these also alter the spectral composition of transmitted light – these flies are superb at making out objects that you would assume naturally blend into the back ground (such as a deer in the wood for example).

If you were to rehydrate the dead fly (we do this by using your average kitchen vegetable steamer or a small amount of wet tissue for a limited time in a sealed container alongside the fly) once more you would be able to see the patterns of their eyes. These patterns can be used as diagnostic features in the field.

Photo showing the dramatic colouration and banding on a horsefly's eyeballs

Horsefly eyeballs

Taxonomy

Globally there are some 4,400 named species, of which the Museum collections contain more than half to three quarters of these described species. We hold 3,084 different Index lot records of species names, but lot records includes both the valid name and also any synonyms (an incorrect new name applied to an existing species) where we hold types for them.

The 3,084 different names therefore is going to be a higher amount than the actual real number of species in the collection. The collection does contain 4,672 primary and secondary types (specimens used for the original description) that makes it one of the most formidable collections globally for this family. At some point I have 290 drawers from the main collection to recurate…

In the UK there are only 30 species of which our collections have 15 drawers (again these need recurating and so this number will expand). There are only two sub-families in the UK but globally there are two more sub-families which contain the spectacular long-tongued horse flies (Pangoninae), and a very few odd-looking, fuzzy grey shore-loving species that have very reduced mouthparts (Scepsidinae). This latter subfamily is still a taxonomic confusion (where have you heard that before?).

Photo showing a drawer full of curated , pinned horseflies, with labels for family and subfamilies

The subfamilies of horseflies (Pangoninae, Chrysopsinae and Tabaninae)

The two sub-subfamilies found in the UK can easily be separated from each other by their wings.

Chrysopsinae

The Chrysopsinae subfamily has boldly patterned wings – either being dark or very dark and only the cosmopolitan genus Chrysops occurs in the UK. The members of this genus are commonly referred to as the deer flies and it contains some of our most annoying biters, often seeking out humans in preference to other animals. These like to bite at the back of our heads and have resulted in me whacking myself silly!

Within this genus there are 4 species in the UK, all of which are very distinguishable with their banded wings. These love green marshes and similar soggy habitats, predominantly in the south. In fact horseflies are generally warmth-loving species and so more frequently occur in the south than the far north. The larvae of these species are often described as Hydrobionts, alluding to their love of water.

  • Chrysops caecutiens (Linnaeus, 1758) – Splayed deerfly
  • Chrysops relictus (Meigen, 1820) – Twin-lobbed deerfly
  • Chrysops sepulcralis (Fabricius, 1794) – Black deerfly
  • Chrysops viduatus (Fabricius, 1794) – Square-spot deerfly (see image below)
Photo of the pinned specimen from above, showing its splayed, banded wings

Square-spot deerfly (Chrysops viduatus)

Abroad this is the genus that are vectors for the filarial worm that causes Loa loa in humans, which causes skin and eye disease.

Tabaninae

The other Subfamily is Tabaninae which is further split into two tribes – Haematopotini and Tababini, where the former has wings with brown/grey flecks or ring shaped markings and the latter has completely clear wings.

Haematopotini

This subfamily only has the genus Haematopota (Meigen, 1803) and these all have distinctly mottled wings and all are commonly referred to as the clegs.

  • Haematopota bigoti (Gobert, 1880)  – Big-spotted cleg
  • Haematopota crassicornis (Wahlberg, 1848) – Black-horned cleg (see image below)
  • Haematopota grandis (Meigen, 1820) – Long-horned cleg
  • Haematopota pluvialis (Linnaeus, 1758) – Notch-horned cleg
  • Haematopota subcylindrica (Pandellé, 1883) – Levels cleg

The last species listed above is a newcomer to the UK and there maybe another two that are yet to be described, and so currently sit under the names Haematopota sp.A and Haematopota  sp.B!

Photo showing the specimen from above, with wings splayed

Black-horned cleg (Haematopota crassicornis)

Tababini

Genus: Atylotus Osten-Sacken, 1876

  • Atylotus fulvus (Meigen, 1804) – Golden horsefly (see image below)
  • Atylotus latistriatus (Brauer in Brauer & von Bergenstamm, 1880) – Saltmarsh horsefly
  • Atylotus plebeius (Fallén, 1817) – Cheshire horsefly
  • Atylotus rusticus (Linnaeus, 1767) – Four-lined horsefly
Photo of the specimen from above with wings splayed

Golden horsefly (Atylotus fulvus (Meigen, 1804)). Photo © Steven Falk

Hybomitra

Genus: Hybomitra (Enderlein, 1922)

These are commonly (and I think rather sweetly) called the hairy-eyed flies. This is not necessarily so obvious in the whole genus but luckily the UK species are all densely haired.

Macrophoto focussed on the head of the specimen

Hybomitra ciureai female head (Minsmere, Suffolk) © Steven Falk

This has been a confusing genus with many misidentifications over the last 100 years. In the image below, you can see some of the changes that have occurred through 60 years of work in the UK. They can be incredibly variable, which has led to much confusion.

Photo of the page from the book depicting the differing names for the various species

Name changes in Hybomitra from 1909 to 1969 from Harold Oldyrods – Handbook for the Identification of British species

 

  • Hybomitra bimaculata (Macquart, 1826) – Hairy-legged horsefly
  • Hybomitra ciureai (Séguy, 1937) – Levels Yellow-horned horsefly
  • Hybomitra distinguenda (Verrall, 1909) – Bright horsefly
  • Hybomitra expollicata (Pandellé, 1883) – Striped horsefly
  • Hybomitra lurida (Fallén, 1817) –  Broad-headed horsefly
  • Hybomitra micans (Meigen, 1804) – Black-legged horsefly (see image below)
  • Hybomitra montana (Meigen, 1820) – Slender-horned horsefly
  • Hybomitra muehlfeldi (Brauer in Brauer & von Bergenstamm, 1880) – Broadland horsefly
  • Hybomitra solstitialis (Meigen, 1820) – Scarce Forest horsefly
Photo showing the pinned female from above, with wings splayed

Pinned female of Hybomitra micans. Photo © Steven Falk

Tabanus

Genus: Tabanus (Linnaeus, 1758)

And if you thought Hybomitra were slightly problematic, Oldroyd describes this genus as ‘large and ill-defined into which a large majority of species are still dumped’. The one thing that you can agree on the is that these are some of the larger species. 

  • Tabanus autumnalis (Linnaeus, 1761) – Large marsh horsefly
  • Tabanus bovinus (Linnaeus, 1758) – Pale giant horsefly
  • Tabanus bromius (Linnaeus, 1758) – Band-eyed brown horsefly
  • Tabanus cordiger (Meigen, 1820) – Plain-eyed grey horsefly
  • Tabanus glaucopis (Meigen, 1820) – Downland horsefly
  • Tabanus maculicornis (Zetterstedt, 1842) – Narrow-winged horsefly
  • Tabanus miki (Brauer in Brauer & von Bergenstamm, 1880) – Plain-eyed brown horsefly
  • Tabanus sudeticus (Zeller, 1842) – Dark giant horsefly (see the drawing near the top of the blog)
Photo showing someone holding a living fly in their fingers

Band-eyed brown horsefly (Tabanus bromius (Linnaeus, 1758)). Photo © Steven Falk

Tabanus sudeticus, or the dark giant horsefly, is as the name implies, is very large. They are the largest of UK species and I have to say the first time I saw them even I was a bit ‘hey, hey – you’re a mighty fly!’

Cars are great at attracting these magnificent creatures as they absorb the heat of the sun and as such are warm and inviting to the fly (very much like a cow you see). This species resulted in me receiving an email from my colleague in the Museum, of which I have had to slightly edit for the sake of the Museum’s reputation (as well as my colleague’s):

‘What the XXXX is this please?

About 25mm long, seen in Torridon, NW Scotland’

Doesn’t matter how long you are studying natural history, there are always things out there to scare the bejesus out of you!

3 thoughts on “Tabanidae – horseflies | Curator of Diptera

  1. Craig

    Very interesting overview. What keys are there available to identify the 30 British species please?

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    1. Erica McAlister Post author

      Hi Craig

      Yes there is – The rather wonderful book called British Soldier flies and their Allies by Alan Stubbs and Martin Drake has a key, illustrations and good colour images.

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  2. Johan Liljeblad

    That’s a really nice overview of tabanid biology and systematics. Can I just point out that not all of those author citations should go within parentheses? Like Tabanus bovinus Linnaeus, 1758 since the species was described in the genus Tabanus.

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