Large Marsh Horsefly (Tabanus autumnalis) Photo by Donna Derrick

Dear Readers, horseflies (from the family Tabanidae) are big, robust insects (the one in the photo was almost an inch long), and have a fearsome reputation for biting – they feed on blood although, as with midges, only the females bite. Still, with a horsefly bite you will definitely know you’ve been bitten – I had an encounter with one while on a field trip to Slapton Sands in Devon many years ago, and it felt rather as if I’d been got at with a staple gun. The mouthparts of horseflies are perfectly evolved for their task, with two stout piercing mouthparts (that look rather like daggers to me) and a tongue designed for mopping up the resultant blood. Some horseflies are noisy in flight so you can at least hear them coming, while others (such as clegs) are quiet and bite without warning. Interestingly, although you might not know that you’ve been bitten by a mosquito or midge until later in the day, the immediate pain from a horsefly bite means that they are usually shooed off (or swatted if unlucky). This in turn means that they need to feed from more than one animal to get a stomach full of blood, and increases the risk of them transmitting disease from one animal to another.

Horsefly (Tabanus atratus) Photo By USGS Bee Inventory and Monitoring Lab from Beltsville, USA – Tabanus atratus, U, Face, MD_2013-08-21-16.06.31 ZS PMaxUploaded by Jacopo Werther, Public Domain, https://commons.wikimedia.org/w/index.php?curid=28757404

Mainly, however, horseflies choose non-human mammals to feed upon – as the name suggests, horses are popular, along with cows, deer and, in other parts of the world, even elephants. Fossil horseflies have been found from well before mammals evolved, suggesting that they may have originally obtained their blood from dinosaurs. However, there is increasing evidence that the striped pattern on zebras evolved, in part at least, to confuse the horseflies that normally feed on them – it’s thought that the patterns make it difficult for the flies to orientate themselves for landing. Experiments where horses wear striped blankets have been shown to reduce the number of horsefly bites.

One striking feature of horseflies is their eyes – they are often absolutely massive, as you can see in Donna’s photograph above. In living horseflies you may be able to spot incredible bands of colour spreading across each eye, though these fade when the insect is dead. As I might have mentioned before, you can tell whether most species of fly are male or female by looking at the gap between their eyes – in the extraordinary photos below, the female has a broad band between her eyes, whilst the male’s eyes are jammed in close together. I’m not sure whether the difference in colour between the sexes is just due to the angle of the photograph, or is true sexual dimorphism. Either way, it’s fascinating to see how beautiful these much maligned creatures are when looked at in the right way.

Female Striped Horsefly (Tabanus lineola) Photo by Thomas Shahan at https://www.flickr.com/photos/opoterser/4915106328

Male Striped Horsefly (Tabanus lineola) Photo by Thomas Shahan at https://www.flickr.com/photos/opoterser/4189239614

So, how does a horsefly spend its days? Of course there are differences between species, but generally eggs are laid in damp places, and the larvae, on emergence, fall either into the water or onto the soil, whereupon they turn into voracious predators, eating all manner of other invertebrates. The larvae then move to drier land to pupate. The males emerge first (we’ve seen this tendency in everything from hairy-footed flower bees and midges to common frogs) and may swarm in large numbers, looking for females. Fortunately, the males are only really interested in pollen and nectar, and indeed several species are important pollinators. The females hatch, mate and go looking for blood (literally). Once they have imbibed enough protein to ensure that their eggs will develop, they will lay them on plants close to water, so that the cycle can begin all over again.

Female horsefly laying eggs (Photo by By Bernard DUPONT from FRANCE – Horse Fly (Tabanidae) female laying eggs, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=40740210

The UK is home to about 30 species of horsefly, and here is one of the prettiest – the Golden Horsefly (Atylotus fulvus), now Nationally Scarce and found mainly in southern England, in particular the New Forest, Hampshire and Surrey. Look at this little beauty!  This one is a male (look at the eyes) and he’s going about his business feeding on the flowers.

Golden Horsefly (Atylotus fulvus) Photo by By AfroBrazilian – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26873272

So, what splendid insects horseflies are! For sure they can be irritating if you’re unlucky enough to get bitten, but they look to me like the Lancaster Bombers of the insect world, big, sturdy and actually rather beautiful. Plus, horseflies are eaten by birds (especially the larvae), and their eggs are  parasitised by a whole range of fungi, nematode worms and parasitic wasps. In the USA adult horseflies are even hunted down by the rather magnificent Horse Guard Wasp (Stictia carolina) which hangs around horse paddocks and takes the flies to provision its larvae. Interestingly, it’s been noted that horses are not disturbed by the large size and rattling flight of the Horse Guard wasps, almost as if they know that the insects mean them no harm.

Horse Guard Wasp (Photo By Howard Ensign Evans, Colorado State University, Bugwood.org, CC BY 3.0 us, https://commons.wikimedia.org/w/index.php?curid=10969866)

And finally, a poem. If you aren’t familiar with Michael Rosen, I invite you to have a look at his extraordinary body of work, from his book ‘The Sad Book‘ about the death of his son, to his many children’s books, to his book about his time in hospital with Covid, during which he was in an induced coma for several weeks and came very close to death. He has done a tremendous amount to encourage children to read, and to combat racism and anti-semitism with his stories and poems that try to tell the stories of marginalised people. But look, he has also written an entomology poem, illustrating the difficulties of identifying flies (and goodness knows there are enough species about to confuse anybody). I hope you enjoy it! PS My guess is that the horseflies that weren’t houseflies might have been deerflies, which are a kind of horsefly. Hope that clears up the confusion 🙂

Flies by Michael Rosen

I know flies. I’ve camped with them.
I heard how they eat. A lot of them land
on things and put down their proboscis that
sits at the front of their heads. Some saliva
comes out and this starts to digest whatever
they’re sitting on. Then they suck the stuff
that they’ve started to digest back up their
proboscis. You can feel that saliva moment
just after they land on your skin, slightly moist,
slightly cool. Then there are the biting ones, that we
call horse flies. Their probosces are like daggers.
They jab that into your skin and suck the
blood up through the dagger. I thought I had
all this figured. The flies that do the saliva thing
are the ones we call house flies and the
bigger house fly type are blue bottles. And there are
some shiny green ones that love horse shit.
And the horse flies come in medium and large,
the medium ones are nippy and when they
land on you, you can hardly feel it, until they
stab you with the dagger. The large ones are
like flying caterpillars, fleshy and angry, and
a bite from them is like being attacked by a
fork-prong. Once I saw one by a swimming
pool waiting to get my shoulder. I grabbed a
flip-flop and threw it and it hit it, first time. End
of horse-fly. I’ve tried a hundred times since
and never got one. That’s it, I thought: house
flies and horse-flies. Then one day we were
sitting at a table and I felt something bite me
and I looked down and all I saw was a fly. But
that kind of fly doesn’t bite. A house fly. Then
there was another. These little house flies
were biting me. I got one and when it fell off,
there was a little drop of blood on my leg. I
picked it up and looked closely: its proboscis
didn’t have the little spongey saliva bit on the
end. It was pointed like a tiny needle. It was a
tiny horse fly disguised as a house fly. It didn’t
say it was a horse fly. It just turned up acting
like it was any old house fly but then did the
horse fly thing in my leg. Not just one of them.
There were hundreds of them. And under the table.
Always under the table.