There are about 400 species of March fly in Australia. Some of these feed on flower nectar and pollen but the majority prefer a helping of blood.

Anyone who’s come into contact with a March fly, also known as a horse fly, is not likely to be very welcoming to the next one they come across.

March flies, common across southern Australia during summer, are known for their short, sharp and stinging bite. Many a time have we felt the shock of a bite on your leg and then been surprised when you look down to find the culprit is a fly—and still sitting there by the way.

Just when you thought the incessant buzzing of the flies in summer was bad enough, now they’re biting us as well.

Black fly with large green eyes

The March fly’s bite can deliver quite a sting. dracophylla/Flickr

Dr David Yeates, the director of our Australian National Insect Collection and an expert on flies (Diptera), says that the March flies are after our blood. Just like a mosquito, the female March fly bites us to get at our blood. It then uses the protein in the blood to develop eggs, which give rise to the next generation of March flies.

So that explains why they are so insistent.

Dr Yeates says that the painful and itchy reaction that can follow a bite is caused by the anticoagulants the fly injects us with when feeding on our blood. The anticoagulants are chemicals in the fly’s saliva that prevent our blood from clotting and ensure a steady flow for them to feed on—again, this is similar to a mosquito.

There are about 400 species of March fly in Australia. Some of these feed on flower nectar and pollen but the majority prefer a helping of blood.

March flies are not too picky about where the blood comes from either; if you’re warm blooded then you’re a target. Horses come in for quite a bit of attention, hence why the flies are also known as horse flies. Dr Yeates says that in North Queensland the flies have even been seen feeding on Crocodiles!

Dr Yeates was recently interviewed by the ABC about March flies, read the interview.

10 comments

  1. These buggers have my Labrador freaking out. They are trying to bite her around the bum. As soon as one lands on her she reacts like she’s seen a ghost & high tails it back into the house.

  2. Got bitten down the beach on two occasions this summer. Horrible things and the lumps I get are horrendous. Must be a particularly bad season. Summer 2021 in Victoria. Keep a lookout. I even spotted one in my garden today. They are coming for me 🙂

  3. Don’t wear dark blue work shirts in SW WA in march fly season, they love them and pierce them with bloodthirsty glee. They are also attracted to other blue items like 200L plastic empty water barrels. Why blue, when yellow was the colour for insect trapping pans? How could they smell our blood? Do we smell to them like horses and kangaroos? Like mosquitoes, do they detect our CO2? Do they detect animal movement by eyesight? It can’t be body heat, they’re at us on 40 degree days. Unlike mozzies they are silent assassins. Who are they in turn feeding? Should we let them feed on us so we can save some threatened creature that feeds on them? Please explain! I don’t want to depend on noxious chemicals to ward them off while working in a vineyard, as much as I enjoy feeling like the Brothers Grimms’ 7-in-one-blow little tailor.

  4. Hi Steve. Loved your comments and could relate. Marchies have hit here at Wonboyn this week (in isolation)
    Yesterday was a great “trophy” day… I swatted 21 during our lunch on the balcony.

  5. In January and February 1974 I did two months field work for my Uni research, on the plains behind the Pilbara Coast, at a place called Sherlock Bay roughly half way between Roebourne and Port Hedland.

    I had previously encountered, I think, two March Flies in my life, at the beach in Adelaide. But at this place they were teeming — about like a dense location for mossies. There were three different kinds; from ordinary bush-fly size to brown ones bigger than a bee. They all gave a hell of a jolt with their bites. They were incessant and everywhere. They were so bad and prevented so much work that the company arranged for the David Grey pesticide company to fly up a specially-formulated, hand-labelled batch of super-strength foam/cream/muck for us to rub all over our bodies as a continuous coating!! This did somewhat reduce the assault for an hour or so after each application, but I’ve always wondered what huge dose of pesticides we absorbed thro our skins.

    But really, the only half-remedy was absolutely constant vigilance — they fly quite slowly, and so if you had your “radar” or “antennae” carefully tuned, and kept pivoting your head in ALL directions (throughout ALL daylight hours) you could spot most of them and swat them. Some still got you though; probably a dozen bites a day for the whole two months. So the worst upshot was this : by the end of the two months I was so thoroughly psychologically “conditioned” by the incessant hyper-vigilance and intermittent stabbing bites, just like a rat with two months of electric shocks in a cage, or like Pavlov’s dogs, that any flying speck, no matter how tiny, innocent or distant, provoked an instant flinching and twisting motion (preparatory to swatting). So the sudden appearance of a butterfly, for example, even if I was standing in a group of people at something formal, would provoke this alarmingly traumatised reaction!!!

    The psychological “damage” lasted for years, only VERY gradually fading, and I’d say that today, even 45 years later, I am still slightly or somewhat abnormally reactive and jumpy to flying insects that get close to me.

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