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.

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.