Flies are a vital attendee at any good Australian Christmas. Their buzz is the symphony of our summer. Their coming is met with an "Australian salute". They always RSVP promptly to barbecues. They are really annoying.

But to the scientists who spend their lives studying them, a group bigger than you think, they are simply amazing.

"And there are many beautiful ones. Even the blowfly - it's metallic green, it's shiny," says Dr Christine Lambkin, curator of entomology at the Queensland Museum.

"We have a reputation for our bush flies that dates back to Captain Cook. He discusses them as being horrendous," says Dr Lambkin.

They are bad, but they are not the worst in the world, she says. That honour would have to go to the malaria-carrying insects of the tropics, says Dr Lambkin. Yes, mosquitoes are a type of fly.

Here's the first fact to know about flies: don't bother trying to swat them.

Flies have two wings plus two small "paddles" that give them extreme aerial agility.

Plus, their brains and nervous systems are perfectly adapted to dodging predators. In the time it takes your brain to command your arm to swat a fly, the fly has already seen you coming, reacted, and got away clean.

The second fact to know about flies: in most species, only the females annoy you. Here's why.

While there are about 30,000 species of fly in Australia, we only really come into contact with four groups: the bush fly, house fly, blowfly, and the mosquito.

Some, like the bush fly, are born from eggs laid in animal dung. They hatch into larvae, which feed off the dung, and then pupate in a cocoon like a butterfly. When it is finished developing, the adult fly literally headbutts its way out.

When they emerge, many have only a short time to live, maybe only weeks. So they get down to business pretty quickly.

"They want to find a mate, mate, produce eggs, and that's pretty much it," Dr Lambkin says.

The females of some species seek out humans because they are starving for protein, which they need to make eggs. The sweat on our bodies - and the moisture around our eyes, mouths and noses - is a great source they are desperate to suck up.

To some flies, we probably look like delicious human milkshakes.

There are many billions of flies in Australia. Beyond trying to sip from our eyeballs, why did they evolve?

"Flies have been here a long, long time, and they'll be here after we go. I'd say flies are probably asking, 'Why did humans evolve?' " says Dr David Yeates, director of the CSIRO's National Insect Collection.

Flies are excellent pollinators. Bees get all the publicity for pollination (producing delicious honey probably helps), but many studies suggest flies are just as important, says Dr Yeates.

They have also succeeded because they are "nature's little recyclers", says Dr Yeates, and they are very good at their job. Female flies are great at finding rotting organic matter, be that dung or dead bodies, to lay their eggs in.

The larvae then eat the matter and grow into flies - which are a great food source for birds.

"It returns it back into the ecosystem so it can be used. It turns a cow poo into bird food," says Dr Yeates.