It is the background sound to sausages being turned on the barbecue, to kids splashing in the pool, to the crack and fizz of opening that first beer – the dull, drone song of the cicada.

Some love the song, others hate it, but for many people, especially those with large backyards, it is ever-present during the warmer months, a sound that seems to come from an insect you can never find, no matter how much hunting you do ... and then, towards the end of summer, the sound ends abruptly and the only evidence of it you can ever find is strange, brown shells that crumple to the touch.

“It is just a normal part of the summer celebration, the soundtrack of summer,” says one of Australia’s few cicada experts, Professor David Emery.

“There used to be loads of them around. I used to catch them and bring the shells home and put them on the Christmas tree.”

While they may evoke sweet associations for us, a cicada’s life is, in fact, one of solitary, years-long darkness punctuated by a sudden festival of sex. Then death. The song we hear at the height of summer is the cacophony of carnal festivities as the creatures go out with a bang.

What are cicadas, and why do they make such a racket? Why does it happen at dusk? And why only in summer?

What is a cicada?

Cicadas are a wonderfully diverse and ancient insect. Fossilised versions date to more than 200 million years. There are about 3000 described species and perhaps 1000, named and unnamed, roosting across Australia – no one really knows the true number.

Australia has by far the highest diversity of cicadas of anywhere in the world (South-East Asia and the Americas are next), probably because the creatures have been here for so long – at least since the last Ice Age, says Emery, a professor of veterinary parasitology at the University of Sydney.

“We don’t know a hell of a lot about them – there are only a limited number of cicada scientists about,” he says. “We don’t really know what the triggers are for them to emerge, except for rain. But what causes the major plagues we see some years, and virtually none next year?”

Today’s cicadas come in all shapes, colours and sizes, ranging in length from about 10 millimetres to 12 centimetres – and each has its own distinct song.

Scientists who study them clearly care deeply about these creatures, and have given them some truly fantastic names: Blue Moon, Black Prince, Yellow Monday. The Floury Baker looks as if it is covered in white dust. As the name suggests, the Cherrynose – also known as the Whisky Drinker – has a red “nose”.

The Greengrocer or Masked Devil (Cyclochila australasiae) is the most common cicada in Sydney and Melbourne. Its luscious, leafy-green shade is just part of the colour spectrum of the Australasiae species, which ranges from yellow (thus the Yellow Monday) to turquoise (the Blue Moon).

A cicada’s life begins as an egg tucked in the cracks of tree bark.

“They take a long time to hatch, usually over 100 days,” although some can hatch in as few as 10 days, says Dr Max Moulds, an entomologist and senior fellow at the Australian Museum.

After hatching, the tiny cicada (known as a nymph) falls from its tree and tumbles to the ground. It weighs so little the fall does not injure it. But rather than flutter away to begin a perilous journey to adulthood, the nymph digs a hole for itself – down towards one of the tree’s roots, which it latches on to.

A Greengrocer will spend six years down here in the depths – although no one is sure exactly how long – feeding on the tree’s sap, becoming fat, brown and misshapen with huge front legs and tiny hind legs, a subterranean creature.

“Most of their life is spent underground,” says Emery. “Depending on size, they only get a week to six weeks above ground. And during that time, their whole energy is spent finding a mate – a very justifiable pursuit.”

Eventually, when a cicada is mature, it emerges from the earth, crawls up the trunk of its tree and holds on for dear life.

This happens between September and November, usually after heavy rain. The cicada then begins to moult. A crack appears along its back and, over about two hours, the creature pushes out through it, emerging as a fully grown, winged and colourful adult.

“They usually emerge at night. They climb to a safer position and dry their wings,” says Moulds. “Usually, by the next day, the males are ready to sing.”

To the human eye, all that remains are brown shells attached to the tree or strewn around its base, the husks of cicadas’ former selves, discarded as they crawl, finally, towards the main event of their lives.

How (and why) do they sing? And why won’t they shut up?

After its years underground, the newly hatched Greengrocer cicada is frantic. It has just weeks to find a mate before it dies. To do that, it uses song. Think of the cicada’s song as a homing beacon.

In the larger cicada species, the female cicadas will fly towards the sound, zeroing in on their mate’s location. The sound also lets a female cicada distinguish between different species. Cicada songs are sufficiently distinctive that scientists use them as the basis for designating between closely related species.

“Every species has a slightly different song, either in frequency or the pattern of the pulses. Some have a continuous sound; others produce little pulses of sound in various combinations. That’s what the females recognise,” says Moulds.

That’s the simple story. But a cicada’s call is also as diverse as a book of songs. Many, including Greengrocers, have a warm-up sound.

“It sounds like a rev-up at the start, like a ‘brum brum brum’,” says Emery, imitating a car engine.

They move into the calling song, as it is known, which for the Greengrocer is a long, monotonous burr. Then, as a female draws closer, the cicada will generally shift to a quieter courting song, calling her to his exact location. “That tends to be more muted, as you might expect – you cannot go shouting in your partner’s ear,” says Emery.

Many cicadas will also make a distress call, which sounds like a sharp squawk, if they are picked up by a bird or a human. Most mate in silence but some excitable species make a lot of noise during sex, the sound “a bit like a distress call, particularly Greengrocers,” says Emery.

The Hairy Cicada, which lives only in Australia’s alpine regions, is one of the world’s oldest cicadas – probably one of those left over from an Ice Age – and is a bit on the quiet side. They don’t make a sound we can hear at all; instead they vibrate branches to send messages to potential mates.

It’s a shame that the type of cicada we’re most used to hearing, the Greengrocer, also happens to have one of the less flamboyant calls. Compare more rowdy types, such as Queensland’s Bagpipe Cicada, which can inflate its abdomen to a huge size and use that air to power its song. The Golden Emperor can sound like he’s yodelling. The Double Drummer produces a deafening ruckus if you’re standing too close. Others emit more offbeat sounds, akin to fishing-rod flywheels or lawn sprinklers.

Still others have structures on the wing and the back of the head they can rub together, akin to playing a fiddle, which sounds a bit like rubbing a pencil across a mesh-wire door. Then there are the “tickers”: they communicate using wing flicks, which sound a little like finger snaps. The males fly around, ticking their songs and listening out for a female wing snap in response.

But the way most cicadas make sound is by using a pair of ribbed membranes on their abdomen called timbal (sometimes spelt tymbal). The insect rapidly lengthens and contracts the membrane using muscles. Differences in the shape and ribbing on the timbal give the species their unique songs.

To get an idea of how this works, think of holding a piece of cardboard between your hands and wobbling it to make a “wup wup” sound. Now imagine doing this with a tiny piece of cardboard up to 10,000 times a second. The “wup wup” turns into a deafening drone. A hollow abdomen acts like a drum, amplifying the sound. The larger the insect the larger the drum and the louder the call, which helps females pick out bigger mates.

It’s long been said the cicada’s call is the loudest of any insect, but Guinness World Records does not keep this data. In 2014, University of Florida entomologist John Petti decided to find out. After consulting his colleagues and reviewing the scientific literature, he came upon the Shrill Thorntree of Africa, which can make sounds of up to 106.7 decibels from 50 metres away.

Yet Moulds has measured Australian cicadas, including the Greengrocer, at over 120 decibels at close range – that’s the level of sound a jet makes taking off.

Why don’t cicadas go deaf? To protect themselves, their ears tighten up when singing.

Why do they sing at night? And in summer?

It’s as the sun sinks that many cicadas really find their voice. Not only do they tend to sing at night but also during hot weather, and as a pack. Why? To escape predators. A single, singing cicada is extremely vulnerable. Programmed to go all out on displaying its ardour, it makes itself an easy target, signalling “come and get me” to suitors and predators alike. Birds, ants, spiders and bats all love a crunchy cicada – and they can follow the sound right to the source.

“If you’ve ever watched currawongs in the evening, when an isolated Greengrocer starts singing, they will find him, they will pick him off,” says Emery.

But if cicadas sing together, odds are only a few will get taken – not ideal, but better than the alternative. As an extra line of defence, the nature of the sounds that resonate from their hollow abdomens makes it hard for humans (but not female cicadas) to tell exactly where the song is coming from. If you get close enough to a cicada, it sounds as if the song is coming from everywhere.

Bladder cicadas, which have enormous abdomens, have extremely dull and resonant songs, making them impossible for birds to find. Instead, cunning currawongs will wait nearby and pick off the females when they start to fly towards the males.

If you really want to find a cicada, try looking on tree trunks in the early morning, particularly sites where you’ve already found shells, suggests Emery. Others sit in a tree’s canopy while still others set up in hedges, shrubs and grass.

“They all have their niche,” he says.

Early in the season, when there are fewer cicadas and individuals are more vulnerable to predation, they will wait for the cover of darkness to sing. But as Christmas nears and their numbers swell, they will boldly belt out their song in broad daylight.

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Why do they start to sing in the heat? Scientists aren’t sure. Perhaps because in extremely hot weather most predators are likely to be sheltering in the shade and not hunting for a meal; or, suggests Moulds, cicadas’ muscles work better at higher temperatures.

Most females mate once before finding a slit in a tree’s bark to lay their eggs. Males, on the other hand, seem able to mate several times before dying.

“My daughter found a Yellow Monday we called Stud because he sang very loudly in our azalea,” says Emery. “He mated three times – that we saw – before he died.”

If they are vulnerable, are they also endangered?

Fewer and fewer Australians get to experience the cicadas’ distinctive summer din every year, says Moulds.

“In the cities, they are certainly under threat,” he explains.

Traditional nature strips, especially those with trees running between footpath and road, are increasingly rare in many inner-city suburbs.

“Now it’s all paved. There is nowhere for the little babies to get under the ground or get up from the ground,” says Moulds. “In metropolitan areas, they are pretty well gone.”

And female cicadas prefer to lay eggs in young trees. Many of the trees that remain in the inner city are old – for a cicada. The insects are also under threat at beaches where spinifex once thrived, providing a home for small cicadas known as sand fairies, which would reliably harass beachgoers sweltering in the sun. Storm surges and the erosion of sand dunes have reduced their numbers, leaving many beaches silent now but for the crash of the waves.