By Julianna Kettlewell
BBC News Online science staff
A swarm of cicadas has emerged after 17 years underground in the eastern US.
The noise is beginning to grow
Trillions of the insects will blanket the landscape in a frenzy of breeding, before dying en masse in June.
The bugs, which have the longest known adult-to-adult cycle, will have to contend with a more developed world than the one they left behind in 1987.
Some scientists say urbanisation is endangering periodical cicadas: at least one population is already extinct and others are at risk, they fear.
This month though, if you are in the thick of the action, it is hard to imagine a time without cicadas.
"There is loud singing in the tree above me right now and the sidewalk is littered with dead bodies," Jenna Jadin, an evolutionary biologist from the University of Maryland, told BBC News Online.
"It's amazing - there is a covering of 'shells' pretty much everywhere."
The eastern states of America have several broods of periodical cicadas, which have either 13- or 17-year cycles.
It is quite an event when an "appointed year" arrives.
This year it is the turn of Brood X, which is swarming over about 14 states including Georgia, Indiana, Kentucky, New York and Ohio - in densities of up to 3,000kg/hectare.
17-YEAR CICADA LIFECYCLE
1. Female lays eggs and dies soon after. Eggs hatch.
2. Bugs or 'nymphs' drop to the ground
3. Nymphs live underground feeding on tree roots
4. After 17 years, nymphs tunnel to surface, crawl up trees and shed skins to become adults
5. Adults mate during May and June of 17th year
"A lot of people are really excited," said Dr Jadin. "But there are people that are a little bit frightened by it, too."
Periodicals spend most of their lives as juveniles, feeding quietly on tree roots and growing slowly.
Then, when breeding age is finally reached, each brood's "patch" will witness a short - but dramatic - onslaught, before the cycle goes full circle.
The males spend their brief time in the sun courting females with high pitched trills, made by vibrating abdominal drums called timbales.
"You hear this constant droning, like loud traffic in the distance, and they make this clicking noise at the same time," Dr Jadin said.
A single brood of cicadas can include up to three species, which meticulously coordinate their emergence to create an impressive swarm.
The reason they synchronise like this, is to make use of the "safety in numbers" principle.
Unlike annual cicadas, which are quick and agile, periodicals are sluggish and apparently dull-witted. They make an easy meal for anyone who fancies it, so blending into the crowd is their only hope.
Cicada casings litter the ground
"With periodical cicadas it is entirely safety in numbers," explained Dr Jadin. "You can walk up to these things and pluck them off a leaf."
Racoons, foxes, skunks - and evolutionary biologists - are enjoying the rare feast.
Dr Jadin, who has actually written a cicada recipe book, admitted: "Chocolate-covered cicadas are my favourite.
"I am a fan of British chocolate myself - so I have been covering them with Galaxy or Cadburys."
Brood X might cut an imposing sight today, but their numbers are not as stable as they might seem.
The world has changed since they burrowed into the ground, 17 years ago, as freshly hatched nymphs.
While they were preparing for their month of glory, their habitats have been paved over by parking lots, enormous shopping malls and large tracts of homes.
Thousands of cicadas, entombed in concrete, will be unable to make it to the surface.
"The eastern US corridor is so developed that cicada habitats have been destroyed," Cole Gilbert, an entomologist from Cornell University, New York, told BBC News Online. "They need gigantic numbers to swamp their predators and survive."
Because periodical cicadas are slow and defenceless, they hit a real danger zone if their numbers fall below a critical minimum - whole populations can collapse.
"I don't want to be an alarmist," continued Professor Gilbert, "but at the edges, where the populations become smaller, they can die out."
He added: "It can happen. There was a brood called Brood XI in Connecticut and I think that one went extinct in the early 70s."