It's been 17 years since the cicada horde known as Brood II last peeked its mandibles above the surface of the Earth, to court, mate and expire by the hundreds of millions. News stories from 1996 recount drifts of shed exoskeletons, so deep in places that driveways had to be cleared by snow shovel.
Now, after a long and celibate hiatus underground, their progeny have themselves begun to show their faces along the East Coast. They, too, will have their brief day in the sun, sowing a new generation and continuing a pattern that has puzzled entomologists for decades.
Why the long absence underground? The curious phenomenon of the cicada's periodical life cycle is the subject of much debate among scientists, who are limited to no small extent by the infrequency of the insect's visits to the surface. Most agree, however, that climate shifts -- notably the rapid warming following the end of the last ice age -- have played a role.
There are seven species of periodical cicadas in North America, four bound to a 13-year cycle, three in a 17-year cycle. All are characterized by black and orange bodies, and males woo their mates with species-specific choruses that can be deafening in large numbers.
The genetic similarity of these seven species suggests a common ancestor in the last 8,000 years. And because emergence seems closely linked to soil temperature and moisture, it is likely that climate has played a role in both regulating their life cycles and cueing their appearance.
If the climate were to shift rapidly, as happened after the mid-Holocene period, it could alter the insect's life cycle again, said John Cooley, a professor at the University of Connecticut's Department of Ecology and Evolutionary Biology.
"Unfortunately, we don't have very detailed records of climate [and brood emergence]," he said. "We do know that they favor warmth and moisture. If it's warming up and getting damper, we could see them move north."
The calm before the swarm
Given the short window of time periodical cicadas have to meet and to mate, synchronous emergence is critical to their reproductive success. Without venom or camouflage, the cicadas' main defense is in numbers -- they simply provide more food than predators can eat.
Stragglers do sometimes emerge ahead of or behind the main mass of the brood, but these tend to be picked off before they can continue their line.
If the climate were to shift powerfully and abruptly enough, however, it might be enough to bring large numbers of insects to the surface at once -- even years outside their scheduled date of emergence. In fact, some scientists believe such an abrupt change may have split the cicada family down the middle, resulting in the two distinct yet closely related branches -- the 13- and 17-year species.
"An extreme climatic stimulus might cue large numbers of cicadas to shift, perhaps even enough to satiate predators," Cooley and colleagues wrote in a 2003 essay on climate shifts and periodical cicada life cycles.
As the planet warms -- and as that warming accelerates due to man-made climate change -- "the cicada may yet reprise its role as climate indicator if its cycle is disrupted by a warming planet," Wildlife Conservation Society entomologist Craig Gibbs wrote in an op-ed last week in The New York Times.
Climate might move even faster than the cicadas, Cooley said. "Their response time isn't instantaneous. If their density is too low [upon emergence], they're not going to make it."
A nuisance, but not a danger
While often confused with locusts -- grasshoppers in a specific swarming phase -- cicadas threaten neither humans or their food supplies. After 13 to 17 years feeding on tubers and roots below ground, they don't eat at all after coming above.