The chorus of a million cicadas

From North Carolina to Connecticut, billions of creatures with eyes the colour of blood and bodies the colour of coal are crawling out of the earth. Periodical cicadas are emerging en masse, clambering into trees and singing a shivering chorus that can be heard for miles.

What makes this emergence truly remarkable, however, is how long it’s been in the making.This month’s army of periodical cicadas was born in 1996. Their mothers laid their eggs in the branches of trees, where they developed for a few weeks before hatching and heading for the ground. “They just jumped out and rained down out of the trees,” said Chris Simon, a cicada biologist at the University of Connecticut.

Those larvae then squirmed into the dirt and spent the next 17 years sucking fluid from tree roots. Now, at last, they are ready to produce the next generation. The adult males are snapping rigid plates on their abdomens to produce their courtship song. The females are clicking their wings to signal approval. They will mate and then die shortly afterward. Their time in the sun is short, but their 17-year life span makes them the longest-lived insects known.

Natural selection

In any given species, the pace of life evolves. Natural selection is constantly shaping its genes, adapting it to its environment. How long a species lives and how much of that life it takes to reach adulthood are evolving just like every other trait.

For periodical cicadas , evolution favours growing up in sync. They can find protection from ravenous birds in huge numbers. There simply aren’t enough birds at any moment to eat a few billion cicadas at once.

This strategy has worked so well, in fact, that cicadas have lost their other defenses. They even fly sluggishly. When errant cicadas emerge in the wrong year, they are quickly eradicated by birds — along with their errant genes.

For a fast-growing cicada, Simon suspects, natural selection favours patience.  Evolution favours a long life in cicadas.

Only some of the periodical cicadas in the eastern United States are emerging at the moment. They’re known collectively as Brood II. In other regions, other broods emerge in different years. Last year, Brood I emerged in Virginia, West Virginia and Tennessee. All told, there are 15 broods lurking in the ground around the United States. Twelve have a 17-year cycle, and three have a 13-year cycle. To study all these broods, Simon and her colleagues usually spend spring travelling around the country. In a new study published in April, Simon and her colleagues reported that the common ancestor of all the periodical cicadas in the United States lived four million years ago.

Switching between life cycles

Its descendants then evolved with remarkable speed. “We know that during the last half million years, periodical cicadas have switched between 13- and 17-year lifecycles at least eight times,” said Simon. “There’s a lot of this life-cycle switching going on.”

Simon suspects the switch happens when populations of cicadas expand into territories where other cicadas are already buried in the ground. The immigrants that keep to their old schedule get wiped out, while any oddballs that happen to match the resident cicadas can blend in and survive.

“They just get sucked into the brood,” said Simon. That would explain how the different species in a brood always manage to follow the same timetable. To do otherwise spells doom.

Four years is, of course, quite a change.  How cicadas can make such a drastic switch remains for Simon and other cicada experts to discover. It’s possible that a minor difference in the DNA of cicadas can trigger it.

This summer, after Brood II is dead and its eggs have rained to the earth, Simon and her colleagues will be probing the genomes of cicadas for that switch, hoping to find another clue to one of the world’s great life cycles.