Staff File Photo by Chad McClure / A Brood X cicada climbs on a blade of grass in Diane Rizzo's yard on Suck Creek Mountain in June 2004, the last time the periodical cicadas emerged from their underground slumber.

After 17 years underground, billions of red-eyed creatures will soon arise from the Earth.

The mass emergence of periodical cicadas is considered one of nature's most extraordinary events — and one of its loudest. According to entomologists, swarms of the flying insect can produce sounds up to 120 decibels — comparable to an in-use chainsaw.

What separates periodical cicadas from all other insects is not just their long lifespan, but also their synchronized emergence. This spring, over the course of a week or two, billions of periodicals across 18 states — including Tennessee — will tunnel out of the soil. They will climb into nearby branches, shed their exoskeletons and, in search of mates, the males will begin to buzz.

Typically, the cacophony lasts only a month or so. But as conspicuous as periodicals are, their life history is largely mysterious.

Here, we burrow into what is known and unknown about the insect.


What is a periodical cicada?

There are 3,000 species of cicada in the world, all belonging to one of two categories: annuals or periodicals. Annual cicadas emerge every spring — and in much smaller numbers than periodicals, which emerge by the billions, once every 13 or 17 years.

Among periodicals, these two life cycles represent two distinct types: the 13-year, more common in the southern U.S., and the 17-year, more common in the north.

Tennessee, however, has both.

This year, the latter will emerge — more specifically, Brood X (say it, "Brood 10"), which is the name given to this particular collection of periodicals.

Essentially, a "brood" is a group of periodicals which belong to the same race (13- or 17-year) and emerge in the same region in a given year. And Brood X is considered one of the largest — with estimates of up to 1.5 million cicadas per acre across – spanning from New York to Illinois to North Georgia.


Why the prime numbers?

Cicadas don't bite or sting, nor are they poisonous, making them vulnerable to the many animals that eat them — from birds and lizards to rodents and snakes.

To ensure the survival of its species, the periodical adapted two clever tactics.

First, their sheer abundance ensures they won't all be consumed — a strategy known as "predator satiation."

Second, their prime-numbered life cycles.

The idea is that by emerging once every 13 or 17 years, cicadas prevent predators from matching their reproductive schedules. As an article by Nature Education further explains, "a 12-year cicada species could be wiped out by a predator species with a 2-, 3-, 4-, or 6-year life cycle because each cicada emergence would be met with a boom in the predator population."

However, all that is just a theory. Why cicadas adapted these lengthy, prime-numbered life cycles is largely a mystery. While they may make it difficult for predators, they also make it difficult for science.

some text
Staff photo by Dan Henry/Chattanooga Times Free Press - May 27, 2011. Discarded cicada skins cling to leaves on a tree at the Chattanooga Riverpark on a late Friday afternoon. Most North American cicadas have a 13 or 17-year life cycle living underground as nymphs for the majority of their lives before surfacing to molt shedding their skins and emerging as adults.


How do they know when to emerge?

Each spring, cicadas wait for the ground to reach around 65 degrees before emerging. Some researchers believe that their preference for warm weather may have contributed to their decade-plus developmental periods — beginning 20,000 years ago, when the last ice age may have kept them underground longer.

In regards to a cicada's ability to keep track of time — the leading hypothesis suggests that cicadas use an internal molecular clock, calibrated by the environment. Cicada nymphs feed on the sap of tree roots. The theory is that each spring, when the trees bud, a message is sent to the nymphs via their roots. After accumulating a respective number of these messages – 13 or 17 – the cicadas wait for the ground to warm, then tunnel to the surface.

In 2002, the University of California tested this theory by starting with peach trees that were supporting 17-year cicada nymphs — and which had already been underground for 15 years. Then, they manipulated the trees, causing them to bloom twice in a year.

Sure enough — the cicadas emerged a year early.


How to ID

Periodical cicadas range from 3/4- to 1 1/4-inch long. They are black-bodied with red eyes, red legs and red wing veins.

Annual cicadas, meanwhile, can grow up to 1 3/4-inch long and are entirely dark green to black — with no red features.


Common misconceptions

Cicadas are not locusts — though the two are sometimes confused due the fact they both swarm in large groups.

Locusts belong to the grasshopper family and are found only in Africa, Asia, Central America, South American and Australia. They are also known for decimating plant life with their voracious appetites.

Cicadas do not damage trees — at least not while feeding.

However, after mating, the female periodical uses a knife-like appendage attached to her abdomen to make small cuts in the underside of branches — which she fills with between 400 and 600 eggs. Cicadas prefer to use deciduous trees — elm, maple, oak, for example — as their "nurseries."

Though mature trees have no trouble supporting a cicada's clutch, young trees can be susceptible to injury. To protect them, the University of Kentucky College of Agriculture recommends wrapping their trunks with netting or cheesecloth, which will prevent cicadas from climbing up to its tender twigs.


What happens next?

Six weeks after the female lays her eggs, the squirmy nymphs hatch — alongside millions of others in nearby trees. Collectively, they drop to the dirt and begin to burrow in search of roots. Throughout their life underground, a periodical cicada molts five times. With each, it becomes larger, transforming from a tiny, termite-like insect into a plump red one. Known as instars, each stage lasts an average of four years, except for the first, the shortest.

During its fifth instar, the cicada emerges from the ground, pale and soft-bodied. It molts once more — the end of its transformation marked by the crisp, brown exoskeleton stuck to the side of a tree.

Then, they sing; they mate; they die. And a new generation begins.