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During summer's hottest months in locations around the world, the whirring buzz of cicadas is a familiar sound. Sometimes erroneously referred to as locusts, summer cicadas are annual visitors, but they have darker, red-eyed cousins that are seen much less frequently — in some cases, close to two decades pass between appearances.

Known as periodical cicadas, these long-lived insects — the longest-lived in North America — can be found only in the eastern half of the United States, surfacing between May and June in cycles of 13 or 17 years, depending on the species. They live near trees, hatching and growing underground as nymphs and living off sap that they siphon from tree roots.

During their years underground, the nymphs molt through five growth cycles, known as instars. Then, when ground temperatures reach 64 degrees Fahrenheit (18 degrees Celsius) at a soil depth of 8 inches (20 centimeters), the nymphs emerge en masse and metamorphose into winged adults, according to the National Oceanic and Atmospheric Administration. [6 Amazing Facts About Cicadas]

The emergence of a population of periodical cicadas is usually synchronized across multiple states. The number of boisterously buzzing bugs varies widely from year to year but can total in the billions; in 2016, periodical cicadas in some areas congregated in densities of 1.5 million insects per acre, the Washington Post reported.

Here's what you need to know about the periodical cicadas that will be emerging in 2017.

Annual cicadas emerge later in the year than periodical cicadas, arriving in late June through August, according to the Magicicada website. The annual variety is typically light green or brownish in color, while periodical cicadas have black bodies, red legs, bright red eyes and red veins running through their large, translucent wings.

Periodical cicadas' bodies measure about .0.75 to 1.25 inches (1.9 to 3.2 centimeters) in length, while annual cicadas' bodies are somewhat bigger, at about 1.75 inches (4.5 cm) long, entomologists with North Carolina State University (NCSU) wrote.

There are approximately 3,000 cicada species, seven of which represent periodical cicadas. Three periodical species — Magicicada septendecim, Magicicada septendecula and Magicicada cassini — have 17-year life cycles, while four species — Magicicada tredecim, Magicicada neotredecim, Magicicada tredecassini and Magicicada tredecula — follow a 13-year life cycle.

Periodical cicadas are identified by the term "broods," which categorizes the insects based on the year in which they emerge, with Roman numerals representing each group. According to NCSU entomologists, there are 30 broods: Broods I through XVII are found predominantly in the northeastern U.S. and have 17-year life cycles, while Broods XVIII through XXX are 13-year cicadas and live mostly in the southern U.S..  

In 2017, Brood VI periodical cicadas are expected to emerge in three states: South Carolina, North Carolina and Georgia. They will appear in Rabun County in Georgia, in Oconee and Pickens Counties in South Carolina, and in Buncombe, Burke, Caldwell, Henderson, McDowell, Polk and Wilkes Counties in North Carolina, with smaller groups anticipated in Ohio and Wisconsin, Cicada Mania reported.

As of June 12, cicada emergence is underway in Georgia and the Carolinas, with additional sightings reported in central Oklahoma, according to Cicada Mania.

The last time this brood was seen in the United States was in 2000. Hillary Clinton had just been elected to the U.S. Senate, the International Space Station was welcoming its very first crew, and Harry Potter and his friends were entering their fourth year at Hogwarts in the newly released novel "Harry Potter and the Goblet of Fire" (Scholastic, 2000).

The periodical cicadas that emerged in 2016 — Brood V, another 17-year group — appeared in parts of Maryland, Ohio, Pennsylvania, Virginia, West Virginia and New York.

Sometimes, different broods overlap, as they did in 2015, when Brood IV (17-year cycle) and Brood XXIII (13-year cycle) appeared across 14 states in total. In May 2017, people living in the Mid-Atlantic region were surprised to see periodical cicadas, which were not expected until 2021, appearing in the hundreds — 1,000 sightings were reported from northern Virginia to Bel Air, Maryland in just two days, the Washington Post reported. Entomologists are collecting data about these early appearances, to determine whether this represents a disturbance in the cicadas' life cycle that could be linked to longer growing seasons due to climate change, according to the Washington Post.

"The year of cicada emergence is cued by what I and others believe to be an internal molecular clock," Chris Simon, a professor in the Department of Ecology and Evolutionary Biology at the University of Connecticut, told Entomology Today.

A periodical cicada's internal clock is likely calibrated by certain environmental cues that signal the passage of a year, "such as the trees leafing out," Simon suggested. This event, he said, changes the composition of the fluid in tree roots that nourishes cicada nymphs in early instars, or developmental stages.

"The accumulation of 13 or 17 years triggers the emergence of fifth-instar nymphs. The day of emergence is triggered by accumulated ground temperature," Simon said.

In fact, entomologist Richard Karban of the University of California, Davis, used this "tree time" phenomenon to get a group of 17-year cicadas to emerge a year early. Karban manipulated peach trees that were supporting the cicada nymphs so that the plants bloomed twice a year rather than just once. The cicadas had already been brooding for 15 years, and the double-blossoming tree roots tricked the insects into "thinking" two years had passed, according to the article published in July 2000 in the journal Ecology Letters.

Adult periodical cicadas enjoy a brief and frenzied mating period that lasts just four to six weeks, the U.S. Department of Agriculture (USDA) explained in a Pest Alert publication. About three to five days after mating, females lay approximately 24 to 28 eggs, which hatch after six weeks. Then, the nymphs burrow underground, and the cycle begins all over again. Brood VI's next expected appearance will be 17 years from now, in 2034.

Adult cicadas can't bite or sting, and aren't toxic; in fact, they are edible, with an "asparagus-like" flavor, cicada expert Gene Kritsky, a biologist at the College of Mount St. Joseph University in Cincinnati, told National Geographic. Unlike the highly destructive locusts that they are sometimes mistaken for, periodical cicadas don't feed at all, according to the USDA. However, females can damage tree branches by carving tiny slits to hold their eggs, the USDA said.

  • Every species of cicada produces a unique song that the males use to call the females, and the bugs can be heard at up to a distance of 1 mile (1.6 kilometers).
  • While periodical cicadas generally have red eyes, some of the insects have white or blue eyes, biologist Patrick Abbot at Vanderbilt University in Tennessee said in a statement. Cicada researcher Roy Troutman has captured photos and video of periodical cicadas that have white eyes, a trait caused by a genetic mutation that strips the color from the insects' eyes and even from their wings, he explained in a video description.
  • The veins at the tips of periodical cicadas' wings can appear to take the shape of the letter "w," according to the Cicada Mania website. People once viewed that letter as an evil omen, The New York Times reported on May 20, 1894, saying, "some superstitious persons have greatly alarmed themselves, most unnecessarily, by thinking this meant war. Others, more sensible, think it means warm weather."
  • By emerging in enormous numbers, periodical cicadas follow an unusual survival strategy, essentially outlasting their predators by providing them with too much to eat, according to a 1993 study published in the journal Ecology. This technique, known as "predator satiation," enables hungry predators to gorge themselves sick on readily available prey, until the predators reach a saturation point where they avoid the cicadas completely. The "synchronized, explosive emergences" of periodical cicadas ensure that even though large numbers of the insects may be eaten, plenty will still survive long enough to mate and lay their eggs, the study authors wrote.

Editor's Note: This article was originally published on January 24. It was updated with information on the current status of the cicada season.

Original article on Live Science.