This summer, prepare for the return of Brood X.
No, that’s not the title of a B-grade horror flick, but it does involve an army of insects rising from the earth Night of the Living Dead–style. Seventeen-year cicadas spend over 99 percent of their lives underground, only to crawl out by the billions on a warm spring night in their 17th year. Then, after dedicating a few weeks to repopulating the brood, their bodies fall back to the ground they emerged from.
Different cicada broods resurface on different years. Brood X—the most widespread cicada brood—last covered the Eastern U.S. from Georgia to New York in 2004. Now the offspring of that last batch are finally ready to emerge as the summer of 2021 approaches. Whether they’ll be in your neighborhood or not, these are 17 facts about the periodical bugs worth knowing.
1. Cicadas are active their whole lives.
Just because we can’t see (or hear) cicadas when they’re a few feet underground doesn’t mean they aren’t keeping busy. “They’re not hibernating ... they’re just slow-growing,” Louis Sorkin, an entomologist with the American Museum of Natural History, tells Mental Floss. According to Sorkin, 17-year cicadas live underground as immature nymphs before reaching full adulthood above the surface. And because they’re still too young to procreate, one major activity occupies their time: eating.
2. Cicadas' diet may explain their long life cycle.
A cicada nymph’s meal of choice is xylem, a type of sap they slurp from tree roots. The fluid is more than 90 percent water and very poor in nutrients. Some cicada experts believe that the insects' lousy diet may be one of the reasons behind its drawn-out maturation process.
3. Cicadas use trees to keep track of time.
The cicada’s super-accurate timekeeping skills have been puzzling scientists for centuries. According to one paper published in 2002, they may be using more than just their biological clocks to count down the years. For the study, researchers transplanted 15-year-old 17-year cicada nymphs beneath a tree that had been manipulated to blossom twice in one season. After feeding on its roots, the insects emerged one year early. This suggests that cicadas use the influx of sugars and proteins from a blossoming tree’s roots to mark the passage of time underground.
4. Math helps cicadas avoid predators.
Seventeen-year cicadas aren’t the only members of the periodical cicada genus. They share the title with 13-year cicadas, who, as their name suggests, live a similar life cycle except it ends four years earlier. Thirteen and 17 may seem like arbitrary numbers of months to stay buried in the dirt, but the numbers share a mathematical property that may help keep the insects alive. Both 13 and 17 are decently large prime numbers, which means they aren’t divisible by any number smaller than themselves. This makes it next to impossible for predators to adapt to the cicadas’ emergences. Stephen Jay Gould wrote in his 1977 book Ever Since Darwin: Reflections in Natural History, “Consider a predator with a cycle of five years; if cicadas emerged every 15 years, each bloom would be hit by the predator. By cycling at a large prime number, cicadas minimize the number of coincidences.” That’s an impressive math trick for an arthropod.
5. Cicadas evolved during the last ice age.
The prime number trick is just one of the reasons some cicadas live to be 17. Louis Sorkin says their long lifespan is likely a leftover adaptation from the last ice age. Periodical cicadas are believed to have evolved during the Pleistocene Epoch at a time when North America’s climate was fairly unpredictable. By staying underground for as long as possible, cicadas had a greater chance of avoiding a cold and deadly summer when it was finally time to come up.
6. Cicadas are triggered by temperature.
When it’s their year to emerge, 17-year cicadas wait until the soil 8 inches beneath the surface warms up to around 64°F. This specific temperature point is their signal to tunnel out of the earth and up the nearest vertical surface they can find, whether it’s a tree, fence post, or screen door. According to Sorkin, they’ll even climb up the leg of a person standing nearby.
7. There are 12 different broods of cicadas.
Not every 17-year cicada emerges at the same time (if they did, the eastern half of the country couldn’t be blamed for taking a prolonged vacation every 17 summers). The insects are grouped into 12 distinct broods spanning from New York to Oklahoma (along with three broods of 13-year cicadas). When a brood procreates, their offspring are the ones who carry on that same brood 17 years later. Entomologist C.L. Marlatt was the first person to assign Roman numerals to the periodical cicadas in the late 19th century. Since then, a handful of the original broods have gone extinct.
8. Cicadas arrive in large numbers.
A single brood can contain billions of cicadas. Simultaneously emerging in such great numbers is a survival strategy known as predator satiation. “[Predators] get completely full and engorge themselves,” Sorkin says. “They can’t catch everything, so you’ll have lots of [cicadas] doing fine and progressing to the next stage of their lives.” This ends up working out for cicadas as well as for the birds, rodents, and reptiles that get a surprise feast whenever they show up.
9. Cicadas are a treat for humans, too.
Humans have a long history of dining on cicadas. Before European colonists arrived in North America, Indigenous people enjoyed roasting them like peanuts. Today the bugs have become something of an unlikely food trend. Not only do they fit into the movement to utilize insects as a protein source, but they’re also gluten-free, low in carbs, and hyper-seasonal. When one Missouri ice cream shop introduced a batch of cicada-flavored ice cream in 2011, it sold out before hitting the display case. During the most recent summer of Brood II, one chef served cicadas at his James Beard-nominated sushi restaurant in Connecticut. If you can’t find cicadas on the menu of your favorite local spot, catching them and cooking them at home is easy enough to do. Sorkin recommends grinding them up to make a flour, or even sautéing the critters with garlic and oil and eating them whole.
But if you want to put cicada on the menu, there are a couple of things to consider. Experts recommend that those with a shellfish allergy avoid the bugs, as should anyone who’s concerned about mercury in their diet: In 2010, researchers found cicadas may contain mercury.
10. Cicadas leave a lot of exoskeletons behind.
Before they can become adults, cicadas must get to a high-up spot to shed their immature skin. From the old exoskeleton a soft, new body unfurls, leaving behind the perfect shell of its nymphal self. Even though they’re harmless, the cicada skins can prove to be a nuisance for some. News stories from Brood II’s 1996 emergence reported shells piling up so high in some spots that people were using snow shovels to clear their driveways.
11. Cicadas are not locusts.
While it’s true that 17-year cicadas do emerge in plague-like proportions, that doesn’t make them locusts. Sorkin says this misnomer was likely spread by European settlers who knew the bugs from the Bible. In reality, locusts are a type of grasshopper, and they tend to be much more destructive than relatively harmless cicadas.
12. Cicadas' songs can reach 100 decibels.
A cicada above ground has one job to do: make baby cicadas. To accomplish this task, an adult male will spend his last four to six weeks of life in furious song. There are three different species of 17-year cicadas and each one has a distinct mating call. The sounds are sometimes reminiscent of chirps, rattles, or high-pitched screams, and when males gather in trees to form a chorus, the noise can exceed 100 decibels. That’s about as loud as a car stereo playing at max volume. The song can be heard by females up to a mile away, and if she likes what she hears, she responds by flicking her wings.
13. Cicadas are attracted to power tools.
The deafening roar of a chorus of cicadas is often compared to the sound of heavy-duty power equipment. The noises are so similar that confused females have been known to swarm around people using lawn mowers, leaf blowers, and other loud tools.
14. Cicadas lay their eggs in tree branches.
After the female cicada has mated, it’s time to lay her eggs. She uses a structure on her rear called an ovipositer to slice Y-shaped gashes on the tips of tree branches. Females can lay up to several dozen eggs in a single branch and as many as 400 eggs spread out over 40 to 50 sites. After incubating for several weeks, the newly-hatched cicada nymphs fall to the earth, where they will spend their next 17 years of life.
15. Cicadas are one of the longest-lived insects.
16. Cicadas have five eyes.
An adult cicada is easily identified by the opaque pair of compound, usually crimson, eyes bubbling out from the sides of its head. But this isn’t the only set of eyes the creature owns. It also possesses a cluster of three smaller eyes hiding in plain sight on the top of its forehead. These eyes are called ocelli and are much simpler than its more prominent pair.
17. Every 221 years, 13-year and 17-year cicadas co-emerge.
The cycles of 17-year and 13-year cicada broods in the same region rarely coincide, which may be an evolutionary tactic to keep them from interbreeding. But you can mark your calendars now: the next co-emergence event is set to take place in Missouri in 2219.