The buzzing of cicadas that marks any Midwestern summer will be even more intense this year as billions or even trillions emerge in a convergence of insects that no one alive has ever experienced.

Two types of periodical cicadas will come above ground at the same time. One brood, or breeding group, emerges every 13 years, and the other comes every 17 years.

The last time this natural phenomenon took place — 221 years ago — electricity hadn’t been invented, horses and carriages were still the main way to get around, and the Great Chicago Fire hadn’t yet ravaged the city.

Here’s everything you need to know about the upcoming deluge of bugs:

When will the cicadas come out?

The periodical cicadas emerge once the soil that is around seven inches into the ground reaches 64 degrees. Scientists estimate that will be in late May or early June this year. In order for the soil to reach the necessary temperature, the air temperature would need to be consistently in the low 70s, which typically doesn’t happen until late spring or early in the summer in the northern part of the state.

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Further south, it’s possible cicadas could emerge earlier because of the warmer temperatures, but it probably won’t be much sooner than the rest of the state, according to Martin Berg, a professor of biology at Loyola University Chicago who has studied cicadas.

The cicadas that buzz in the area every year usually don’t emerge until later in the summer, so it’s not expected that the two will overlap.

Where will the cicadas be?

When the cicadas emerge from their years feeding underground, one brood that emerges every 17 years will be centered in northern Illinois, while the 13-year brood will be found in the southern part of the state. The two converge near Springfield, but there aren’t likely to be many, if any, places where both broods can be found at once. Even so, you won’t know because the broods look and sound identical.

“If you look and you zero in on Springfield, Illinois, that’s where the two broods come close, but they don’t necessarily overlap,” according to John Cooley, who studies cicadas at the University of Connecticut, one of the top institutions for cicada research. “They come real close but there aren’t gonna be places that have double densities or anything like that”

People in Chicago may see or hear cicadas, but they’re more likely to be found in heavily wooded areas or places with more natural landscapes instead of manmade structures. Some suburbs that are bound to see a more dense populations of cicadas include Winnetka, Des Plaines and Elmhurst.

Though this type of cicada is only found in North America, they can be found in several states in the U.S.

Illinois, though, is “the epicenter of everything great,” Cooley said.

“You can literally drive Illinois top to bottom and be in cicadas the whole way … that doesn’t happen very often,” Cooley said.

How many will there be?

It’s hard to predict just how many will be out, but one rough estimate from 1956 suggests there will be as many as one million bugs per acre in areas where they come out. By that logic, Illinois could be home to around 37 million cicadas. Experts say with so many states involved, the total number could amount to billions, or by some estimates even trillions of the insects.

How rare is this year for cicadas?

The last time the two broods of cicadas emerged at the same time was 1803. Beyond that, of the 5,000-6,000 species of cicadas around the world, fewer than 10 are periodical, according to Cooley.

Scientists and researchers, like Kacie Athey of the University of Illinois, are gearing up for the rare occurrence and encourage others to appreciate its uniqueness.

“We talk about this as being a once-in-a-lifetime thing where two are coming out at once, it’s more than that … the next time we can expect to see two broods to come up is 2245,” Athey said.

Could the two broods interbreed? If so, will that bring rise to 15-year cicadas?

Not likely. Two different scientific papers predict that the offspring would have life cycles of 13 or 17 years rather than cycles of intermediate length. And they would basically look the same as their parents.

“If hybrid offspring were produced, then they would emerge alongside the next generation of one or both of their parental broods and be indistinguishable from cicadas that were not hybrids,” UConn researchers write.

What do they look like? How big do they get? Will I notice them?

The periodical cicadas are around an inch long, smaller than the ones we see each year. They have large, red eyes with orange wings and legs. The cicadas we see yearly are larger and greenish in color.

“Take pictures,” Berg said. “Those things have been underground for 17 years, just realize that. Just go out and sit and listen, that really makes a big impact.”

What is that noise?

The buzzing that hovers in the air while cicadas are out is a type of mating call the males make. The “singing” can reach 90-100 decibels, which is about the same volume as a lawnmower or a shouting conversation. The singing starts about five days after the cicadas emerge.

What do cicadas do while they’re underground? Why do they come out?

When they’re born, cicadas hatch from their eggs and immediately make their way underground. It’s an innate action, Athey said. They stay there for years, depending on how long their species remains underground, and feed on plant roots. They don’t cause any damage to the plants.

The length of time in the ground for each brood is determined by genetics. The reasoning for 17-year cicadas staying underground so long is largely a mystery, as researchers haven’t found a definitive reason why some cicadas are periodical and others aren’t.

When it’s time for them to emerge above ground, they “basically come out, mate, lay eggs and die,” Athey said.

Are cicadas dangerous?

Cicadas are harmless to humans and other wildlife. They don’t bite or sting, and they don’t damage anything except some young trees. They’re not toxic or poisonous, either.

Do animals eat them? Will this help other species survive?

Other animals can eat cicadas and have “positive ecosystem consequences,” according to Berg. They could be consumed by most animals that eat insects, including squirrels, spiders, birds and snakes.

Will the cicadas harm trees in my yard?

Cicadas lay eggs in small branches or twigs on trees, using part of their body to carve a place for the eggs to stay. For younger trees, this can be dangerous, but most trees won’t be harmed. If you have a tree less than five years old, wrap the branches in mesh to avoid damage to the younger trees. Experts say there is no reason to use insecticides in response to the emergence.

Can I eat cicadas?

Yes! Cicadas are edible and some recipes for snacks like cookies and popcorn incorporate the insects.

However, people with seafood allergies should steer clear. They’re not a threat to people with the allergy unless eaten, so being outside around cicadas isn’t a concern, but eating them could be dangerous.

When will they go back into the ground?

The cicadas that come up from underground won’t go back beneath the surface after they emerge. After coming up from underground, they individually have a lifespan of a few weeks. Overall, the entire brood of cicadas will be out for a period of around six weeks. Once hatched, their offspring will immediately go underground to start their period of feeding before emerging again in another 13 or 17 years.

When the cicadas die out, their decaying corpses leave an infamous smell. As they decay, the corpses return their nutrients to the soil where they die.

Will climate change impact cicadas in future years?

Because the cicadas come out when the soil temperature reaches a certain threshold, it’s possible that the warming of the climate could cause them to come out earlier and earlier each cycle.

There’s also the factor of the unpredictability of Midwestern weather. If there’s a period of warm weather followed by a cold snap, it’s possible the cicadas could be harmed.

“Insects are really responsive to temperature,” Berg said. “If they come out too early and then we have a cold spell, it could be pretty devastating to the population.”