Billions of cicadas are set to appear in a rare 'double brood ...

If you're not a fan of giant flying hideous insects, it may soon feel like the end of the world as we know it! But only if you live in certain parts of the country.

The birds are back and the flowers are back, but for the first time in centuries, billions of cicadas are also about to be back.

It will be a confluence of creatures the likes of which hasn't been seen in the United States since Thomas Jefferson was president - and won't happen again until 2245.

"17 and 13 doesn't overlap too often," said Frank Krell, Ph.D. entomologist at the Denver Museum of Nature and Science.

Dr. Krell is alluding to, in a few weeks, the brood of cicadas that emerges every 13 years and the brood that emerges every 17 years are about to pop out of the ground at the same time, which last happened in 1803.

It's a rare emergence of insects some are referring to as "cicadapocalypse."

The 13-year group, known as Brood XIX, or the Great Southern Brood, is the largest periodical cicada brood, stretching across the southeastern United States. The Northern Illinois Brood, or Brood XIII, emerges every 17 years.

Though the idea of a cicadapocalypse may seem foreboding, experts predict that the two broods won't overlap significantly, and the bugs themselves, while loud and numerous, are harmless. Here's what you need to know going into cicada season.

What to know about cicada broods

This spring's bugs are part of a genus, or group, of cicadas in the eastern US known as the Magicicada, or periodical cicadas. Three species emerge on a 17-year cycle, and four species are on a 13-year cycle. (Scientists have long debated the significance of these numbers, which are both prime - some researchers have suggested that emerging on these prime-numbered years makes the periodical cicadas less likely to be killed by predators that have 2- or 3-year life cycles, but the jury's still out.)

The pattern periodical species follow is different from that of "annual" cicadas, which don't actually have an annual life cycle, even though you can see them every summer in much of the United States. The nymphs, or babies, of annual cicadas spend two to five years underground, slowly growing, until they're ready to emerge. There are just so many overlapping generations that there appears to be a steady stream of these cicadas every year.

It's easy to tell annual and periodical cicadas apart. Annuals tend to emerge later in the year than periodicals. For instance, the "dog day" annual cicadas in the genus Neotibicen tend to show up in the dog days of summer, around August, whereas the periodicals make their appearance in the spring. While there are numerous species of annual cicadas, many of them are large and greenish. Periodical cicadas are smaller and mostly black, with bright red eyes and orange-tinged wings and legs.

Cicadas are divided into groups called broods based upon when they emerge. A brood can contain cicadas from multiple species. As long as they're adults in the same 13- or 17-year cycle at the same time, they count as members of the same brood.

When and where will the cicadas emerge?

This spring's periodical cicadas will make their appearance when the soil temperature 8 inches deep reaches 64 degrees Fahrenheit (about 18 degrees Celsius). It will likely happen sometime in mid-May. The individual bugs' adult life cycles are just a few weeks, but their emergence will be staggered, so there will be about six weeks of cicadas.

That month-and-a-half period will be jam-packed with loud singing, mating and then dying, like "the most macabre Mardi Gras that you've ever seen," Larson said.

Parts of the Midwest and Southeast are due for cicadas this spring. Northern Illinois, along with southern Wisconsin, eastern Iowa and northwest Indiana are likely to see bugs from Brood XIII; central and southern Illinois, most of Missouri and scattered areas of Kentucky, Tennessee, Virginia, Maryland, North Carolina, South Carolina, Georgia, Alabama, Mississippi, Louisiana and Arkansas are due to get Brood XIX bugs.

There are some areas of central Illinois where the two broods' geographic ranges have historically been close to each other and could potentially overlap. However, predictions of a cicadapocalypse - in which Brood XIII and Brood XIX show up at the same place at the same time - are probably an exaggeration.

"We're not even sure that they're really going to overlap," said Dr. Chris Simon, a professor of ecology and evolutionary biology at the University of Connecticut. Her research group at the university maintains a website of cicada information, which includes maps showing where the broods have historically emerged.

The double emergence of Broods XIX and XIII is rare, occurring every 221 years (when the 13-year and 17-year cicadas overlap, as 13 times 17 is 221). These two broods haven't been aboveground at the same time since 1803, and after this year, they won't be reunited until 2245.

However, the co-occurrence of different cicada broods, somewhere in the United States, isn't quite as rare. It last happened in 2015; it'll happen again in 2037.

Preparing for cicadas

Even though a major overlap of the two cicada broods is unlikely, only getting one brood in an area still means countless bugs.

"You should expect lots and lots of cicada exoskeletons to be covering your trees and shrubs. You should expect to hear lots and lots of noise," Larson said. The insects are likeliest to be in wooded areas near water, he added.

While the sheer volume of insects, along with their distinctive jackhammer-loud sounds and bright red eyes, might give some people pause, Larson notes that cicadas are harmless. They don't pose a risk to garden plants. However, if you have young trees, cicadas could potentially damage them when the insects cut into branches to lay their eggs. You can mitigate this harm by covering the trees with cicada nets.

Cicadas won't bite or sting you or your pets. If your dog eats a cicada or two, he said, the animal will be just fine.

Dogs aren't the only ones tempted to nosh on cicadas; people have eaten them for thousands of years. "They have kind of a natural, sweet nut flavor," Larson said. (If you're allergic to shellfish though, you should avoid eating cicadas - a protein in shellfish that's tied to allergies is also present in many insects.)

If you live in an area with cicadas making an appearance this spring, you can download community science apps to help researchers studying these bugs.

"The main thing we want people to know is that they should download the Cicada Safari app, which is free on the web, and all they have to do is photograph whatever cicadas they see," Simon said. Those photographs are sent to scientists, who then map where and when the cicadas are emerging: information vital for scientists studying how climate change affects cicadas and predicting future cicada activity.

Beyond the bigger scientific story of cicadas, Larson said he hopes people will embrace cicada spring simply because it's a rare chance to see some of the world's most unusual bug behavior.

"These are some of the coolest insects in America," Larson said. "I really hope that people will appreciate this for what it is: this unique natural phenomenon that you don't get anywhere else. It's beautiful."

ABC News and CNN contributed to this post.