The incredible emergence of billions of cicadas across America

Don't be alarmed if you hear a buzz in the trees this spring. Trillions of cicadas are expected to come out of their underground dens in the United States in what scientists are calling a "phenomenal" emergence. Billions of the insects will be crawling and flying around in what scientists say is a phenomenon that hasn't occurred for nearly 200 years. They forecast that the 13-year cicada brood will emerge in numbers as high as 1.5 million per acre in places like Chicago, Cincinnati and St. Louis, and the 17-year brood will emerge in numbers as high as 2 million per acre in states including Georgia and Tennessee.

These periodic cicadas have spent anywhere between 13 and 17 years maturing underground before emerging to mate above ground. They will start to come out of the soil in large numbers, beginning in late April and into May, staying above ground for only a few weeks this spring. Their emergence will be short-lived, but impactful.

While cicadas don't directly pose a danger to humans, the bugs can be a nuisance as they fly around and cling to trees, shrubs and houses. When they crawl out of the soil, they can leave behind their burrows, creating holes that can be a tripping hazard. And their short lifespan and vast numbers mean that they can be an easy feast for predators.

These periodic cicadas are one of about a dozen species that regularly emerge in attempts to synchronize reproduction on a large scale. This event, known as a "brood," happens at different intervals for different species. This spring, two broods, the 13-year Great Southern Brood and the 17-year Northern Illinois Brood, will appear together for the first time since the 1800s. And the dual emergence of these two broods, which happens once every 221 years, will not occur again until 2245.

Here's what you need to know about these bizarre bugs:

• Where you'll find them: They live in the soil of every state in the US except for Alaska and Hawaii. Periodic cicadas thrive in climates with a warm spring and lots of plants and trees.
• Why they're here now: Soil temperature determines their timing. Males start singing to attract females, and that's how you know they're emerging.
• What they look like: They are big, black-and-orange bugs with red eyes. Adult cicadas vary in size, but some can be more than an inch long.
• What they sound like: They make a very loud noise. Males of the species have a special noise-making organ called a tymbal. When they flex their muscles, it makes a noise to attract females.