Within the next month, billions of insects will appear in parts of 15 states — including Virginia, Maryland, Delaware and Pennsylvania, as well as D.C. They’re called cicadas (pronounced suh-KAY-duhz), and they’re about the size of a miniature candy bar.

If you’ve never seen a cicada, get ready to be impressed. The critters have huge, see-through wings and bright red eyes. When a lot of them get together, the insects can create a loud buzzing sound that might make you think there’s a generator running somewhere in the neighborhood.

So where do all these bug-eyed beasts come from? How do they know to arrive at the same time? And should we be scared? KidsPost asked Matt Kasson, a scientist who studies cicada diseases at West Virginia University.

The first thing you need to know about cicadas is that they’re harmless.

“They don’t really have any defenses,” Kasson says. “Their only defense is their numbers.”

Kasson said that while there will definitely be billions of cicadas buzzing around shortly, the true number could be “in the hundreds of billions, maybe even pushing into trillions.” But having lots of cicadas is also probably why they never evolved the ability to bite or sting.

“They basically fill up every predator on the planet when they emerge, so every bird, snake and fish within range will just basically gorge themselves on cicadas, and yet there will still be plenty left to persist,” he says.

This group of cicadas is officially known as Brood X (X is the Roman numeral for 10). You might think they are newborns, but these bugs are older than you are. That’s because they’ve been hiding below ground for nearly 17 years.

Kasson explains that cicadas undergo a metamorphosis (change), sort of like a butterfly, only they don’t have a larval or caterpillar stage. Cicada eggs hatch into nymphs, which spend more than a decade below ground sucking juices out of tree roots. Then they pop out of the ground, climb up a tree trunk, and transform into adults with wings. (When the emergence happens, you’ll be able to collect tons of cicada exoskeletons, or shells, which are left over from the transformation. If you’re brave enough, you can also hold the adults. Just be gentle.)

The adults live four to six weeks. In that time they will mate, and the females will lay eggs in tree branches. By July the adults will be gone, and the nymphs will start falling to the ground to burrow into the soil.

There are two reasons so many cicadas appear at the same time. First, the nymphs can count, and they keep mental notes of how many years have passed by the starting and stopping of tree sap that occurs each winter. They are also sensitive to temperature and will wait until the soil reaches a comfy 64 degrees before coming out to mate and lay eggs.

While some people dislike the appearance of cicadas, Kasson says it should be a cause for celebration. Periodical cicadas, which is what we call the cicada varieties that emerge in huge batches every 13 or 17 years, occur only in the United States.

“So I would say just marvel in the fact that you’re witnessing something that no one else in any other part of the world gets to see,” says Kasson. “And it’s right in your backyard.”

Bittel is a freelance journalist who often writes about animals. His children’s book, “How to Talk to a Tiger . . . and Other Animals,” will be published April 13.