These states can expect to see a 17-year periodical cicada emergence this spring

With spring will come warmer temperatures, flora in bloom -- and millions of screaming insects emerging from their resting place beneath the ground.

Three species of cicada that only emerge once every 17 years are gearing up to spring to the surface in droves, when they will plague several regions on the East Coast with swarms of insects that are not visible for the majority of their lifetimes.

Cicadas make sure they are seen and heard during these events. The sounds of millions of singing cicadas will soon fill the air. Cicada nymphs and exoskeletons will litter the ground. Adult cicadas will be seen flying through the air and resting on trees and plants, experts told ABC News. In addition, cicadas are large and "very active," said John Cooley, an entomologist at the University of Connecticut.

What to know about this year's periodical cicada emergence:

Brood XIV will emerge this time

Brood XIV, the second-largest periodical cicada brood, last emerged in 2008, Tamra Reall, an entomologist at the University of Missouri, told ABC News.

Since then, the nymphs of those periodical cicadas have been underground feeding on tree root tap and waiting to come out, Reall said.

Brood XIV was first discovered by European colonists in the 1600s, who assumed the swarm of insects was akin to a biblical plague before they realized the pattern of emergence, Cooley said.

Periodical cicadas differ from annual cicadas that emerge every year, the experts said.

Annual cicadas have a green coloring but periodical cicadas are black and orange, Jody Gangloff-Kaufmann, an entomologist and associate director at the New York State Integrated Pest Management Program at Cornell University, told ABC News.

In addition, annual cicadas tend to emerge in the "dog days" of summer -- in July and August -- rather than in the spring, Cooley said.

Those who witness emergences can upload their accounts to Cicada Safari, a citizen science program that tracks cicada events, Cooley said.

When and where to expect a cicada emergence

A large swath of the eastern U.S. -- from Massachusetts down to Mississippi and Georgia -- will start to see cicadas emerge once temperatures begin to warm up, Gangloff-Kaufmann said.

States like Kentucky, Ohio, Indiana, Tennessee and North Carolina can expect to witness the emergence of Brood XIV this year, the experts said.

The emergence will be triggered once the soil temperature about 6 inches below ground reaches 64 degrees -- typically about mid-May to late June, depending on the region, Gangloff-Kaufmann said.

The event typically happens "suddenly" over days or weeks, Cooley said.

Why you shouldn't fear (or kill) cicadas

The emergence of cicadas is "just like out of a sci-fi movie," Reall said.

First, holes -- or exit tunnels -- start to appear in the ground, and the nymphs will be seen popping out before they look for something to climb up, whether it be a tree or fence post, Reall said.

Once the nymphs find somewhere to stop on their climbs, they molt and the adult cicada will emerge, Cooley said. The males will begin to sing to garner the females' attention, Reall said.

"We, as people, can actually observe this magical part of the insect metamorphosis," Reall said.

After the adults are active for a few weeks, the females lay their eggs, the adults die, and the nymphs remain underground until it's time for them to emerge again, more than a decade later.

"It's kind of shocking how fast it all disappears," Cooley said.

However, it's important to remember that cicadas are not dangerous, Gangloff-Kaufmann said.

Cicadas are not a threat to humans to animals, the experts said, imploring those who encounter them to avoid killing them -- whether by squishing them or spraying them with pesticides. They don't bite, and if your pets ingest them, they will be fine, the experts said.

"People tend to be terribly afraid of insects, and these guys are big, but they are harmless," Gangloff-Kaufmann said.

In addition, cicadas are beneficial to tree health because the holes they create can lead to better moisture drainage into the soil and some natural pruning, Reall said.

"These are a natural part of the forest," Cooley said.