(WHTM) — The Brood X cicadas emerge from the ground once every 17 years, to the amazement — and, depending on how you feel about bugs, possibly dismay — of Pennsylvanians. Between their nearly two decades underground and a handful of weeks on the surface, these bugs keep busy throughout their lives.

Brood X, pronounced “brood ten,” actually includes three different species of periodical cicadas. While their impressive numbers make them a notable presence above ground, they’re also an important force below it.

“We know that they pretty significantly affect a lot of soil processes,” says Keith Williams, community engagement coordinator at the Lancaster Conservancy. “They aerate the soil, they provide a lot of nutrition to the soil…they bioturbate, so they’re turning the soil over, which is another important process.”

In their 17 years underground, the cicadas go through five larval instars, or developmental stages, molting between each phase. And they feed on xylem, which is the fluid that goes from trees’ roots up into the rest of the plants.

That xylem is how experts think the bugs are able to keep track of the 17 years between each emergence. The xylem stops flowing each winter and then starts moving again each spring. Williams explains that the cicadas probably count those pulses to keep track of time.

After the 17th pulse, the cicadas get ready to come out. They dig tunnels up to the surface, sometimes creating mud chimneys several inches tall at the tops of their holes. When the ground reaches about 65 degrees, they emerge en masse.

Sometimes cicadas create mud chimneys like this one when they dig tunnels up to the surface.

The massive number of cicadas that emerge at once is a survival strategy, Williams explains. They “flood the market” with prey so that even if predators catch them, there are still plenty left to reproduce and carry on the species.

After spending years and years underground, the cicadas are only on the surface for about a month, give or take a few weeks. They molt one final time after leaving their tunnels, leaving behind their empty brown exoskeletons. Then the male cicadas fly up into the trees and start calling for mates.

The three different species of Brood X cicadas all emerge at once, but they don’t interbreed, says Williams. They’re able to differentiate between the species by their unique songs.

Once the cicadas mate, the female lays her eggs at the ends of twigs in trees or bushes. Williams says this might hurt a young plant, it doesn’t do much harm to most trees. The ends of the twigs might turn brown, but the tree itself will survive.

After they reproduce, the adult cicadas die. The eggs stay in the branches for around a month as the babies develop into their first larval instar, then they drop out of the plants and go right back underground to wait another 17 years.

While the unusually large and plentiful cicadas may be unnerving or annoying for some, Williams hopes people will try to appreciate the critters. “I would encourage people to just enjoy this really rare event — this is one of the most amazing phenomena in all the planet that occurs right in our backyards — and just embrace it and enjoy it rather than fear it.”