A brood of "17-year cicadas" is set to descend on southeastern Pennsylvania, New Jersey, Delaware, and other parts of the East Coast this spring.

Fifteen different broods, or generations of periodic cicadas, appear in the United States in 13- or 17-year cycles.

Unlike the cicadas that are around every summer, these ones emerge en masse so thickly they can cover trees, and their songs can even drown out public events.

This generation, called Brood II, last appeared in 1996. An Academy of Natural Sciences field observation from June of that year taken in Gloucester County, N.J., recounts thousands of cicadas flying from tree to tree around an intersection.

"They were singing, and the whole area hummed with their sound," the field notes read. "In the air, 100 or so laughing gulls were flying about, snatching the cicadas on the wing."

Brood II cicadas take 17 years to complete their life cycle.

"They live underground for that entire time, and they're totally living off the fluids off of the roots of plants and trees," said Greg Cowper, curatorial assistant in the department of entomology at the Academy of Natural Sciences of Drexel University.

They remain underground until a warm spring evening, 17 years into their life cycle, when the soil temperature reaches 64 degrees. The cicadas burrow out from underground, making exit tunnels to the surface, and nearly an entire generation emerges in a single evening, Cowper said. They scale nearby trees or bushes, shed their old skin, get their wings, and prepare to mate.

"The whole point about coming above ground is to reproduce," Cowper said.

Male cicadas create their characteristic songs to attract a mate, and during these periodic emergences the sound can be intense.

"Just like when I was a teenager, I would rev up my Mustang and try to get girls. They're making this auditory response to bring females," Cowper said.

A few weeks after the long prepared-for emergence, the cicadas die, leaving their eggs to start the cycle all over again.

Cowper's Academy of Natural Sciences records show Brood II has historically appeared in the Philadelphia area in late May or early June. Cowper predicts the most cicada activity locally to be in South Jersey, especially Gloucester County.

In total, eight broods appear in different parts of Pennsylvania in different years.