The humble vibrato of summer will crescendo a bit earlier this year in the U.S. South. Billions of cyclical cicadas will be out in full force starting this May, following a 13-year lull.

Having dropped to the ground from treetop eggs during the Clinton administration, the so-called "Great Southern Brood" (aka brood XIX) of cicadas spent more than a decade in a nymph stage underground nibbling on tree roots. But this year, the hefty hemipterans will all be roused from their burrows as the soil warms to crawl, for the first time, into the daylight in search of mates.

And 13 years in the making, the males' signature cacophony is certainly persistent. As St. Louis Zoo entomologist Jane Stevens explained in Scientific American in 1998, "The daytime noisemakers are indefatigable in calling for a mate."

How do they spend so long in their dank borrows, five to 46 centimeters belowground? Scientists are still trying to crack this and other cicada mysteries. But a 2009 study found that symbiotic bacteria that live inside cicadas' cells help produce essential nutrients that are otherwise lacking in the insects' earthy diets.

This brood isn't the only one that passes years underground—and by no means has spent the longest getting ready to make their entrance. There are about 15 broods of periodical cicadas (of the genus Magicicada), which surface in different regions of the U.S. in different 13 or 17 year cycles. The brood X "Great Eastern Brood" cicadas, for example, spent 17 years getting ready for their 2004 debut. The trusty annual crops of cicadas usually crawl out of the ground later in the season, often after the cyclical cicadas have laid their eggs and died.

Researchers still aren't sure why some of these riotous insects emerge in long and odd-yeared cycles. One hypothesis is that it helps to reduce competition for resources by minimizing the frequency with which broods are active at the same time. With each brood arriving in such large swarms, the sheer numbers might also help increase the odds that even if millions are picked off by predators, millions more will survive to reproduce. Birds are among the beneficiaries of these big periodic cicada years, along with scientists—and, apparently, hungry biologists. As Gene Kritsky, editor in chief of American Entomologist told NPR, "You can eat them. They taste to me like cold canned asparagus, very green."

Even if you choose to savor the song rather than the flavor of cicadas this spring in the South, rest assured that it's not likely to be as loud as 1998, when both the 17-year and 13-year cicada species overlapped—an event that happens only once every 221 years.

As for brood XIX, we'll expect to see—or at least hear—them again in 2024.

Image courtesy of Wikimedia Commons/Jonrev