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Magicicada: Scientists Study Swarming Cicadas

May 26, 2004 at 12:00 AM EDT
The largest group of periodical cicadas -- called Brood X -- emerged this month after 17 years underground. Jeffrey Brown investigates the biology and culture of these cyclical insects.

RAY SUAREZ: Finally tonight, an insect makes an amazing return. Jeffrey Brown has our science unit report.

JEFFREY BROWN: It’s called “magicicada” the magic cicada because it performs an astounding feat: Disappearing for many years in a regular cycle, and then suddenly reappearing in unbelievably huge numbers.

This spring, the largest group of periodical cicadas, called Brood X, has emerged for the first time in 17 years, and is overwhelming much of the East and Midwest, from the Mississippi to the mid-Atlantic coastline. No one can say exactly how many there are– estimates range as high as ten trillion, perhaps the largest insect emergence on earth– and cicada mania is in the air. Fun for at least some children, not for others.

CHILD: They look ugly and they’re scary.

JEFFREY BROWN: Web sites to track sightings, cicada ice cream cones– not real cicadas– and cooking recipes– real cicadas. Most excited of all: entomologists who study the insects. One is Mike Raupp of the University of Maryland, who took us to a suburban neighborhood near his campus for a cicada life-cycle tour, beginning with their emergence from the ground.

MIKE RAUPP: Here, look at this. This is incredible. They built a tunnel or an escape chimney out of the ground. These will be anywhere from 8 to maybe 12 or 18 inches deep. They are simply waiting for the soil temperature to reach the mid-60s. That’s an indication that it’s going to be warm enough for the adults to get up out of the ground, move into the vegetation, molt, fly, mate, sing and complete the biological imperative.

JEFFREY BROWN: Under every one of these rocks, a bunch of little holes, right?

MIKE RAUPP: Well, look, this is incredible, maybe in a square foot, 17. In certain locations, there can be as many as 100,000 to 1.5 million cicadas per acre. This is a really good site right here.

JEFFREY BROWN: A whole world under our feet.

MIKE RAUPP: Yeah, and it’s been there working along for 16 long years.

JEFFREY BROWN: Now, what are they doing for all those years?

MIKE RAUPP: That’s a great question. They’ve been attached to the roots of trees and other plants underground, and they feed on a tissue called xylem. This is a nutrient-pore tissue that carries water, minerals and some nutrients up to the top of the tree.

JEFFREY BROWN: Once out of the ground, a cicada heads for plants or trees, and sheds its outer skin. What I keep wondering: How do they know when it’s time to come up?

MIKE RAUPP: Well, underground, they are deprived of any kind of light cues. So what we think is they actually attach to the roots of trees and measure the annual fluxes in nutrients, and perhaps hormones that occur in the plant. They have a biological clock or a molecular clock that allows them to count those 17 years.

JEFFREY BROWN: Notice Raupp said, “We think.” There’s plenty about these cicadas that remains uncertain. Most interesting, perhaps, is why they return in such numbers, and why they have such curious prime-numbered life terms– periodical cicadas are on a 13- or 17-year cycle. What’s for certain is that when they do appear, a mass slaughter takes place; for dogs, squirrels and birds, it’s a feast.

MIKE RAUPP: They’re incredibly easy prey. They have virtually no defense. Right now, everything on this planet wants to eat a cicada.

JEFFREY BROWN: But millions do survive, and that’s led scientists to theorize that the mysterious ways of periodical cicadas are a survival strategy.

MIKE RAUPP: The whole game on the periodical cicada is to emerge in massive numbers simultaneously, and simply overwhelm the predators that want to eat them or that are there to eat them. There are enough cicadas that will emerge to basically fill everybody’s belly and still leave enough to reproduce and perpetuate the species.

JEFFREY BROWN: So, overwhelm the enemy in such vast numbers, and the goal is just survive.

MIKE RAUPP: Exactly.

JEFFREY BROWN: As many as can survive.

MIKE RAUPP: Exactly. That’s right. It’s a safety numbers game. The strategy is called “predator satiation.” You simply make the predators that are out there so full that there is a number that will actually survive. Very bizarre, very clever strategy and, by golly, it sure has worked for tens of thousands of years.

JEFFREY BROWN: Indeed, humans have taken notice of cicadas for thousands of years. Their red eyes were said to come from staring into the fires of Hades underground. Ancient poets, including Homer, referred to the cicada’s song, performed exclusively by the males.

MIKE RAUPP: You can tell the male because he has a blunt abdomen. It’s rounded on the back, blunt abdomen. But if we lift up the wing and look very carefully just beneath the wing, we’ll see an organ called the tymbal organ. That white membrane at the end of my thumb– you can see it vibrating– is how he makes that sound, and it’s only the male that will call.

JEFFREY BROWN: Holding a male to his microphone, Raupp allowed us to get a cicada sound bite.

JEFFREY BROWN: So, just that vibration makes the sound we’re hearing?

MIKE RAUPP: It’s incredible. It’s the tymbal organ, and also the abdomen is largely hollow. We believe it acts as an amplification chamber to amplify this noise and make a sound that can reach as high as 80 to 85 decibels. That’s the sound of a large truck going down the street, or a garbage disposal. It’s pretty noisy.

JEFFREY BROWN: When millions of these guys are up in the trees together, it’s a regular din. And it’s all about one thing.

MIKE RAUPP: These are very romantic arthropods. The males will chorus, and this basically helps different species of cicada to go to the same tree. There are three species that will emerge simultaneously in this Brood X. Once they are sorted out by species, he will shift over to a series of courtship songs, which basically try to convince that special someone that he should be the father of her nymphs.

JEFFREY BROWN: You can hear the differences in these three examples as the male sings a distinct song to attract females of his own species. (Cicada “songs”)

MIKE RAUPP: If he’s successful, she will signify her acceptance with a little fan dance with her wings, allow him to mate, and then he’ll go off and find other ladies. She’ll be left with the task of laying the eggs in the small branches. After about four to six weeks, those eggs will hatch and the little tiny nymphs, now about the size of small ants, will take an incredible leap of faith out of the tops of these trees, down to the earth below, burrow down anywhere from one to three feet underground, and spend the next 16, almost 17 years, feeding on tree roots underground.

JEFFREY BROWN: The cycle continues.

MIKE RAUPP: The circle of life.

JEFFREY BROWN: For their part, the adult cicadas die quickly, soon after they’ve done their mating and breeding duties. So, 17 years underground, four to six weeks above, and it’s over. But while it’s happening, it’s a bonanza for scientists.

MIKE RAUPP: It’s an unparalleled opportunity to learn about nature, ecology, evolution, behavior, all the various aspects of biology — so a million mysteries, a million unanswered questions.

JEFFREY BROWN: So while you’re waiting for the next Brood X, what happens to you for the next 17 years?

MIKE RAUPP: Well, I’ve got a subterranean office, and I’ll go down there for 16 years and write papers and grants, and hopefully in the 17th year they’ll let me back up to tell more bug stories. (Laughter)

JEFFREY BROWN: Cicada mania is expected to last until the end of June, when the Brood X adults are all dead, and countless little nymphs are tunneling back into the ground to await their next appearance in 2021.