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The Buzz on Brood XIX

Posted By kirkwoj On September 2, 2011 @ 2:10 pm In Bright Ideas, Summer 2011 | No Comments

Millions of 13-year cicadas emerged in Nashville in May,  emitting sound levels at 85 to 88 decibels and clogging up cooling systems all over campus.

Millions of 13-year cicadas emerged in Nashville in May, emitting sound levels at 85 to 88 decibels and clogging up cooling systems all over campus.

And on the subject of bugs—Nashville’s largest brood of cicadas emerged in May and hung around for five or six weeks, blanketing the campus like thousands of tiny helicopters. Besides their practice of appearing on 13- to 17-year cycles, cicadas are best known for the buzzing and clicking sounds they make, which can be amplified by the sudden emergence of millions of the insects to an overpowering din.

This crop of cicadas, known as Brood XIX, operates on a 13-year cycle. Males are responsible for the noise, which they produce by vibrating ridged membranes on their abdomens.

Todd Ricketts, associate professor at the Vanderbilt Bill Wilkerson Center for Otolaryngology and Communication Sciences, measured the cicadas’ sound level at peaks of 85­ to 88 decibels outside Vanderbilt University Medical Center on May 24. These levels are comparable to a motorcycle or subway at 25 feet. Stand too close for too long, and these vocal insects could damage your hearing, Ricketts says.

Cicadas caused problems with cooling systems across campus, says Mark Petty, assistant vice chancellor for plant operations. Most Vanderbilt buildings have cooling towers, which essentially push air over chilled water. Cooling-tower motors emit a frequency that amounts to a siren’s song for the insects.

“The cicadas end up dying in large numbers in and around the cooling towers, and their little carcasses plug up the strainers,” Petty explains. “We have to shut down the tower to clean it out, and while the tower is down, the building doesn’t receive any cooling.”

The cicadas’ appearance also gave new life to an urban legend that researchers were paying as much as $3,000 for specimens of rare blue-eyed cicadas. While most cicadas have bright red eyes, a very small percentage has blue or white eyes, confirms Patrick Abbot, associate professor of biological sciences, “but the idea of anyone paying for them is a recurrent myth.”

During the last cicada emergence in 1998, Glenn Webb, professor of mathematics, became interested in why cicadas emerge only at yearly intervals that are prime numbers. He created a model with periodic cicadas and hypothetical predators on two- and three-year life cycles, and found that by emerging every 13 or 17 years, the cicadas better ensured their survival.


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