So-called mute cicadas are not so silent

By Kathy Keatley Garvey

Are “mute” cicadas really mute? If so, how do they communicate and attract mates?

A team of scientists including Christian Nansen, agricultural entomologist at UC Davis, answers these questions in a new paper, “How Do ‘Mute’ Cicadas Produce their Calling Songs?” in the Feb. 25th edition of PLOS ONE.

Cicadas in the genus Karenia lack the specialized sound-producing structures, called tymbals, that characterize most cicadas, according to Nansen and colleagues Changquing Luo and Cong Wei, both of Northwest A & F University, Shaanxi, China.

But the word mute is misleading, says Nansen. “They do indeed produce sounds.”

The researchers found that Karenia caelatata produces impact sounds by banging the leading edge, or costa of the forewing against the operculum, a lid covering the insect’s equivalent of an ear. The operculum of K. caelatata is larger than in other cicadas and extends past the edge of the body.

When the male mute cicadas are at rest, the wings are held back over the body with the trailing edge of the wing is locked into a groove on the animals back. When the male wants to make some noise, he lifts his abdomen from the tree branch and rapidly opens and closes his wings. With the back edge of the wing locked in place, the leading edge beats against the hard operculum to make a clicking sound. It’s somewhat like beating a drum while other cicada species with tymbal mechanisms play an orchestra of diverse and loud sounds.

“The new sound-production mechanism expands our knowledge on the diversity of acoustic signaling behavior in cicadas and further underscores the need for more bioacoustic studies on cicadas which lack tymbal mechanism,” Nansen and colleagues concluded in their abstract.

Video: How the mute cicada sings (ScienceNews)

Cicadas, also known as “tree crickets” (from Latin cicada), are among the most widely recognized of insects due to their large size, usually 2 to 5 centimeters or more, and loud sounds, sometime as high as 120 decibels. Theirs is among the loudest of all insect-produced sounds. Cicadas live in warm climates, from temperate to tropical. Immature cicadas spend most of their lives sucking juice from tree roots. The adults suck plant juices from stems.

The best-known North American genus, Magicicada, has a long life cycle of 13 or 17 years and emerges in great numbers.

Cicadas damage cultivated crops, shrubs, and trees, mainly from females scarring tree branches where they lay their eggs. In many cultures, cicadas are a delicacy on the menu.

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