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by Marat Moore
It's a sonic phenomenon-and a rare one. In May, one of the longest-living insect species in the world, the periodical cicada known as Brood X, emerged by the billions in 15 states in the mid-Atlantic and Midwest.
Cicada sounds have been compared to the whirring of "flying saucers" in sci-fi films, and to the sound of millions of clacking castanets.
After feeding on the sap of tree roots for 17 years, Brood X nymphs encased in hard, cinnamon-colored shells tunneled through the earth and crawled out dime-sized holes at the base of the trees that had fed them.
The white, red-eyed nymphs then climbed trees and other vertical structures, and struggled out of their shells. Stunned and silent, they were immobile until their wings dried, shiny and transparent, and their bodies turned dark. At maturity they are about 1.5 inches with large red compound eyes on each side of their head-and ready for their ancient mating ritual of song.
Mating Songs
Male cicadas use a complex apparatus for singing. Males produce sounds through tymbals, a pair of ribbed membranes at the base of the abdomen. Contracting internal tybal muscles cause the tymbals to buckle inward and produce a sound pulse. The high-pitched, rapid clicks resonate through a large air sac in the insect's abdomen and other structures to control sound volume and quality. The upward angles of the wings form a megaphone-like chamber that further controls the sound, which can travel for more than a quarter of a mile.
Females respond with a "wing flick," which creates a sound that can vary from a gentle rustle to a sharp pop that is heard and seen by the males.
Both male and female cicadas have organs for hearing. A pair of large, mirror-like membranes, the tympana, not only receive the sound, but amplify the sounds produced by males. The tympana are connected to an auditory organ by a short tendon. When sound leaves the tympana, it's nearly 20 times louder than the clicks produced by the tymbals. When a male sings, it creases the tympana so it won't be deafened by its own sound.
Males cluster in "chorusing centers" that move from place to place as they migrate in their search for a mate. They produce waves of sound that increase in intensity, then ebb, in a cycle of a few seconds. Cicadas sounds have been recorded at levels of more than 90 dBA. In an area of mature woodland in Greenbelt, MD near Washington, DC, sound levels recorded at a distance of eight feet with a Radio Shack Sound Level Meter measured a 3-second interval of sound ranging from 80 dBA to 89 dBA, as loud as a power lawn mower. More than two and a half hours of continuous exposure to 90 dBA noise can cause hearing loss, according to ASHA's audiology unit.
Cicada songs have been celebrated for centuries, long before power tools provided a basis for comparison. Japanese haikuists honored their songs, and Homer compared the discourse of "sage chiefs exempt from war" in ancient Greece to the sounds of the cicadas. Legend has it that the black "W" on their wings stands for a time of war.
Once mating has occurred, the females lay their eggs in twigs by slicing through the wood, which can kill young trees,. The eggs hatch in six to 10 weeks, fall to the ground, and dig their way into the earth to sip the sap of tree roots for another 17 years, when they will again emerge.
Check back for a follow-up story in 2021.
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