Calm before the song

Natural History, June, 2004 by Erin Espelie

An insect's age is typically measured in days or weeks, not years. Yet the periodical cicada ("locust" is a common misnomer), like the one poised here atop its recently shed "skin," lives alone in the soil for many years. Underground in their flightless nymph form, the cicadas slurp tree sap, biding their time. Then, after thirteen or seventeen years (depending on the species), the nymphs crawl to the surface and metamorphose into red-eyed adults. Incredibly, all the cicadas in the same brood--a population connected geographically--emerge in synchrony. Billions of them take over many forests across eastern North America for a few raucous weeks of mating before they die.

Leon G. Higley, an entomologist and photographer from the University of Nebraska-Lincoln, observed the 1998 emergence of a seventeen-year cicada brood. The insects were so thick, he recalls, that "you crunched them underfoot and were pelted by them from above" So perfect is the cast of the cicada's shed skin in Higley's photograph that a mold of the insect's respiratory vessels, or tracheoles, is visible (see the white stringy-looking strand of chitin at the left of the image).

Males announce the mating frenzy with deafening noise; the loudest choristers attract mates. Higley and his family had to shout to be heard above the 120-decibel calls. Females cause significant tree damage when they rake back the bark of twigs to lay their fertilized eggs. A few weeks after all the adults have died, new nymphs drop to the ground and burrow down under their parents' corpses for a long wait.

Such a long development period, followed by such a brief window for mating puts all periodical cicadas at risk. Yet they are far from vulnerable. The insects Higley photographed were so dense near the Platte River, northeast of Lincoln, that predators could gorge themselves without making a dent in the ability of the brood to reproduce. Another evolutionary hypothesis is that the thirteen- and seventeen-year cycles help the cicadas avoid predators with shorter, multiyear life cycles. Both 13 and 17 are prime numbers, divisible only by themselves and 1. A predator in sync with the cicadas one year could not benefit from them again soon enough to become a threat.

This June the largest known brood of seventeen-year periodical cicadas, dubbed Brood X, will emerge across fifteen states in the midwestern and eastern United States. Their next hurrah won't be until 2021. Don't miss out!