Nature Journal

15. Periodical Cicadas

121 ß chapter fifteen Periodical Cicadas This piece is based on my one (so far) personal encounter with periodical cicadas. In 1998, Dr. W. Mike Howell and I were conducting a fish survey in western Jefferson County, Alabama, when we were taken aback by an incredible, eerie sound—like metal dragging on concrete. It was easy to follow that sound to its source, a woodlot of small trees teeming with cicadas. It was also easy to see that the woodlot had been timbered and severely altered during the thirteen-year cicada life cycle. So what two successive generations of insects experience may be totally different things. Every thirteen years—precisely every thirteen years—near the beginning of May, the woodlands of Alabama waken to an eerie, incessant din. The strange, numbing noise is quickly traced to thousands of two-inch-long, blackbodied , red-eyed creatures that literally arise overnight, crawling up from deep below the soil surface to perch and sing. And just as suddenly, toward the end of the same month, they completely disappear. Often mistakenly called “locusts” after their voracious biblical cousins, periodical cicadas are among the world’s 122 ß chapter fifteen longest-lived insects. They spend most of those lives tucked safely underground , supping on the juices kindly provided by tree roots and slowly developing through five instar stages. But every thirteen (or seventeen) years, in response to two-million-year-old molecular triggers, they rouse from seeming slumber, crawl slowly to the surface, and shed their final nymphal skin. Upon reaching this brief state of adulthood, male cicadas cluster in enormous aggregations called choruses, vibrating a pair of abdominal membranes to produce a droning “song” whose decibel (or annoyance) level can reach 100. (A New England naturalist, in 1669, complained of “such a constant yelling noise as made all the woods ring of them, and ready to deaf the hearers.”) Females, attracted to the chorus, are quickly mated; they then slit twigs with their blade-like ovipositors and deposit tiny, white, spindle-shaped eggs. By the end of June, the eggs hatch into first instar nymphs, raining to the ground and burrowing toward the tree roots. Their parents’ bodies—finally silent—remain scattered and decomposing above. This bizarre life cycle lasts either thirteen or seventeen years—but nothing in-between—with the longer cycles reserved for more northern climates. The nomenclature of periodical cicadas, as standardized in 1893, places them in thirty broods designated by Roman numerals. (Starting that year, numerals were assigned by the year of appearance.) But not all broods have been occupied, and some have become extinct. Fifteen are currently recognized—twelve of the seventeen-year cycles (I–X, XIII, and XIV) and three of the thirteen-year cycles (XIX, XXII, and XXIII). Brood XIX occurs in Alabama and adjoining states, appear- periodical cicadas ß 123 ing most recently in 1959, 1972, 1985, and 1998. (If you would like to pause, at this point, to reflect on your own life and the changes that took place between those years, please do.) Why would such weird “now-you-see-’em-now-you-don’t” strategies evolve? The generally accepted idea is that periodical cicadas benefit from their periodical nature through predator satiation. There’s safety in huge numbers; and, no matter how greedily birds, squirrels, turtles, snakes, and spiders dine upon them, they just can’t eat ’em all. And by emerging in perfect synchrony, and by staying site-specific, adults are assured of plenty of potential mates in the immediate neighborhood. (While mistakes are made, the occasional straggler who wanders too far, temporally or spatially, is quickly consumed.) But why the magic numbers 13 and 17? This question, long puzzling to evolutionary biologists, was finally satisfactorily answered in 1997 (the Year of Brood III, if you follow the Periodical Cicada Zodiac). According to theorist Jin Yoshimura, climate cooling during ice ages led to the slowing of nymphal development—cooler soil temperatures stretched the original seven- to nine-year cycle (found in some modern cicadas) to twelve to fifteen years in the South and fourteen to eighteen years in the North. Although breeding groups developed, their timing patterns were not precisely fixed; insects from neighboring broods could meet and mate, producing asynchronous hybrids bound for extinction. Patterns based on non-prime numbers—12, 14, and 15 in one group and 14, 15, 16, and 18 in the other—came into phase and hybridized too...