Deep in the dirt from North Carolina to Connecticut, billions of cicadas have been waiting 17 years for a few weeks of sunshine, singing and sex in the treetops. Their time has finally come, and they began turning up Friday in Calvert and St. Mary's counties.
These revelers hatched back in 1979 and have spent the intervening years awaiting the right moment to make their appearance.
For squeamish humans, of course, there's never a "right" time for millions of big, noisy bugs to crawl out of the dirt.
The 17-year cicadas' dawn-to-dusk, 80-decibel mating calls can drive people nuts. Their sheer numbers, clumsy flight and surprise landings have freaked drivers and rattled wedding guests. And their carcasses can pile up in smelly heaps faster than eager cats and dogs can eat them.
But scientists are fascinated by the bugs' longevity and their mysterious 13- and 17-year life cycles -- both numbers with intriguing mathematical and evolutionary significance.
The good news for cicada-phobes is that this onslaught may not affect the Baltimore area. John D. Zyla, 32, a naturalist at Calvert County's Battle Creek Cypress Swamp nature center, thinks the 1996 batch is confined mostly to Southern Maryland.
Calvert and St. Mary's countians have told him the cicadas "were everywhere" in 1979. "They were very loud," he said. In the coming weeks, he hopes to pinpoint their range.
America's "periodical cicadas" first were reported by 17th-century New Englanders, who said the "flies made such a constant yelling noise as made all the woods ring of them, and ready to deaf the hearers."
They are called "periodical" because they emerge from the ground in vast numbers to mate only once every 13 or 17 years. But they don't all appear and disappear everywhere on the same schedule.
Different "broods," or populations of periodical cicadas, emerge in different years, in different places. This year's emergence in Southern Maryland is part of what entomologists call Brood 2, and they last emerged in 1979.
In May 1987, the Baltimore region was aswarm in cicadas when another population, part of Brood 10, crawled from the ground and began clamoring in the trees for sexual fulfillment. Their offspring are due back in 2004.
American entomologist Charles Marlatt first assigned the broods' numbers beginning with those that emerged in 1893. Some have since become extinct. Others are insignificant.
Brood 2 emerged in 1894, 1911, 1928, 1945, 1962 and 1979.
Tree-cutting, paving and development are reducing their range, but there are still 12 sizable broods of 17-year cicadas, and three broods of 13-year cicadas in the eastern United States.
Since Marlatt did his work, research and DNA analysis have shed light on some of the mysteries of the insect, often mistakenly called "locusts." Evolutionary biologists guess at others.
Why, for example, do cicadas live so long? And why do they emerge every 13 or 17 years? Is it an accident that each life span is a "prime" number, evenly divisible only by one and itself?
The cicadas' six species all belong to the genus Magicicada. They are distant cousins to the more familiar two- and three-year cicadas, some of which emerge each July or August.
Magicicada spend all but a few weeks of their lives underground, sucking on tree roots. After somehow noting the passage of 13 or 17 years, they dig neat, round, escape shafts to the surface. When the soil warms to 64 degrees or so, they all emerge in a
Finding a vertical perch, the nymphs split their dry skins, and pale, winged adults climb out to begin an urgent search for mates.
Each brood contains several species, and they have just a week or two to sort each other out. The males call by vibrating two round organs called tymbals under each wing. Multiplied by millions, the songs become a lusty din that attracts members of like species to treetop "chorus centers."
After they mate, the females disperse, and each one slips up to 600 eggs into cuts in the tips of tree branches. In six to eight weeks, the eggs hatch, the nymphs drop to the ground and burrow into the soil to feed for another 13 or 17 years. Their parents die.
The cicadas' egg-laying wilts nearby leaves and can damage orchards. But otherwise they're harmless, and homeowners are urged not to run for the bug spray.
No one knows why cicadas evolved their unique life cycles. But scientists have lots of ideas.
"It's all speculation," said Dr. Chris Simon, an associate professor of evolutionary biology at the University of Connecticut and author of a summary of scientific thinking on the periodical cicadas.
"My idea is they lengthened their life cycle to become synchronized," she said. The nymphs develop at widely varying rates. Because there is safety in numbers, the faster ones evolved an ability to delay their emergence until the others "catch up."
"To do that, you have to extend your life cycle," she said.
Others believe the cicadas' ancestors got into an evolutionary race with some cicada-specialized predators. Time underground lengthened until the shorter-lived predators could no longer synchronize their own life cycles with the cicadas' emergence.
Prime-numbered life cycles would have provided the cicadas with a nifty mathematical refuge.
When a hypothetical 12-year cicada emerged, it could be met each time by specialized predators with life cycles of one, two, three, four or six years. By evolving a 13-year cycle, it would face repeated predation only from enemies with one- or 13-year cycles.
Simon said one predator -- a fungus -- has solved the problem. Its spores fall from infected adults and rest in the soil for 13 or 17 years until the next generation of nymphs emerges.
Other scientists say cicada ancestors may have had life cycles of many lengths. But evolution would have favored prime-numbered cycles because their emergence would intersect less often with those of nonprime cicadas.
For example, 12-year cicadas and 14-year cicadas would encounter each other every 84 years. Prime-numbered 13-year cicadas would only encounter the 14-year cicadas every 182 years. Today's 13- and 17-year cicadas emerge together only once in 221 years.
Their greater isolation in time would minimize the prime-numbered cicadas' opportunities for cross-breeding, which would produce hybrid offspring with a variety of cycles, scientists suggest. It also standardizes the cycles of the prime-numbered cicadas, and they emerge in greater numbers.
The cicadas' huge, synchronized emergence -- as many as 1.5 million in a single acre -- is a survival strategy biologists call "predator satiation." It means hungry birds and squirrels overfeed before they can finish off all the cicadas, leaving more to reproduce.
It has allowed cicadas to become "predator foolhardy" -- big, obvious, noisy and slow. It means stragglers that emerge a year early or late tend to be gobbled up.
But not always. Sometimes they form new broods.
Scientists have observed that a cicada brood in a region usually emerges a year before the brood that abuts it to the south. They theorize that they once were one brood, but that a late spring freeze caused the more northerly nymphs to count one more "winter" than their southern siblings.
Likewise, 17-year cicadas sometimes emerge four years early, an "acceleration" scientists say may be prompted by underground crowding.
Often these populations are wiped out by predators. But Simon said her research has found adjacent 13- and 17-year broods that are genetically identical. That suggests such accelerations can become permanent and create a new 13-year brood.
Pub Date: 5/19/96
Naturalists are asking for help defining precisely where 17-year cicadas emerge in Maryland.
Available data are vague and "the distribution patterns we see don't represent necessarily the distribution of a species. They represent the path of the collectors," said Dr. Richard C. Froeschner, 80, an emeritus entomologist at the Smithsonian Institution.
Anyone who sees or hears cicadas from now until mid-June is asked to box a selection of the insects, noting the date, location and circumstances, and send them to Froeschner at the Smithsonian Institution, Mail Room 105, 10th St. and Constitution Ave. NW, Washington, D.C. 20560.
Naturalist John Zyla, at the Battle Creek Cypress Swamp Nature Center, is accepting reports at (410) 535-5327, or by mail, c/o Calvert County Courthouse, Prince Frederick, 20678.
An educational program at the nature center -- at 10 a.m. June 1 -- will include a "crunch brunch" with some choice cicada recipes. Yes, they're edible.