The bit that everybody missed was hardly hidden. Male cicada choruses swell to thousands—tens of thousands—and flirt with the subtlety of jets shrieking in for a landing. And thousands of charmed females respond.
When periodical cicadas emerge, once every 13 or 17 years, local outdoor restaurants languish. People threaten to move or go mad. The din has inspired commentary from the greats of natural history such as Charles Darwin, as well as from just about every local newspaper reporter in search of a spring feature.
So, why were two graduate students recently able to identify important components in the mating calls of periodical cicada that no one had ever described?
John Cooley and David Marshall, now at the University of Connecticut in Storrs, may have been lucky. But they’re also part of a band of researchers who’re working hard to push periodical cicada science to new heights and discover overlooked peculiarities of some of the continent’s most in-your-face insects.
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Much of what a cicada scientist does is explain what periodical cicadas don’t do. They’re not able to sting or bite. They don’t kill trees, and, most especially, they’re not 7-year locusts. Not only does the periodical cicada live more than 7 years, it doesn’t even belong to the same family as the locust, which is related to grasshoppers.
Some 2,400 or so species of certified cicadas stretch around the world, mostly in dry lands. The most famous—the ones inspiring all the headlines—live underground for years, then burst out in a synchronized mating frenzy in spring.
These so-called periodical cicadas have driven taxonomists into their own frenzies. One major camp has labeled one genus Magicicada, made up of six well-known species plus a newly discovered one.
“They’re the only cicadas with red eyes” in most of North America, explains Chris Simon, also of Connecticut. The red can shade to orange or salmon, and one in a million pops up with blue or white eyes. These dramatic colors, however, differentiate periodical cicadas from the humdrum, dog-day cicadas.
These, in the genus Tibicen, come out later in the summer and sport dark or olive eyes. They spend a year or more underground before breaking out for raucous courtship. However, since some of these cicadas emerge every summer, people don’t get too excited.
Most insect guides state flatly that Magicicadas, which live only in North America, are the only periodical species. However, long-time cicada researcher Maxine Heath has uncovered tantalizing patterns among the Okanagana species that she has been following for years in Arizona. Some of these show a population boom every 11 or so years, which might represent an early step toward periodicity.
So far, Heath hasn’t described any species with clear-cut periodicity like Magicicada‘s. Still, clarifying such patterns can take a whole career. Her lab work has shown that one species needs years to mature. Also, as Simon explains, for many Okanagana species in question, “only Maxine can tell them apart.”
Eggs in trees
Periodical cicadas lay their eggs in trees, literally. The female’s stiletto-like egg-laying tube rips a cozy gash in a twig and fills it with eggs. When they hatch, the youngsters take a long tumble to the ground and burrow into the soil.
“They’re not sleeping,” emphasizes Gene Kritsky of the College of Mount St. Joseph in Cincinnati, trying to stamp out another myth. Immature cicadas spend the next 13 to 17 years tunneling through the ground and drawing watery liquid out of trees’ xylem, the system of slender vessels that pulls water from the roots to the leaves. Young cicadas must use their considerable sucking muscles to overcome the upward whoosh in the piping.
Just how the young cicadas keep track of their years underground has prompted a variety of notions. Simon wonders whether the seasonal changes in the plants might give cicadas their calendar.
After more than a decade underground, the cicadas work their way closer to the soil surface. Temperatures probably trigger the final breakout, with emergences of members of a species clustered within about a week. After all those years underground, they have only 4 to 6 weeks to mate before they die.
The young cicadas often crawl up through the surface at the end of a day and then spend about an hour wriggling free of the exoskeleton of their fifth and final juvenile stage. The adult body emerges white, but by morning, it has turned black with orange-veined wings. The abandoned, crackly brown exoskeletons often give people their first clue that cicadas have emerged.
The insects have blanketed areas at densities of more than 3.5 million per hectare, consisting of perhaps 500 kilograms, or half a ton, of biomass per hectare.
The new adult needs a few days before it gets down to serious mating. Its body must harden for it to make noise and for its genitals to hold their shape. Then, males begin chorusing.
Simon, who spent part of May camping amid cicadas emerging in North Carolina, testifies that the first males can start around 4 a.m. These early bugs often do subside, but they renew their efforts around dawn.
She can identify the periodical cicadas with her eyes closed. These include three pairs of sister species, a 17-year and a 13-year version of each basic form. The two Magicicada -decim “sound like a flying saucer,” she says, giving a breathy, whirring whistle. She’s known police to pinpoint M. -decim hotspots when people inquire about neighboring “fire alarms.” The new species, which is not yet formally described, has a similar sound but at a different pitch. M. -cassini males sound “like little ratchets,” and M. -decula males “cheep, cheep” until so many of them cluster that they sound like someone throwing water into hot fat, says Simon.
One of the great eras in cicada science came in the early 1960s when Monte Lloyd, now in New Orleans, and his colleagues mapped broods and later dug up immature 17-year cicadas at various stages. The researchers discerned that the insects typically take a 4-year rest during their underground growth.
The number four raised exciting possibilities to explain cicada habits. Lloyd and his colleagues speculated that losing the 4-year rest for one generation through some developmental quirk might allow a brood to emerge early.
If enough insects came out of the ground ahead of schedule and mated with each other, they might found a brood 4 years out of synch with their original cohort. The two broods could share the same ground but not meet as adults because they emerge in different years. A string of such happenstances might explain how the United States came to have 12 separate broods of 17-year cicadas.
That idea got a big boost in 1969, when Chicago reverberated with hundreds of thousands of cicadas—apparently 4 years early. That off-year brood died out, but the idea lived on.
For example, Simon remembered the 4-year gap when contemplating cicada abdomens. Within what was supposedly a single burst of 13-year cicadas cycling in the Midwest, she found some with darker undersides than others. Genetic analysis has shown that some of these 13-year species are actually 17-year cicadas that somehow lost their 4-year rest break forever and now carry on as a new kind of 13-year cicada.
Simon marvels at the luck that enabled her to figure out why the formerly 17-year species—now the 17-minus-4-year species—remained separate instead of interbreeding with true 13-year cicadas coming out at the same time. During the peak of the season in 1998, she mentioned the problem to Cooley and Marshall, who were on a major cicada-observing road trip. They just happened to have phoned from a zone where the longtime 13-year and the 17-minus-4-year broods overlapped.
Years of analyzing cicada mating calls had tuned the travelers’ ears to nuances generally appreciated only by female insects. The cacophony around them, with its contributions from both species, sounded funny, they reported.
When Cooley and Marshall dissected the uproar, their instruments detected two prominent pitches in the chorus. Auditions of individual insects traced the sounds to the different species.
If the different pitches of their calls keep the species from mating with each other, and if those pitch differences lessen outside overlap zones, cicadas could provide a classic case of what evolutionary biologists call reproductive character displacement, says Simon. In theory, similar species sharing a habitat develop exaggerated differences in some characteristic as a hint for finding Mr. or Ms Right Species.
The pitch difference in mating calls makes a particularly tidy story, Simon notes, because the calls differ most in the zones where species overlap. Farther from this boundary, where even an indiscriminant cicada wouldn’t find the wrong species, the calls don’t reach such extremes.
Kritsky likewise was thinking of Lloyd’s 4-year rest break when he designed a class exercise in 1991. He had students read Lloyd’s paper and write predictions to seal in envelopes of what they would find when they then dug up some 17-year cicadas partway through their life cycle in an orchard on campus.
The immature insects that the students unearthed seemed to be much further along in their development than he and the students had expected. Kritsky speculated that the insects had somehow skipped their rest period and would therefore emerge 4 years early—in late spring of 2000.
He kept monitoring the cicadas’ progress. “No matter what I was teaching, the students knew we were going to go dig up the orchard,” he says.
Over the years, he also searched for historical records of cicada emergences. He’s collected some 8,000 from the standard scientific literature, diaries and letters preserved in historical societies, and even Governor William Bradford’s 17th-century records of life in Plymouth colony.
In 98 percent of the records, cicadas emerge either on time, 1 year off-target, or 4 years early.
In early May this year, Kritsky was still predicting the great early Y2K emergence. He appeared on local Cincinnati television on May 3, announcing that the event could happen soon in Cincinnati. Finally on May 7, he heard about an early-bird cicada. This year had been expected to bring out a brood centered in North Carolina, but Kritsky received reports of cicadas bumbling out of the ground in the suburbs of Washington, D.C., and his much-studied orchard in Cincinnati.
For the first time, a scientist had predicted a speedup, and he’d been right.
The early arrival of cicadas should please all the animals that prey on the insects. Cicadas are what Simon calls “predator foolhardy.” That is, very easy to catch.
Cooley and Marshall are investigating the only cicada enemy that has adopted the multiyear lifecycle of its prey. Spores of the fungus Massospora cicadina infect immature cicadas as they burrow into the soil. The insects shed their last exoskeletons and emerge to mate as infected adults.
The fungus spreads somewhat like a venereal disease. Among the cicadas, infected females mate but don’t lay eggs, and infected males show interest in males as well as females.
Adults that pick up the fungus from an ill-advised partnering develop nubbins on their abdomens that eventually come loose and fall to the ground, where they infect young cicadas and begin the cycle again.
The fungus may lower insect-population numbers by confusing cicadas about their gender. Cooley’s preliminary evidence suggests that the males that were infected as juveniles try to mate with other males. Their efforts don’t do much for cicadakind but do spread the fungus.
Even without confusion introduced by a fungus, the workings of normal cicada courtship have been puzzling. Males shriek away in rowdy companies, but researchers hadn’t worked out which elements were appealing and just how the enticed females would signal their interest.
Cooley and Marshall recently happened onto one of their major clues when they caged females that had never mated.
“After several days, they were getting hyperreceptive,” Simon says. The females could hear a male calling, and the researchers noticed that they all flicked their wings, making a little click, in unison.
That turned out to be the “Let’s talk” signal. A male ends each of his screeches with a quick downward slur. Marshall and Cooley then found that a female intrigued by a particular suitor flicks her wings at a certain interval after this final sound. The researchers learned that courting males will give an interference buzz to keep other males from crowding in on the romantic moment.
Cooley has perfected his own whistling downward slur to the point of winning feminine clicks of appreciation.
Males that earn interested clicks shift their courtship song to a second style, and if that also works, the female lets them approach. The males then shift into a final serenade.
The males can be coaxed to sing to researchers who give the right encouraging clicks.
Knowing what to whistle and when allows a noncicada to try out the ritual. “You can have cicadas all over you,” Simon says.
Her tone implies that little else could be as entertaining.