© Chris Simon, University of Connecticut
Two cicadas mate.
It's a bit tricky, getting a tiny drop of Super Glue in exactly the right place on a cicada's thorax. Martin Wikelski must affix his microtransmitter far enough forward so that it doesn't interfere with her wings, because her wings, and how far she flies with them, are why Wikelski, a physiological ecologist at Princeton University, is spending a cloudy May morning watching ungainly red-eyed insects struggle out of their exoskeletons and creep up the trunks of trees whose roots they've been feeding on since 1987. Occasionally he plucks a cicada, glues on the 300 mg electronic burden, and gently places her (or him) back on the tree trunk to continue lumbering upward.
Being a cicada researcher is like being an astronomer on a NASA mission: It's a commitment to years and sometimes decades of waiting, followed by a brief frantic...
BROODING OVER DIFFERENCES
There are three groups of
In 1998, while still grad students at the University of Michigan, John Cooley and David Marshall (both now at the University of Connecticut at Storrs) built on previous work by Chris Simon (also at Storrs) and discovered a new periodical species. Cooley recalls that they were driving south to continue their study of cicada mating systems. Brood XIX, a 13-year brood, had just emerged, and the students started hearing what were supposed to be
"They didn't sound right to us," Cooley says. They tape-recorded the ruckus and looked at it on a spectrogram, using a computer to analyze the sounds. The typical
They soon noticed that cicadas making high-pitched sounds never made low-pitched sounds, and they also had dark abdomens. Cicadas making low-pitched sounds had light abdomens. After the previous Brood XIX emergence in 1985, Simon and Andrew Martin had shown that the midwestern and southern cicadas, then both known as
A WHOLE NEW BUG
How new periodical species arise poses a mystery in light of the insect's survival strategy. Researchers describe them as "predator foolhardy": Some cicadas can flee hungry birds, but the sluggish periodicals are easy snacks. Instead, they survive via predator satiation. They emerge in the billions, so numerous that predators can't eat them all. "They combine the idea of safety in numbers with the idea of not being around for a long period of time," explains Simon.
Tabitha M. Powledge
A Brood-X cicada emerges from its exoskeleton.
Satiation as a strategy makes speciation difficult. Individuals pioneering a new trait are few in number; chances are they would be picked off quickly and fail to pass on the trait. Randel Cox, an earth scientist at the University of Memphis, and Chris Carlton, now director of the Louisiana State Arthropod Museum in Baton Rouge, suggested that
Marshall, Cooley, and Simon say that scenario might be possible if 13-year males don't wander far, because in order to survive, their
WATCHING THE CLOCK
Cicadas can indeed modify their periods. Developmental sloppiness is extreme in the periodicals, which frequently drop or add an entire four-year instar. In 2000, a large premature emergence of some of this year's Brood X occurred, which Kritsky says he predicted as early as 1991. And, Kritsky reports, some Brood XIV cicadas, not due out until 2008, instead emerged this year with Brood X. "Brood XIV cicadas emerged this year from a tree that was not planted in 1987, so it had to be an acceleration."
"Periodical cicadas seem to have flexibility in their life-cycle length that is environmentally triggerable, or can be environmentally cued," says Marshall. Researchers speculate that the shift may have something to do with weather or nutrition, but they don't really know. This plasticity might provide a bridge, Marshall says. If many 17-year cicadas are cued to come out in 13 years, and if the cue remains over many generations, natural selection can favor genes that make the new life cycle stick.
A key issue is how far cicadas travel, and whether males travel farther. Marshall, Cooley, and Simon argue that their life-cycle shift model of
Enter Wikelski, who has used his microtransmitters mostly to track birds. In his collaboration with Simon, he says he radio-tagged 10 male and 10 female cicadas using 300 mg micropip transmitters, and he and his students checked the location of each one every few hours using handheld scanners. After 11 to 13 days, one transmitter was removed from an apparently sick insect and four simply vanished; Wikelski thinks the unlucky four were either eaten or run over. The researchers recovered 15 transmitters, 10 of them still attached to corpses, within 100 meters of the trees where they had tagged the cicadas. No differences between males and females were observed. Wikelski's conclusion: "Those critters are the most stationary creatures I know of."
So perhaps cicadas aren't enthusiastic travelers, which would support the hypothesis that
Moreover, this year both Kritsky and Cooley observed cicadas that had migrated at least a quarter mile. Kritsky reports, for example, that no cicadas emerged in a Cincinnati housing development built in the '90s. But by mating time, "So many cicadas had flown into the area that cicada choruses were measured at 96 dB. Hearing damage [can occur at] 93 dB."
Wikelski is working on a smaller transmitter, which he says will be ready in time to track dragonfly migration in the fall. "Obviously it's only the first generation of this kind of investigation," he says. Next time, he forecasts, "I'm sure we'll have much better technical help to understand much more about this dispersal." They have to wait only until 2007.
Tabitha M. Powledge