What’s the News: In another glorious reminder of how weird nature really is, it’s time to get ready for the swarm: This May, after spending 13 years underground, huge populations of cicadas will emerge in the southern U.S. to molt, sing their riotous mating tunes, and breed. It’s a brief coda to their long adolescence in burrows 30 cm beneath the soil—by July, they will be dead, and their children will be beginning their years of exile from the surface.
What’s the Context:
- While there are plenty of cicada species that send a generation to the surface every year, cyclical cicadas (of the genus Magicicada) come out en masse after 13 or 17 years. Scientists believe that this strategy evolved as a way to overwhelm predators—when there are so many cicadas around at one time, a good many of them will probably survive.
Cyclical cicadas live in tribes called broods that occupy certain geographic areas (see map)—the brood that’s swarming this year, called brood XIX or the Great Southern Brood. It was last seen in 1998. (Go ahead, check the math.)- Scientists have puzzled for decades over the fact that some cyclical cicadas live for 13 years and others for 17. One model built to study cicada evolution assumes that a single gene could determine whether a brood is a 17-er or a 13-er, with the 13-year gene being dominant over the 17-year gene—see that paper here. But the real-life genetics are still unclear.
- Cicadas have been known to deviate occasionally from their routine. In 2009, brood II burst out four years early, surprising scientists (and folks living in the Atlantic states, where brood II is located). They are very sensitive to soil temperatures, and scientists think that mild winters might accelerate cicadas’ development and trigger early swarms.
- Like solar eclipses and planetary transits of the sun, overlaps between 17-year and 13-year cicada broods are easily calculated: brood XIX and 17-year brood IV should next emerge simultaneously in Missouri, where their territories overlap, in 2219.
The Future Holds: The first cicadas have been spotted in Georgia already—keep your eyes peeled for more in the coming weeks.
Image credit: tlindenbaum/flickr. Map credit: College of Mount Joseph
