Brood II returns better understood
For a few weeks in 2013, a chorus of headlines about the raucous reproduction of periodical cicadas just about drowned out the real cicada news.
This was the year for a cohort of big, reckless, ruby-eyed bugs to break out of the soil for their first and only chance to mate after 17 years of sucking plant roots in the dark. Synchronized generations of three Magicicada species designated as Brood II reliably emerge every 17 years in a swath of the U.S. East Coast from the southern Appalachians to New York (SN: 7/13/13, p. 26).
In just about any year, one of the 15 numbered Magicicada broods emerges somewhere. But Brood II is big and pops out in cities with major news outlets. The 2013 breakout would have been really big science news only if it hadn’t happened.
Still, a synchronized emergence of Magicicada species is one of the great spectacles of nature. These are not the annual or dog-day cicada species, which appear each year in late summer. Out of several thousand cicada species on the planet, only the seven kinds of Magicicada, which live just in the eastern and central United States, make their loud, synchronized appearances at 13- or 17-year intervals. Broods don’t overlap much in any particular grove of trees. So a chance to experience the surging chorus of a particular backyard’s periodical cicadas comes only a few times in a human life.
What actual scientific cicada news there was in 2013 didn’t get much media attention. Chris Simon of the University of Connecticut in Storrs and colleagues pooled DNA data collected during 30 years to create a bizarre but beautiful genealogical tree of Magicicada’s evolutionary history.
It’s a long, strange story, almost musical in the way patterns echo each other in independent branches. One example: Roughly 4 million years ago, Magicicada ancestors started splitting into three distinctive forms (Decim, Cassini and Decula), the researchers reported in April. Then each of these groups split into both 17-year and 13-year forms.
Brood II, like most other broods, mixes splinter populations of all three original groups, even though they don’t breed with each other. It’s evolutionary weirdness that the well-trained ear can hear. Cassini-type males rev like electric carving knives. Deculas cough out rasps. Decims whistle-drone like B-movie spaceships. Anyone who missed the distinction this time can hear it all again in 2030.