北美17年蝉提前4年出土

Swarms of cicadas are unexpectedly crawling out from under trees from North Carolina to New Jersey. The red-eyed insects are almost impossible to miss; they fly around lazily, plunking into backyard barbeques and crashing into cars. They litter the ground with their crunchy husks as they molt. Most noticeably, they chirp en masse for their mates, producing a relentless, shrill buzz that is recognized as a song of summer. And within a month they are gone.

Different populations, or broods, of "periodical" cicadas emerge in distinct geographical regions during specific years, after spending a 13- or 17-year span growing underground. (Some "annual" species just emerge yearly.) Scientists were expecting to see Brood VI bugs in South Carolina and Georgia, which happened, but they got a surprise when Brood X cicadas also started appearing in North Carolina, Virginia, Maryland, the District of Columbia, Ohio and Indiana last week -- four years earlier than anticipated.

Experts suspect a warming climate, with more warm weeks a year during which the underground nymphs can grow, could be triggering some cicadas to emerge ahead of their brood. "Temperature is everything," says Marlene Zuk, an entomologist at the University of Minnesota. "When temperature changes, insects don't just feel hot or cold. Their whole body doesn't function normally." And cicada nymphs may be growing to a threshold size so quickly that their internal biological clock is miscalculating when it is time to emerge, says Keith Clay, a biologist and cicada expert at Indiana University Bloomington. To calibrate these clocks, periodical cicadas likely rely on a variety of environmental cues such as changing seasons and ground temperature, he says. Nymphs feed on the xylem fluid (sap) from tree roots, and changes in the fluid composition as trees leaf out each spring may also help them gauge the passage of time. Entomologists reached this conclusion back in 2000 when they artificially sped up the blooming cycle of peach trees supporting cicada nymphs that were in their 15th year and tricked the insects into emerging a year early.