for National Geographic News
After 17 years underground, billions of cicadas are about to descend upon the U.S. Midwest, crowding the trees and filling the air with their distinctive mating call.
But the usually punctual Brood XIII bugs are emerging about a week ahead of schedule—which has some scientists pondering how a changing climate might alter the cicadas' little-understood life cycle.
"The fact that our Aprils are warmer than they have been in the past is apparently encouraging the cicadas to emerge a week or so earlier than they have in the past," said Gene Kritsky, a biologist and cicada expert at the College of Mount St. Joseph in Cincinnati, Ohio.
A Puzzling Cycle
Cicadas in the nymph stage subsist on sap from tree roots. When they emerge as adults—every 13 to 17 years, depending on the brood—they climb trees, shed their skin, and wait for their new exoskeleton to harden.
The adult female cicadas deposit their eggs in the trees. After four to six weeks, nymphs hatch and fall to the soil, where they burrow in for a lengthy stay underground.
(See related: "Cicada Facts: Understanding the Invasion" [Updated May 21, 2007].)
But scientists don't fully understand why the insects follow such a pattern. It may be a species survival behavior: Since there are simply too many for predators to eat, some are able to survive to reproduce.
More is known about the climatic triggers that draw the cicadas aboveground.
"We know that the soil temperatures need to be around 64 to 65 degrees Fahrenheit (about 18 degrees Celsius) for about three to four days before they will emerge," said Daniel Summers, an entomologist at the Field Museum in Chicago, Illinois.
"That is probably the one thing that we do know."
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