How 17-year cicadas keep track of time

Seventeen-year periodical cicadas (Magicicada spp.) require 17 years to dev elop underground and all individuals at any location emerge synchronously w ithin several days. The mechanisms that animals use to keep track of time a re poorly understood and nothing is known about how cicada nymphs emerge af ter precisely 17 years. We altered the seasonal cycles of trees supporting cicada nymphs and thereby induced premature metamorphosis of the associated cicadas. This indicates that cicadas accomplish a consistently accurate 17 -year preadult development time by counting host seasonal cycles and not ei ther by the passage of real time or by the accumulation of degree days.