Seventeen-year periodical cicadas (Magicicada spp.) require 17 years to develop underground and all individuals at any location emerge synchronously within several days. The mechanisms that animals use to keep track of time are poorly understood and nothing is known about how cicada nymphs emerge after 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 either by the passage of real time or by the accumulation of degree days.