Scientists believe that the cicada’s unusual lifecycle is an adaptation to save the species from being wiped out by predators. A predator can’t specialize in cicadas the way an anteater specializes in ants and termites, because it will starve before there’s a good crop of cicadas. When cicadas do emerge, they do so in such huge numbers that they can afford predators glutting themselves on newly hatched insects.
Scientists believe that nymphs know when to come out of the ground by counting the ebbing and flowing of the sap they feed on. When 13 or 17 cycles have been completed and the soil reaches 65 degrees Fahrenheit, the nymph emerges.