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Periodic cicadas are cicadas which spend all but a very few weeks of their long lives silent and immobile underground. Then almost simultaneously millions of nymphs emerge from holes in the earth and, within hours, become gregarious flying adults. Within a short time however, the adults of that brood are all dead, leaving behind eggs that will hatch to become a new underground generation, one that will emerge again in. The "emergence" is coordinated over extended periods of time - 7, 13, 17 year cycles are thought to reflect a strategy of avoiding other species on different cycles to avoid mating confusion and cross breeding . The numbers as prime numbers mean that for example being on a seven and a 13 year cycle two species would only emerge together once in every 200+ years. Wheres if they were on 2 and 4 year cycles they would meet far more frequently.