Why do cicadas emerge all at once after living underground?

Periodical cicadas are famous for emerging after 13 or 17 years underground, depending on the brood. Once soil temperatures reach the right threshold in spring, huge numbers of nymphs surface, molt into adults and begin mating. Synchrony is an anti-predator strategy: when so many cicadas appear at once, birds and other animals cannot eat them all, so many survive. The 13- and 17-year cycles are prime numbers, which may reduce the chance of matching predator population cycles. These insects are a classic example of extreme life-cycle timing in nature.