A brood spends its whole juvenile life on tree roots, then emerges within a few days across a whole region. Local densities have been recorded above a million individuals per acre. The adults are slow, conspicuous and defenceless, and every predator in the forest eats them, but the supply so far exceeds appetite that most cicadas survive to breed. The strategy is called predator satiation, and it only works if emergence is synchronised and rare.
The prime intervals are the part still argued about. One explanation is that a prime cycle minimises how often the cicadas coincide with predators or parasites on shorter cycles of their own. Another emphasises avoiding hybridisation between thirteen and seventeen year broods, since primes share a common multiple only at 221 years.
What is documented rather than inferred is the accounting. Broods are numbered, their ranges mapped since the nineteenth century, and occasional four-year-early emergences recorded without the schedule collapsing.