Cicadas are winged insects of the superfamily Cicadoidea. There are over 2,500 species of cicada, native to temperate and tropical regions across the globe. While most cicadas are an inch or two long, members of the Pomponia and Tacua genera can be over half a foot with an 8inch wingspan. Cicadas live on tree sap, sucking up the sweet stuff by puncturing the bark with their straw-like proboscis; as they have no defenses, the only way a cicada might hurt a human is by mistaking them for a tree and going in for drink.
To better attract mates, males’ synch their songs to amplify the effect. Once a female is fertilized, she carves slits into the tree bark, laying her eggs inside. Upon hatching, cicada nymphs fall from the trees and burrow underground; they remain buried until it’s time to mate, surviving by drinking from tree roots.
Annual cicada nymphs emerge each summer; molt to reach their winged adult form; then fly into the trees to mate, lay eggs, and die. But some species – like North America’s Magicicada – are periodical instead of annual. Once underground, Magicicada nymphs won’t emerge for another 13 or 17 years (depending on the species). When the time comes, populations of these bugs synchronize their emergence and mating – sometimes in tremendous groups called broods. Brood X (the Great Eastern Brood) first emerged in 2004 across Illinois and Michigan, and from New York to Georgia; they make their next appearance in 2021. Likewise, after a 13-year hiatus, Brood XIX (the Great Southern Brood) has recently reemerged across the majority of the American South.
Cicadas' natural predators include wasps, birds, rodents – even humans. By breeding in such large numbers, cicadas benefit from predator satiation: there are so many of them, most aren’t eaten and get to reproduce. Yet the periodical Magicicada gain an additional, more mathematic advantage. You may have noticed that 13 and 17 are prime numbers. Imagine a predator with a four-year life cycle happens to be around when a 13-year cicada brood emerges; because 13 is prime, it will be another 52 years until that predator gets to taste another cicada. Whatever your feelings on math – if you happen to be a cicada – numbers are a literal lifesaver.