Cicadas Have an Existential Problem

When the cicadas of Brood X start to swarm the United States in their billions, try to look beyond their overwhelming numbers. Instead, focus on just one of them. Despite appearances, that individual cicada will be a swarm unto itself—the insect and a community of organisms living inside it. Their lives have been so tightly entwined that they cannot survive alone. Their fates have been so precariously interlinked that their future is uncertain. And their relationship is so unusual that when John McCutcheon first stumbled upon it in 2008, he had no idea what he had found. Sitting in a basement laboratory and staring at the data, his reaction was less Eureka! he told me, and more How did I mess this up?

Many insects harbor beneficial bacteria called and . Between them, they produce 10 amino acids that are missing from the cicadas’ diet of plant sap. Because those amino acids are essential, so too are the bacteria. Without them, the cicadas can’t survive. The opposite is also true: Inside the cushy confines of their insect hosts, endosymbionts eventually lose the genes they’d need to exist independently. They become forever tethered to their insects, and their insects to them.