Surviving on plant sap as cicadas, aphids, and some other insects do is like living on a diet of sugar water—there’s just not enough protein. These insects solve this problem by teaming up with bacteria that live inside their bodies and produce protein building blocks called amino acids. This relationship has existed for tens of millions of years. But about 5 million years ago, in one species of cicada (pictured), one bacterial partner split into two species, researchers report in the current issue of Cell. At first, the bacteria species’ two genomes were similar, each having all 137 genes and functioning independently. But over time, different genes in the two species disintegrated, such that they now rely on each other as well as on the cicada to survive. Overall, 64 genes have been lost in one or the other microbe, and losses continue, the researchers note. These kissing-cousin microbes occupy different types of cells within the organ they live in and share that organ with another bacterium. So together with the cicada, they’ve turned a three-way partnership into a foursome.