The citrus mealybug looks like a walking dandruff flake, or perhaps a woodlouse that’s been rolled in flour. It’s also the insect version of a Russian nesting doll. If you look inside its cells, you’ll find a bacterium called Tremblaya princeps. And if you look inside Tremblaya, you’ll find yet another bacterium called Moranella endobia.

The two bacteria aren’t just passing hitchhikers. They’re symbionts— constant fixtures of the mealybug’s cells, and necessary for its survival. The trio cooperates to manufacture essential nutrients, such as amino acids. This involves a chain of chemical reactions, and it takes enzymes from all three partners to complete every step. Imagine a single production line with machines from three different manufacturers. Raw ingredients enter;