Lots of organisms rely on symbiotic relationships, in which two species rely on each other for survival and one lives inside the other. But citrus mealybugs enjoy a triply symbiotic relationship unlike any we've ever seen...with one absolutely crucial exception.
Microbiologist John McCutcheon of the University of Montana and biologist Carol von Dohlen of Utah State University teamed up to study how symbiosis helps the citrus mealybug survive. The bug must turn plant sap into nutrients, but it lacks the ability to do this on its own. It relies on the bacterium Tremblaya princeps to handle the conversion process...except this bacterium can't do it all either.
Instead, Tremblaya princeps handles one part of the conversion into amino acids, and then it hands things over to a second, smaller bacterium Moranella endobia. This bacterium lives inside Tremblaya princeps, and neither bacterium possesses the necessary complement of biomachinery to turn the plant sap into nutrients. So this means the mealybug relies on both bacteria equally for food, Moranella needs to be housed inside Tremblaya, and Tremblaya must survive inside the mealybug. It's the first triple symbiosis we've ever seen.