Buchner didn’t press the matter further. But when other scientists later analyzed these globules, they started getting very odd results. If the mealybugs swallowed antibiotics, the globules would rupture along with the bacteria inside them. Peculiar. The mucus also seemed to contain many of the elements of an actual cell. Also peculiar. And genetic studies showed that they contained DNA from two separate lineages of bacteria. That, at least, was explicable: Buchner’s globules probably contained two types of symbiotic bacteria rather than one.

In 2001, Carol von Dohlen tested this idea by studying the citrus mealybug—a tiny insect that looked like a lozenge dipped in icing sugar. Von Dohlen fashioned two fluorescent molecules—one red and one blue—that would each stick to DNA from one of the two bacteria. If the two microbes did indeed share the same living quarters, their respective glows should have blended into a sea of purple.

That is not what happened. Instead, von Dohlen saw red dots against a blue background. The red probe had stuck to the bacteria in the globules. But the blue probe was sticking to the globules themselves. These mucus-filled spheres weren’t enclosing two kinds of bacteria. They were bacteria.

Von Dohlen had discovered that the citrus mealybug is a living Russian doll—or perhaps a microbial turducken. The bacteria living in its cells have more bacteria living inside them. It contains multitudes, and its multitudes contain more multitudes. The bigger microbe was eventually named Tremblaya, and its inner companion was called Moranella. And then things got even weirder.

In 2011, von Dohlen teamed up with geneticist John McCutcheon to sequence the genomes of the two microbes. Both were very small, as is often the case with bacteria that find their way into insect cells. In the cozy confines of their hosts, these microbes can afford to lose genes that they would normally need for an independent existence. Tremblaya has even lost a group of supposedly indispensable genes that were there in the last common ancestor of all living things, and are found in everything from bacteria to bats. There should be twenty of them, and Tremblaya has none. It survives because the insect around it and the Moranella within it compensate for its genetic shortfall.

This convoluted set-up developed gradually. Tremblaya was first of the two partners to colonize mealybugs: it’s there in all the species from one particular lineage, and there are some mealybugs that carry it and it alone. Snug in a bug, it began jettisoning genes. In the citrus mealybug, Moranella joined the partnership. The duo became a trio, and Tremblaya continued its slide into genetic pauperdom. As long as any gene exists in one of the partners, the others can afford to lose it.

That’s abundantly clear when you look at genes for making nutrients. For example, it takes nine genes to make an essential amino acid called phenylalanine. But none of the three partners makes all nine. Tremblaya can build 1, 2, 5, 6, 7 and 8; Moranella can make 3, 4, and 5; and the mealybug alone makes the 9th. As I wrote in my new book, this reminds me of the Graeae of Greek mythology: the three sisters who share one eye and one tooth between them. They are still distinct entities, but they’re each like thirds of a single whole. They cooperate to make nutrients that they all rely upon, and none can survive without the other.