Von Beeren and Kronauer were also relentless. They would watch the ants for hours at a time, sitting in fold-up chairs in the pitch-black jungle, and peering at the legions through headlights. One day in the spring of 2014, they realized that some of the ants looked a little odd. “The abdomens reflected the light differently, and the color was a little different,” says von Beeren. “Then, we noticed that they looked like they have two abdomens.”

They collected some of these dual-derriered insects and put them in a vial. Back at camp, Kronauer shook the vial… and the back-up backside fell off. It was a beetle. “And it blew our minds,” says von Beeren. After working with USDA entomologist Alexey Tishechkin, he realized that the bonus-butt beetle was new to science. And he named it Nymphister kronaueri, after his colleague Kronauer, who helped to discover it.

Nymphister kronaueri, biting onto an ant’s waist.

(Courtesy of Daniel Kronauer)

The beetle is far from the only member of the army ant entourage. Despite the evident danger, at least 550 different species follow the marching legions, and around 300 depend on the ants for their survival. Together, they represent the single largest association of animals centered on a single species. Birds pick off insects that flee the ants. Parasitic Stylogaster flies shoot harpoon-like eggs at bolting cockroaches. Flesh flies lay eggs in injured victims that have somehow escaped. Tiny mites ride on the ants, with each species specializing on a different body part: one sucks blood from the base of the ants’ jaws, another rides on the feet, and yet another has only ever been found riding on their eyes.

And there are lots of beetles. Tetradonia actually attacks the ants, racing in to seize passing workers. Some groups mimic the ants’ appearance, even evolving thin and un-beetle-like waists. Others mimic the ants chemically, using ‘appeasement glands’ to produce pacifying smells. Yet others rely on armor instead of subterfuge. One group, the histerids, have solid, spherical bodies, with grooves and slots into which they can retract all of their appendages. “They’re like walking tanks,” says von Beeren. “The ants have no point of attack.”

“However, only a few beetles have really gone all in and said: Screw it, we're going to literally ride around on the very ants that want to murder us,” says Ainsley Seago, a beetle expert at CSIRO. The ptiliids, for example, have evolved a shield-like shape that can deflect snapping jaws, and so can sit safely on an ant’s back. From there, it gets a free ride, constant access to rich food, and protection from predators.

But the new species that von Beeren discovered—also a histerid—goes one step further by resembling the abdomen of its host. “This is utterly bananas,” says Seago. “But the histerids would be the ones to come up with that strategy, given that they're so, so good at compressing their whole body into a tiny, compact pod.”