Army ants are so vast in their legions, and so destructive in their appetites, that they have been billed as “nature’s Mongol’s hordes” and “the Huns and Tartars of the insect world.” The comparison in apt. But then again, neither Genghis Khan nor Atilla the Hun rode around with people sitting behind them, clamped around their waists, pretending to be their butt.

Eciton burchellii, the best-studied species of army ant, does have one such hanger-on. It’s a newly discovered beetle that hitchhikes on ants by clamping its jaws around their waists. This bizarre stowaway is red and spherical—exactly like the ant’s backside. From above, the ant looks normal. From the side, it looks like it has a bustle. A prosthetic posterior. A gluteus extraneous. A beetlebum.

Christoph von Beeren and Daniel Kronauer discovered the beetle, while studying army ants in the rainforest of Costa Rica. Every day, these insects fan out from temporary nests or bivouacs, in a raiding party that can include up to 200,000 individuals. They attack everything. Small creatures like other insects, spiders, and scorpions are dismembered, and eaten. Larger ones like snakes and birds can’t be eviscerated, but they can be stung to death if they are overwhelmed. If the ant army has a drumbeat, it’s the relentless rustling of other animals trying to flee.

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.”

In fairness, von Beeren isn’t sure if the beetle is actually a mimic. The ants themselves are blind, so the beetle could look like anything for all they care. Its appearance might protect it from more visually oriented predators like birds, but von Beeren only ever saw it accompanying the ants at night. Then again, he also found that its outer layer has an unusual microscopic structure that looks very similar to those of the ants. Perhaps its disguise lies not in looking like an ant’s butt, but feeling like an ant’s butt.

“We don’t know a lot about it,” says von Beeren. “We don’t know what it feeds on, or how well it integrates into army ant society.” He didn’t even know that he had seen it before. Working back through some old collections, he realized that he had actually picked up the beetle in 2013, before he realized what it was. “They’re very well camouflaged,” he says.