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EvolutionEvolution

This Rare Beetle Bucks Family Tradition—By Sucking the Life Out of Ants

Their closest relatives hide in burrows to ambush their prey. But Ozaena beetles have taken a different tack: infiltrating ant nests.

ByKatherine J. WuNOVA NextNOVA Next
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Camponotus ants are juicy treats for the larvae of Ozaena beetles, which set up shop in their nests and suck unsuspecting ants dry. Photo Credit: April Nobile / AntWeb.org / CC BY-SA 3.0, Wikimedia Commons

She hadn’t eaten in days.

While her cousins in the next room over had eagerly scarfed down a smorgasbord of treats, she refused each item placed before her. And the situation was getting dire: If her hunger strike continued, it could soon spell trouble—not just for her own body, but for another life burgeoning within.

Or, at least, that was the idea. There was no way to tell if she was actually pregnant—or, really, even that she was a she. But entomologist Wendy Moore, the de facto caretaker of this stubborn Ozaena lemoulti ground beetle, had chosen to roll the dice.

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In her two decades of field work in the state of Arizona, Moore had collected fewer than ten individuals of this species, all full-grown. Their rarity made this bug, and whatever cargo it might carry, incalculably precious. A windfall like this might never come again.

So Moore persevered. Nothing she offered—mealworms, waxworms, flies—seemed to suit the beetle’s taste. But then Moore remembered that the Emory oak trees that Ozaena beetles are known to frequent always seemed to be teeming with the tawny bodies of carpenter ants.

Into the chamber went a handful of wriggling ants. And though Moore never saw the beetle take a single bite, the ants’ thick, segmented bodies slowly began to disappear. Then, one morning, Moore peered into the beetle’s studio apartment to find the greatest gift of all: a tiny, beautiful Ozaena egg.

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An Ozaena lemoulti beetle egg. Eventually, this egg might have hatched a parasitic larva that would suck the juice out of ants in their nests. Image Credit: Andrea Di Giulio, Universita Roma Tre

Barely a millimeter and a half long, the egg would soon hatch into the first Ozaena larva ever scientifically described. Today, in a study published in the journal PLOS ONE, Moore and her colleagues report that these rare and exquisite larvae are noteworthy for another reason, too: Just like their mothers, these insect infants appear eerily well-equipped to feast on the innards of unsuspecting ants—and may do so by infiltrating their nests.

This finding flies in the face of Ozaena evolutionary history. These bugs belong to a group of beetles that’s best known for a macabre hunting method called burrow trapping. Take their close relatives in the Goniatropus genus: After hatching, Goniatropus larvae will dig headfirst into a patch of soil, plugging the hole to its burrow with the hypersensitive, disc-like appendage that fans out from its rear end. When an insect stumbles over this butt-borne umbrella, the disturbance prompts the larva to rear up and ambush its prey like a living landmine. After latching onto its victim, the larvae will drag it into the burrow to be eviscerated and consumed.

But when Moore peered at her lone Ozaena larva under the microscope (sadly, its tenure on this earth was short-lived: just days after its birth, it was boiled and preserved for further study), she found that it bore none of the traits the burrow-trapping lifestyle demands.

Gone were the critical sensory structures that burrow-trapping beetles touted on their rears, as well as the powerful jaws that enabled typical burrow-bound larvae to grasp onto their prey. The Ozaena larva also lacked the classic, U-shaped body type of their close relativesa flexible physique that allowed burrow-bound larvae to bend nearly in half to reach insects wandering past their bums—and it was completely blind. Despite the place it held on the beetle family tree, Ozaena was no garden-variety burrow trapper.

So how, then, were Ozaena sustaining themselves?

When the researchers split open the guts of eight previously preserved, fully grown Ozaena beetles, they found them sloshing with the bodily juices of carpenter ants—the same ones that had reinvigorated Moore’s lethargic mother-to-be. This in combination with the oddball larval physique, Moore explains, pointed not to the varied diet of a burrow trapper, but that of a highly specialized parasite that subsisted exclusively on ants throughout its lifetime.

Ozaena are by no means the first beetles to take advantage of ants. A distantly-related group—Paussus beetles—have actually achieved a small degree of infamy for this behavior. Paussus beetles are the Trojan horses of the insect world: After conning their way into ant nests, female Paussus will lay eggs that hatch into sweet-smelling larvae, thought to exude an irresistible bouquet of ant attractants. Lured in by the scent, ants approach—only to be punctured and sucked dry by the ravenous, vampiric larvae. Paussus adolescents thus spend their early days gorging on an essentially unlimited, self-sustaining supply of protein-rich food.

When beetles invade ant nests, they initiate “an exploitative relationship,” says Brittany Owens, an entomologist at the Louisiana State University Agricultural Center who was not involved in the study. “The ants really get a bum deal.”

And from the beetle’s perspective, it’s a pretty cushy lifestyle compared to, say, the toils of burrow trapping. “If you are able to socially integrate inside ant colonies and gain acceptance inside nests, you have no predators,” says Joseph Parker, an entomologist who studies beetle-ant relationships at the California Institute of Technology who was not involved in the study. “There are infinite resources for you to feed on. You’re in a really safe, stable environment. The payoff is really big.”

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So there’s a big motivation to make the switch from burrow to nest. But beetles can’t just integrate into ant society willy-nilly. The change requires some substantial evolutionary rejiggering: Ozaena’s ancestors, which gave rise to burrow trappers, likely split off from the Paussus line at least 100 million years ago. This means Ozaena had to find its own way ant-ward—a feat that required these beetles to shed millions of years of adaptations that had tailored their relatives to burrows, and instead take on traits that would allow them to raid unfamiliar nests, lay their eggs in the presence of strangers, and meet their nutritional needs with only one source of food.

The shift is so difficult and so extreme that it’s only happened a few times in beetles, Moore says. And if Ozaena beetles have indeed traded their lineage’s long history of burrow trapping for a life of juice-guzzling luxury, they would be the only members of their subfamily that parasitizes ants in the United States—making them the ant-munching black sheep of the family.

“I’m always amazed by the adaptations that these insects show,” Parker says. “Your jaw hitting the floor is the standard response.”

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Ozaena beetle larvae (top) lack many of the specialized structures that their close relatives in the Goniatropus genus (bottom) use for their burrow trapping lifestyle, including a disc-like appendage at their rear ends that helps the larvae sense when prey passes over their burrow. Image Credit: Wendy Moore, University of Arizona

Despite their similarities, though, Ozaena isn’t simply a Paussus copycat—and in their time apart, Ozaena has come up with its own strategies to make the most out of nests. For instance, while Paussus larvae are pretty much immobile blobs that must wait for their meals to come waddling by, the Ozaena specimen boasted unusually long legs that might have helped it scurry around the nest to actively hunt its prey.

“When we think about beetles, we only think about adults,” Moore says. “But larvae aren’t just big bags of goo. They represent this entire hidden world.”

Besides, “it’s the most vulnerable stage,” Moore says of the larvae. “If they don’t make it, the species doesn’t make it.”