Army ant colonies are home to a treasure trove of raided food and helpless juveniles that other insects would love to feed on. But one does not simply walk into an army ant colony and start eating.
Ill-prepared intruders would face swarms of aggressive ants eager to defend their nest. That's why many insects — such as silverfish, mites and wasps — have developed armor or clever disguises to raid ant colonies.
Several species of rove beetles, a diverse family of tiny elongated insects, take this to the extreme.
These beetle species are able to infiltrate ant nests through an elaborate mimicry scheme, evolving ant-like antennae and legs, ant-like smells and even ant-like behaviors to assimilate within the ranks of their hosts.
In the last 60 million years, this beetle-to-ant disguise has emerged 12 different times in various genera of rove beetles around the world, according to research published this month in Current Biology.
"There's nothing else quite like it in biology," said Joseph Parker, the Columbia University evolutionary biologist who led the study. "These beetles have independently evolved to live with army ants. They sort of morphed into the shape of their ant hosts and their behaviors have changed, too."
Parker and his colleague, Munetoshi Maruyama of Kyushu University in Japan, spent a decade in the field searching for the tiny insects among thousands of marching army ants and collecting them for later DNA analysis. Many of the species they found were new to science.
By reconstructing the evolution of the ant-mimicking rove beetle subfamily Aleocharinae, the researchers found that the insects descend from a common ancestor that lived 105 million years ago in the early Cretaceous Period. This is around the same time flowering plants and modern mammals appeared.
The ant impostors' ancestor would have looked more like today's "free-living" rove beetles, the ones that do not live as parasites inside ant colonies.
As army ants and termites conquered the tropics during the Cenozoic Era, at least 12 separate lineages of rove beetle took on ant-like characteristics.
The researchers call this an ancient example of convergent evolution, in which unrelated or separate groups of organisms follow seemingly parallel evolutionary paths.
Rove beetles' ant mimicry is surprising, in part because of the complexity their adaptions. Convergent evolution is generally limited to a single trait, such as the shape of eyes or wings.
It's also unusual that the beetles developed these disguises over such a long period of time.
Usually, convergence takes place over just a few million years. That's because the species, though separate, share a similar genetic blueprint that allows their evolution to follow similar courses.
That's not the case for the rove beetles, whose ancient ancestor split into separate groups more than 50 million years ago. In that time, each beetle species became more genetically distinct. Even so, more than a dozen rove beetles still managed to evolve their own ant-like disguises.
All of these beetles evolved to survive in a very particular niche, in this case the inhospitable colonies of army ants. In other words, the beetles had no freedom to evolve different ways of life, he said.
"If the selective conditions are right and the starting material is right, then evolution can be extremely predictable," Parker said. To live in an ant colony, "you presumably have to obey specific rules. That's smelling like an army ant and behaving like an army ant."
The ant impostors' last common ancestor likely passed on a set of adaptations that predisposed the insects to eventually develop traits that approximated the appearance, smells and behaviors of various army ant species.
But first, the beetles needed a reason to venture into a dangerous army ant den: the nest's ant brood and scavenged arthropods are an attractive source of food. To survive such a hostile environment, the beetles' defensive gland on their abdomen allowed them to secrete an irritating chemical, called a quinone, that allowed them to defend themselves against aggressive ants.
"They can walk into an ant colony, eat their resources and sort of blast them in the face with quinones," Parker said. "They can get in the door because they can chemically defend themselves."
Finally, rove beetles possess a body plan that's much easier to remodel into the shape of an ant. Free-living rove beetles have a relatively short wingcase and a flexible abdomen, so they're only a few steps away from being ant-like, Parker said. Then all the myrmecoid beetles needed was elongated antennae and legs.
Each myrmecoid rove beetle is adapted to parasitize a single species of ant. The beetles probably live their entire lives inside the ant colony, although no one has ever found their larvae, Parker said.
It's also unclear whether the ants ever realize that there's an impostor among their ranks. Parker said myrmecoid beetles have defensive modifications, including thickened, club-like antenna and sturdier body segments in case they do get caught.
It's possible, Parker suggested, that the ants and beetles are engaged in an "evolutionary arms race." As the impostors feed off the ants' brood, perhaps the ants' ability to detect the parasites has also evolved.
Parker said more specialized parasites typically don't undermine the hosts they depend on. The beetles probably only produce enough offspring to propagate the species — although more research is needed on the impact the impostors have on their host colonies.
"Maybe intruders who can't keep apace were detected and killed," Parker said.
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