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A scanning electron microscopy image of the wood-cutting mandibles of a young Melissotarsus worker.

A. Khalife et al., Frontiers in Zoology 10.1186 (2018)

Evolution turned this ant into a living drill

Anyone who’s attempted to cut down a tree by hand knows just how difficult it is to chop through living wood. It turns out wood-boring ants do, too—so they’ve transformed themselves into bizarre, living drills. A new study reveals that extreme adaptations unlike anything seen in other ants let them carve complicated tunnel networks in their host trees.

Not much is known about Melissotarsus ants—native to continental Africa and Madagascar—because they’re only a few millimeters long and never leave the carved galleries of their trees. Inside, the ants are thought to herd sedentary scale insects for food, eating their tasty wax secretions or even their meat. Worker ants have two pairs of back legs that perpetually angle upward and a bulbous head loaded with silk glands (a unique feature among ants). Entomologists have long thought these features must assist with the ants’ unconventional lifestyle, but they weren’t sure exactly how.

“It was not obvious how they could derive the strength to chew live wood,” says Christian Peeters, a research biologist at Sorbonne University in Paris and senior author on the study.

So Peeters and his team took a closer look. The researchers removed ant-inhabited branches from trees in Mozambique and South Africa, sending them back to the lab in Paris. There they combined x-ray microtomography (a type of 3D x-ray imaging for tiny objects) and high-powered microscopes to visualize the ants’ skeletomuscular system, focusing on the anatomy of the head, jaws, and legs.

It turns out that their big domes house more than just silk glands—huge muscles fill the head, anchored to short, sharp mandibles, the team reports in Frontiers in Zoology. These muscles provide the jaws with enormous chiseling power that can tunnel through hardwood. In contrast, ants with weaker jaws typically have to make do with settling in rotten wood or tunnels already excavated by boring beetles. That’s because chewing dry wood—whose fibers are brittle and easily broken—is easier than chewing through healthy, moist wood, Peeters explains.

Artist’s conception of a Melissotarsus worker boring a tunnel. While tunneling, these ants brace themselves against the tunnel walls using strong, specialized legs and basitarsi “heels” to anchor themselves in place.

A. Khalife et al., Frontiers in Zoology 10.1186 (2018)

Even the jaw opening muscles are stronger than those of any species of ant known, a characteristic Peeters thinks may be useful in pushing wood debris out of the way while tunneling.

The researchers also found that the mandibles themselves were remarkably well-suited to a life of chewing. Their wide base made them efficient levers, and analysis of their tips revealed high concentrations of zinc embedded within the exoskeleton.

Zinc-reinforced “heavy element biomaterials” like these are common in invertebrates, says Robert Schofield, a biophysicist at the University of Oregon in Eugene who was not involved in the study. They’re found in body parts that sustain heavy use, such as spider fangs and marine worm jaws. The nanoscale clusters of zinc are bound into the chitin matrix, imparting hardness without increasing the risk of breakage. For ants that depend on these tools to build and eat with, that’s pretty important. “If a sharp tip gets damaged, then they're dead,” Schofield says.

The legs of Melissotarsus workers are also superbly adapted. The researchers found that the legs—perpetually bent close to the body—have strong muscles for bracing against tunnel walls. The “basitarsus” of the ants’ feet (analogous to a heel) is also enlarged, and—with the addition of peglike bristles—provides extra grip when bearing down. This keeps workers rigidly anchored in place, counteracting the intense chewing forces. But the adaptations come with a cost: The ants’ legs are so dramatically modified that the insects can no longer walk on a flat surface (see video, below).

“This is a great paper on an amazing ant,” says Andy Suarez, an entomologist at the University of Illinois in Urbana, who was not involved in the study. “This is the only example I know of where an ant has evolved a lifestyle … living in [the] wood of living trees that requires workers to be able to tunnel through the wood itself.”

Melissotarsus has evolved an “irreversible commitment” to life inside the trees, Peeters says, forsaking the outside world to tend to their scale insect herds. The findings illustrate the extraordinary results that evolutionary specialization can produce, turning once highly mobile ants into tireless power tools.