Leaf-cutter ants are covered in rock crystal armor never before seen in insects

Leaf-cutter ants owe their name to their Herculean exploits: they bite foliage and carry unwieldy pieces, like green flags several times their size, long distances to their colonies. There, they chew the leaves to feed the underground mushroom farms. Along the way, the insects brave all kinds of predators and regularly engage in wars with other ants.

But these insects are even more resistant than previously thought.

A new study shows that a species of leaf-cutting ants from Central America has a natural armor that covers its exoskeleton. This shield-shaped coating is made of calcite with a high magnesium content, a type found only in another biological structure: sea urchin teeth, which can crush limestone.

The bones and teeth of many animals contain calciferous minerals, and crustaceans, such as crabs and lobsters, have mineralized shells and other parts of the body. But before this discovery, no type of calcite had been found in any adult insect.

In leaf cutter ants, this coating is made up of thousands of tiny, plate-shaped crystals that harden their exoskeleton. This “armor” helps prevent insects from losing limbs in battles with other ants and prevents fungal infections, according to a Nov. 24 article in the newspaper. Nature communications.

The discovery is all the more surprising as the ants are well known. “There are thousands of articles on leaf-cutter ants,” says study co-author Cameron Currie, an evolutionary biologist at the University of Wisconsin-Madison.

“We were really delighted to find [this in] one of nature’s best studied insects, ”he says.

Although this article has only examined one species, Acromyrmex echinatior, Currie and his colleagues suspect that other related ants also have the biomineral.

Long before the evolution of humans or their immediate ancestors, around 60 million years ago, leaf-cutting ants invented their own form of agriculture. These underground fungal farms are the result of a symbiotic relationship that provides food for the ant larvae and protection for the fungus, and each species of ant has its own species of fungus.

Some of the 50 or so leaf-cutter ant species, including the ant in the study, also harbor symbiotic bacteria to keep their gardens from being infected with other harmful fungi. This microbe envelops young workers; As they meander through their mushroom gardens, bacteria secrete chemicals that kill invading fungi.

A former postdoctoral researcher at the Currie lab, Hongjie Li—Now a researcher at Ningbo University in China — began to study these bacteria and was quickly intrigued by the strange, tiny crystals covering the ants’ exoskeletons. He convinced geologists to help him study the mineral-like material, using several types of imaging techniques, including electron microscopy, to characterize the composition.

When Li got the results one autumn morning in 2018 showing that the ants were covered with a type of biomineral that had never been seen in any insect, he was ecstatic.

“There was stone on the ants,” Li said. “I found rock ants!”

Li says the ant’s armor is very similar in composition to the mineral dolomite, except a little harder.

Like all insects, ants have chitin exoskeletons, which are tough and flexible. To see if this extra layer of biomineral acted as extra armor, Li and his colleagues first raised ants in the lab with and without the biominer layer. (If the ants are separated from their colony as pupae and reared under certain conditions, they do not develop the coating.) They then did several tests.

One such experiment involved pitting these ants against a slightly larger but closely related species in “ant wars,” Li says. In an hour of battle, the “rock ants” lost three times fewer games. of the body than those without mineral coating.

Next, the researchers exposed the insects to a pathogenic fungus, which can infect ants, and which is linked to fungal species that cause “zombie” behavior. After six days, all of the mineral-free ants were dead. But only half of their armored relatives had died.

Yet another experiment has shown that the exoskeletons of these ants are more than twice as hard when they contain biominerals than when they do not.

The mineral coating also expands as ants age. Young ants tend to frequent fungal gardens and do not face a high risk of attack by other ants or predators, and have little need. By the time they start to feed in the world outside of mushroom gardens, they have a thicker coat than when they were young, Li says.

Andrew Suarez, an entomologist at the University of Illinois, says it’s especially exciting to see this type of mineral in an exoskeleton, given that similar minerals were previously found in more isolated and specialized structures, such as teeth.

“It would be like having your body covered in [tiny crystals] enamel, ”says Suarez, who was not in the study.

“I enjoyed this article because it documents something new: biomineralized skeletons in insects,” says Andrew Knoll, a biomineral expert at Harvard University.

“A number of arthropods make calcium and carbon exoskeletons, including … crabs and lobsters and extinct trilobites, but extending that to fully terrestrial insects is really interesting.”

Scientists also claim that these types of biomineral crystals could have manufacturing applications in the future, such as coatings or nanocrystals that enhance or prevent corrosion of various materials.

For now, say the scientists, it’s a matter of understanding the role these minerals play in the ants themselves – and seeing if there are other armor and biominerals as yet unknown.

There probably is, Currie said. “If you don’t know the biomineral for this species, what does that say about the 99.9 percent of insects that have received little or no study?”