Scientists are starting to understand and solve a mystery involving different species of ant.

Scientists have known that one species of ant, Formica archboldi, like to decorate their nest with the skull or head cases of several types of trap-jaw ants. They find this quite bizarre as trap-jaw ants come equipped with potent stingers and gigantic mandibles that can snap closed like a bear trap. They mouths have some special features such as allow the ant to catapult to freedom when facing attack.

Diminutive F. archboldi ants, native to Florida and surrounding states, nevertheless succeed in taking them down and nabbing their heads—but how? “Something weird was going on but no one had looked into it yet,” until now, says Adrian Smith, a research biologist at North Carolina State University.

In a new study published in the journal Insectes Sociaux, Smith has shown that F. archboldi ants subdues the other ants with a quick and efficient spray of toxic acid. This is curious, most ants that possess the ability to spray acid use it as a last resort self-defence mechanism. F. archboldi ants, however, goes on the attack. “They just seem to be sort of ready for the fight,” says Smith.

No one is sure why the headhunters should be such effective predators of trap-jaw ants, but it may have something to do with a waxy layer of scents ants produce called cuticular hydrocarbons. When Smith sampled these scents from F. archboldi, he found that they matched the species of trap-jaw ants they co-occur with almost perfectly.

A strange but fascinating story