For decades, scientists have puzzled over why the nests of a little ant called Formica archiboldi are littered with the skulls of other ant species. The most common heads found are those of fierce, predatory trap-jaw ants (Odontomachus). Some researchers have wondered if F. archiboldi somehow inherits trap-jaw ant nests, or if it's a specialized predator itself.
According to a study published in the November 2018 issue of the journal Insectes Sociaux, the answer is much more interesting. Like other members of the Formica family, F. archiboldi can spray formic acid from the tip of its abdomen when threatened. F. archiboldi's acid isn't stronger than that of other ants, but its aim is better. It goes on the offensive, hunting and spraying trap-jaws, which are immediately immobilized and can be dragged into the nest for dismemberment.
But how does this "headhunter" ant get close enough to predatory trap-jaws to spray them? One possibility is scent. Ants "see" their world through more than 400 smell receptors, four to five times as many as most other insects. They recognize each other via a waxy scent layer on their exoskeletons. Headhunters' smell matches that of trap-jaw ants almost exactly — but only the trap-jaw species in their region. Some researchers think this allows headhunter ants to approach trap-jaws "unsmelled."
But maybe F. archiboldi smells like the larger, scarier ant for a different reason. A kind of kidnapper ant also coexists with headhunters and trap-jaws. This ant sneaks into headhunter nests, murders the queen, bathes in her fluids as a disguise and then lays lots of kidnapper eggs in a great circle of ant life. Smelling like trap-jaw ants may help headhunters hide their nests from kidnapper ants.