An interesting snippet on "Head Hunter" ants, "Trap-jaw" ants and "Kidnapper" ants.
One species of small, rust-colored ant known as
Formica archboldi likes to decorate its nests with skulls, or head cases, of several kinds of trap-jaw ants (hence the moniker "Head hunter" ants). This is unsual, because trap-jaw ants come equipped with potent stingers and gigantic mandibles that can snap closed like a bear-trap. So how do they do it?
They subdue their quarry with quick, highly efficient sprays of toxic acid. No one is sure why the headhunters should be such effective predators of the larger trap-jaw ants, but it may have something to do with a waxy layer of scents the ants produce called cuticular hydrocarbons which matched the species of trap-jaw ants they co-occur with almost perfectly. This meant that they could infiltrate the nests of these trap-jaw ants, spray the unsuspecting ants with toxic formic acid to paralyse them and then snip off their heads. The next question is why do they do it?
There is a genus of ants called
Polyergus. Their old common name was "slave-making ants", but are now referred to as "kidnapper ants".
Polyergus are parasitic on the closely related genus
Formica. They have lost the instinct for carrying out even rudimentary brood care, and even for feeding themselves. Once a kidnapper ant queen identifies a headhunter ant colony, it sneaks in, murders the incumbent queen, and then “bathes in her bodily fluids”. The imposter then uses this newly acquired scent profile to avoid detection while she lays a bunch of eggs. These hatch into a generation of workers whose only job is to go out and capture more Formica ants, mainly pupae, in massive raids.
Because each kidnapper ant species typically parasitizes just one species of Formica ant, there is a good evolutionary incentive for the Formica ants to keep changing their scent profiles in an attempt to throw the kidnappers off their game. It is possible that this is why the headhunter ants have evolved to smell like trap-jaw ants, the latter of which are not parasitized by the kidnappers. The question still remains, why do these ants surround their nests with the heads of their prey? The headhunters have only been found to mimic native trap-jaw species, not ones that have been recently introduced. This means that they have also co-evolved. The other possibility, therefore (unproven), is that they use the heads of their prey to mask themselves in the scent of their local prey species.