There’s More Than One Way to Decapitate An Ant

Tropical rainforests are home to lots of ants, which means that they’re home to lots of injured ants. Ant colonies frequently fight each other, leaving behind battlefields strewn with wounded workers.

That’s good news for phorid flies.

There are some 4,400 species of phorids. Although their lifestyles are diverse, a surprising number of them specialise in decapitating ants (or bees). The females lay their eggs inside their victims. When the maggots hatch, they move towards the ant’s head and eat its brain and other tissues. The brainless ant stumbles about in a fugue for weeks until its head eventually falls off. Sometimes, that’s because the fly has inflicted so much damage. In other cases, the maggot deliberately releases an enzyme that dissolves the connection between the ant’s head and body.

But in the rainforests of Brazil and Costa Rica, Brian Brown from the Natural History Museum of Los Angeles County has found a phorid that beheads ants in an entirely new way.

Brown is an authority on phorids and especially the ant-decapitating, bee-killing varieties. He has discovered around 500 species of them. In the field, he lures them in by crushing ants and other insects with forceps, and watching what arrives. Some phorids feed directly off the injured insects. Others lay their eggs inside. But one species, Dohrniphora longirostrata, did something different.

A female will land near a wounded ant and circle it. She darts in and out, touching the would-be victim and occasionally tugging on its legs and antennae. The goal, it seems, is to check just how incapacitated this incapacitated ant really is. If it’s too active, she retreats to find an easier mark.

Eventually, she climbs onto the ant and jams her mouthparts into its body. “The fly was nearly constantly in motion, probing from several angles,” writes Brown. “They made in-and-out, as well as rotational head movements.” In other words, she’s sawing.

A housefly’s mouthparts end in a spongy pad for soaking up fluids; in place of that, D.longirostrata has an extremely long proboscis, almost as long as its entire body. Under the microscope, the tip looks like a murderous Swiss army knife, with one fiendish spike and a couple of serrated steak knives. The fly jams this tip into an ant, cutting and severing. After some time, it yanks the head clean off.

The headhunting females typically dragged their bounty away, so Brown isn’t clear about what they actually do with the heads. They probably lay eggs inside. But the team sometimes saw the females slurping up the contents of the heads themselves, and upon dissection, these females never had mature eggs. Perhaps they need to gorge themselves on ant heads before their eggs can develop.

Brown has been crushing ants and watching flies for 30 years, in eight countries across South and Central America. In all that time, he has only ever seen this fly attacking wounded trap-jaw ants. It won’t target healthy workers that could easily overpower and kill them—the strike of a trap-jaw ant is among the fastest movements in the animal kingdom. It won’t attack injured crickets, grasshoppers, or termites either. It’s extremely picky, which means that even without the intervention of forceps-wielding biologists, there must be a lot of injured trap-jaws lying around the forest.

There are between 50 and 100 species of Dohrniphora flies and many co-exist in the same forests. Maybe they’re all picky specialists, each one decapitating its own preferred ant.

Reference: Brown, Kung & Porras. 2015. A new type of ant-decapitation in the Phoridae (Insecta: Diptera). Biodiversity Data Journal http://dx.doi.org/10.3897/BDJ.3.e4299

More: The world’s smallest fly is a phorid; at half a millimetre long, it could sit on a housefly’s eye, and probably decapitates really tiny ants.

Thanks to Alex Wild for the hat-tip.