This Fly Hijacks an Ant’s Brain — Then Pops Its Head Off

We humans were just so proud of ourselves when we invented the guillotine after millennia of experimenting with how best to take heads off shoulders. Maybe a bit too proud, what with that whole Reign of Terror thing. Unbeknownst to those over-enthusiastic revolutionaries, evolution already had produced its own perfect beheader — a tiny fly whose larvae burrow into ants, take control of their minds, and eventually sever their heads from the inside.

These are nature’s flying guillotines: the epically named ant-decapitating flies of the genus Pseudacteon.

Twenty years ago, Sanford Porter, then an entomologist with the University of Texas, was in South America studying fire ants and discovered their numbers were a fraction of those of their invading comrades to the north. Here in America, these two introduced species, the red and black fire ants, cause billions of dollars each year in agricultural damage, pest control costs, and sweet, sweet profits for hospitals treating their excruciating stings.

So Porter searched for a natural enemy that might be keeping southern populations in check. Following a tip from a colleague, he began seeking out fire ants fending off attacks from tiny flies. He gathered some of these besieged individuals and returned to the United States, where he soon began finding maggots in the ants’ bodies. “And around about two weeks [after that] I found that the heads would fall off,” he told WIRED, “and lo and behold I could see the pupa inside the ant’s head.”

The flies he’d observed weren’t hunting the ants. They were much too small for that. Apparently not to be bothered with the stresses of parenthood, they were infesting the creatures with their young. Here, take this for me, the flies seemed to say, I’ve got a lot going on in my life right now.

Here’s how it works. Attracted by the smell of the fire ant’s alarm pheromone, the female ant-decapitating fly hovers a few millimeters from her target. “When they get into just the right position, they dive in,” said Porter, who is now with the USDA Agricultural Research Service. The fly has a sort of lock-and-key ovipositor, the shape of which varies widely between species, “and once that’s fit onto the ant’s body, around the legs somewhere, then what happens is that there’s an internal ovipositor that looks like a hypodermic needle, and that hits probably in the membranes in between the legs,” firing a tiny torpedo-shaped egg into the ant.

In only a few days the egg grows considerably inside the ant before hatching. The resulting maggot works its way through the ant into its head, where it will live for several weeks on the host’s bodily fluids, while maybe from time to time sarcastically asking what the ant is thinking about.

The lock-and-key ovipositor of a Pseudacteon pradei female.

Now, we can safely assume this is relatively distressing for the ant. Indeed, ant colonies maintain something called social immunity to isolate and exile such individuals whose behavior is deemed suspicious — for the good of the community. Yet strangely “the ant runs around, behaves outwardly normally, but even at this point the fly maggot is taking control of the behavior of the ant,” said Porter. Its confederates are none the wiser.

You see, as it develops, the larva needs its host to remain in the relative safety of the colony, where food is plentiful. Porter isn’t yet sure exactly how the maggot accomplishes this mind control, though it’s surely some sort of chemical release. (Did you catch that, CIA? A mind-controlling chemical. Query the NSA for a full transcript of my phone call with Porter for more details.)

But things get even stranger when the larva mercifully decides it’s time to kill its host. Around 24 hours before the maggot is ready to pupate, it finally mind-controls the ant out of the colony. In order for it to develop properly, the pupa needs high humidity, “so what they’re looking for is a place down deep in the leaf litter or plant litter or somewhere that they can dig down in where it remains moist and doesn’t get too hot,” said Porter.

“When they take over the behavior of the ant, that’s what the ant begins to seek,” he added. “When ants normally die, they end up in hot dry places because that keeps fungi and other pathogens from hurting the other ants. But that’s not what happens when they’re under control of the parasite.”

Astoundingly, this strategy has developed totally independently in the Cordyceps fungi, which invade ants’ minds in a similar manner to steer them to unfathomably precise positions in the rainforest to die. Two such completely unrelated organisms developing the same adaptation independently is known as convergent evolution, and this particular convergence is all the more incredible considering that fungi aren’t even animals. This ant-control adaptation has developed not merely between a family or order of organisms, but between kingdoms.

A Pseudacteon curvatus female with hook-shaped ovipositor.

The larva next eats away the brains and muscles and glands, completely hollowing out the head. It then pushes in and out of the ant’s mouth to create an exit route, and begins to pupate. To seal this hole, the pupa builds up a hardened tip that is the exact size of the mouth opening of the ant, protecting it as it continues to develop. And a few weeks later, a new tiny ant-decapitating fly finally crawls out of the ant’s disembodied head like Athena sprouting from Zeus, only Zeus got to remain alive.

This remarkable lifestyle, Porter theorized, is what keeps South American fire ant populations in check on their home turf. Without these specific flies in the U.S. (our native species have their own parasitic flies that don’t attack non-native species), the invasive fire ants run amok. So why not just import the southern flies to control the southern ants?

You might think this is a really bad idea, like bringing in cats to fix a rat problem and then having to bring in dogs to control the cats and pretty soon you’ve got grizzly bears running around making a mess of things. But as a parasite, ant-decapitating flies are extremely host-specific. Native varieties here in the U.S. attack native fire ant species, but very rarely touch invasive species. And likewise South American ant-decapitating flies only parasitize South American fire ants, so, theoretically, importing them wouldn’t pose a threat to our native varieties.

Read more at Wired Science