No, this is not a a scene from Empire of the Ants (above) or a preview for a new horror movie (though it is horror-movie-level stuff). Some parasitic species in the Cordyceps group of fungi are infamous for crawling into the bodies of unsuspecting insects. Once the fungus has invaded, it controls the host like a zombie master until death. Now we finally have an idea how this fungus manipulates its victims — and it isn’t through the brain.
Ophiocordyceps unilateralis might be the most vicious of these fungi. There’s a reason for the ophio, which translates from Greek to “snake.” This homicidal fungus actually doesn't take over affected carpenter ants’ brains, as was assumed before an aptly titled study called "Zombie Ant Death Grip Due to Hypercontracted Mandibular Muscles," recently published in Journal of Experimental Biology. What actually happens is more bizarre.
You’d probably assume something weird is happening in the brain of an ant that loses control over its body and makes for the tops of plants, clamping down on leaves and twigs in a death grip until it dies. Scientists who studied frozen ants in various stages of infection found out that their brains were unaffected. Turns out O. unilateralis actually manipulates the mandibles by damaging muscle fibers, forcing them to contract until they are so swollen they can no longer move.
By the way, like gruesome murder trophies, the ant corpses stay attached to wherever the fungus took them long after death. It’s kind of like an exoskeleton boneyard.
The team who conducted the study said that “both motor neurons and neuromuscular junctions appear to be maintained,” meaning the brain was left alone, but they “found evidence of precise penetration of muscles by fungal structures and the presence of extracellular vesicle-like particles, both of which may contribute to mandibular hypercontraction.”
After the ant has been zombified, O. unilateralis hangs out in the mandibles until it dies, then creeps back out to seek out more ants, because it’s apparently a serial killer. Victims might have something going for them. Something else the scientists found in infected areas of the unfortunate insects were bead-like structures all over the fungus filaments. These could be evidence of a natural defense against the fungus, but even if it is, it wasn’t a strong enough weapon to eat the predator before the predator ate them.
At least this thing doesn't have an appetite for humans.