Over half of all species may be parasites.
The most devious among them are the behavioral manipulators — parasites that get into other critters’ bodies and mind-control them.
The most bizarre being the zombie ant.
A parasitic fungus grows throughout the ant’s body, including through muscles. This may allow it to individually manipulate the movement of each muscle to control the ant like a puppet. Because ants will quarantine individuals in the colony that seem sick, the fungus orders the ant to crawl up a tree and bite onto a twig. The fungus then kills the ant and erupts out of the back of its head as a stalk, raining spores onto the rest of the colony below.
Worms, too, are master manipulators.
These are nematomorphs, AKA horsehair worms, from the mountains of New Mexico. They need to get into the water to breed, so they use crickets as vehicles.
And this is a horsehair worm erupting out of the abdomen of a cricket. Here it had been absorbing nutrients, growing to several times the length of its host.
When it reaches maturity, it releases chemicals that convince the cricket to jump into water. Not a great idea for the cricket, obviously, since it could drown. It’s at this point that the worm drills out of the abdomen and wiggles to freedom. It’ll mate soon after emerging, while the cricket usually survives — so long as it isn’t eaten by a fish.
This little beauty is called a jewel wasp.
It’s the brain surgeon among the parasitic manipulations. It ruins the lives of cockroaches.
It then guides the brainwashed roach into a den, where it lays a single egg on the victim. (You can see the egg here stuck on the roach’s leg.)
The egg hatches into a larva, which pierces the abdomen and feeds on the roach’s juices, before drilling into its body cavity and erupting as an adult wasp. Thanks to its mother’s craftiness, it’s got a good head start in life.
Crabs aren't safe from the zombifiers either.
A bizarre barnacle called a rhizocephalan invades their bodies and grows as a root system throughout their tissues. It also grows outside the shell on the rump of the crap. The parasite then tricks the crab into thinking that rump is its own eggs, so the crab preens it to keep it clean. Then when the parasite releases its larva from the bump, the crab dutifully shakes the youngsters free, as if they were its own.
But no parasite could ever manipulate us, right? Our brains are just too complex?
Rabies is actually a manipulative parasite. Rabies is a virus that typically infects mammals like raccoons, invading their brains and turning them hyper-aggressive. This helps the virus spread itself around, since it’s transmitted by bite.
Because we too have a mammalian brain, the virus can manipulate our behavior as well. Victims in the throes of rabies infection will lash out at their caretakers, before inevitably perishing.
Matt Simon is a science writer at Wired magazine, where he specializes in zoology, particularly of the bizarre variety, and the author of The Wasp That Brainwashed the Caterpillar. He is one of just a handful of humans to witness the fabled mating ritual of the axolotl salamander. He lives in San Francisco.
Plight of the Living Dead: What Real-Life Zombies Reveal About Out World — and Ourselves is out now.