From films like “Night of the Creeps” and “Slither” to video games such as “Resident Evil 4” and “Halo,” sci-fi horror is crawling with parasitic lifeforms who hijack their host bodies, creating zombie slaves to spread themselves even farther. Little do many people realize, this phenomenon is a scientific reality, and happening all around us in some of the most unexpected places.
Possibly the world’s weirdest insects, male strepsipterans are gnat-sized flying critters with huge eyes, fine senses and lifespans of only a few hours. They exist solely to find the female and mate, which can be quite tricky, because the female Strepsipteran is a limbless, eyeless, bag-like parasite living inside the body of another insect, such as a fly, bee or even a preying mantis, with only her head sticking out of the host’s body to breathe. To find herself a male, the parasite will release her mating pheromones on the wind and force her insect host to wait patiently in an obvious and convenient location, such as the tip of a long leaf or twig. Try to imagine having to stand around for hours while the face on your back flirts with strange, tiny men. Days later, she’ll upchuck a bunch of live larvae on the next flower you visit, the perfect place to infect even more insects.
9. Fish Flukes
For most fish, evading predatory birds is as simple as swimming just beyond the reach of a beak…so just how do so many fish end up in the gullets of pelicans and cranes? A huge portion of the average seabird’s diet consists not of normal, healthy fish, but fish under the influence of parasitic worms. Sticklebacks, for example, suffer from the tapeworm Schistocephalus solidus, which grows so large that the host becomes swollen and sluggish. It also changes the host’s coloration to be easier to spot, and finally, alters the host’s behavior to swim near the surface. The worms feed the fish to the birds, and the birds spread the worms to new lakes and rivers in their droppings.
8. Gordian Worm
Once known as “horse hair” worms because they would appear mysteriously in horse troughs, Gordian worms spend their parasitic larval stage within the bodies of insects, especially crickets, but spend their non-parasitic adult stage in water. Crickets aren’t known for their swimming ability, but try telling that to a parasitic nematode (really, try it. They don’t even comprehend English, it’s ridiculous.) When it’s time for adulthood, the worm compels its cricket to seek out the nearest body of water and dive right in. The confused cricket usually drowns, while the worm wriggles free to find itself a mate.
Cordyceps are an entire genus of fungi which develop in the bodies of various insects. Every species has a different host, and will eventually kill the victim to sprout into a tiny mushroom and release its spores. To better propogate themselves, many species take control of their victims shortly before mushroom-time, forcing them to climb up high where the spores can spread farther. They even initiate this at the ideal time of day to infect their preferred hosts; Cordyceps of houseflies, for instance, kill their victim around dawn, when the air is nice and moist for germination and new flies are just hatching from their pupae.
Sacculina is technically a type of barnacle, a crustacean just like its crab hosts, but it was at one time mistaken for a fungus. The female begins her life in a microscopic, shrimplike swimming stage, but will discard more than 90% of her body when she locates a crab, reducing down to a blob of raw cells which grow “roots” throughout the host and eventually create a small opening for the male sacculina to enter and mate with her. If the host crab is a female, it gets tricked by the parasite into carrying, nurturing and spreading larval Sacculina as if they were its own little crablings…and even if the host crab is male, Sacculina transforms its body and mind to function just like a female anyway.
Related to tapeworms, Leucochloridium inhabits the body of a snail but must complete its life cycle in the body of a songbird. Birds don’t find snails to be particularly appetizing and wouldn’t normally notice them lurking in the shadows, but the parasite is able to reverse the snail’s behavior so that it seeks the open sun, and more disturbingly, it warps the snail’s appearance to resemble something tastier. Leucochloridium’s colorful, pulsating “brood sacs” grow within the snail’s eyestalks, transforming them into what resemble fat, striped caterpillars or maggots. Birds spot the lure from the air, rip the snail’s face off, and end up spreading the parasites around in their droppings. The snail, meanwhile, will grow back its tentacles to repeat the grim process again and again.
While this tapeworm relative doesn’t pull any fancy mind control, it does perpetuate itself by transforming its host into a monster; the victims here are tadpoles, and the larval parasites – which look oddly tadpole-like themselves – will tamper with the tadpole’s development into a frog to create horrific deformities. Infected frogs may have any number of deformed arms and legs at awkward, random angles, making it extremely difficult for them to swim or hop. The only purpose of this extreme transformation is to get the frog caught and eaten by – surprise, surprise – a predatory waterbird for a free flight to the next pond.
3. Lancet Fluke
Another one that messes with ants, the adult lancet fluke inhabits the body of a cow, releasing its eggs into the host’s feces. Snails, who happen to enjoy a nice hot cow pie, end up eating the eggs and getting infested with worm larvae. The snails react to the larvae by spitting them back out in big balls of slime, and these wormy slimeballs smell incredibly delicious to passing ants. Once eaten by an ant, the worm waits until nightfall – when it’s nice and cool – and forces the ant to climb a blade of grass, bite down on the tip, and raise its butt into the air. This is the perfect position to get swallowed by another cow, and if the ant doesn’t get swallowed? The worm releases control in the morning, allows the ant to live a normal day of anthood, and repeats the whole process night after night. It’s just like a vampire, if vampires awoke every night trying to get eaten by cows, so actually nothing like a vampire. Nevermind.
Though related to the harmless fruit flies breeding in the world’s neglected fruit bowls, Pseudacteon flies have a far more sinister appetite. The female lays her egg in the body of a living ant, and the tiny maggot will eventually move into the ant’s head to devour its brain. This won’t kill the victim, but will cause the ant’s (technically dead) body to wander aimlessly for days on end, until the ant’s head simply drops off from its body. The maggot will use the severed head as a pupation chamber, transforming into a new fly and finding itself a mate.
There are many, many species of “parasitoid” wasp whose larvae develop in the bodies of other insects, particularly caterpillars, and there are many of these which can alter their host’s behavior, but Glyptapanteles may be one of the most shocking. Like other parasitoid wasps, the larvae will eventually eat their way out of their caterpillar host to spin cocoons and develop into adults, but in this case, the process does not kill the caterpillar. Instead, the partially eaten host will stand guard over the wasp cocoons, cover them in layers of silk and flail viciously at tresspassing insects. When the parasites are finished their metamorphosis and emerge from their cocoons as wasps, the zombie caterpillar finally dies of starvation and exhaustion.