"My mommy said there are no monsters," Newt explains to Ripley in James Cameron's Aliens. "But there are."
Newt is talking, of course, about the monstrous creations of HR Giger - carnivorous, parasitic horrors whose kin will be scaring viewers of Prometheus over the next few weeks. But nasty as they are, these creatures don't come close to the unpleasantness of some real life parasitic creatures. We look at six of the worst.
Among the better known of the really nasty parasites, these are the little critters that led Charles Darwin to doubt the existence of a beneficent God. There are several species with similar behaviours, each targeting a different kind of host, but some of the worst prey on caterpillars. After landing on a target, they inject their eggs underneath its skin, using a virus to disable its immune system so it can't destroy them. As many as 60 larvae will hatch there. When they do, they'll start eating their host's flesh, but will carefully spare its vital organs and primary muscle tissue so that it can go on feeding and growing - on their behalf.
When they reach full size, making up about a third of the caterpillar's body mass, the larvae grow sharp teeth and slice their way out through its skin - but before doing so, they mess with its brain. The mortally wounded caterpillar spends its last hours defending the larvae from potential predators as they cocoon themselves and metamorphosise into adult wasps, ready to start the process all over again.
As any movie buff should know, picking up strange women can be a risky business. It's particularly risky if you're a crab and the woman in question is a young, slug-like sacculina parasite. After finding a gap in your exo-skeleton she'll pierce it with a sharp tube and dissolve her own body, injecting herself into yours. She'll then feed off your blood and gradually develop tendrils that snake around every part of your body, making it answer to her commands rather than your own.
When she's fully in control, sacculina produces a knobbly crown on top of a crab's shell which she uses as her nest. There she will essentially ride the crab until the end of its days. She'll castrate it and use the space this leaves to produce her own young. The crab's last action will often be to waft them away into the ocean where they can look for fresh hosts.
Not all parasites kill or control their hosts; some simply force them into a form of physical intimacy from which they can never escape. This little crustacean is attracted to the gills of fish. Swimming inside them, it gradually claws its way through the folds of flesh and into the mouth. There it attaches itself to the fish's tongue and drinks its blood. Eventually, with its blood supply stolen at the root, the tongue decomposes and drops off. The crustacean then secures itself where the tongue was and acts as a substitute, remaining attached to the now dependent fish for the rest of its days. It will lay its eggs there and have the fish spit them out into the surrounding waters - usually in the middle of a shoal of potential new victims.
From insects and crustaceans, we turn to the humble protozoan. Leishmaniasis is a simple organism but no less destructive for it. It lives out one stage of its life in the sandfly. They mostly come at night - mostly - so if you're in the jungle and you're bitten after dark, and the bite doesn't heal but instead turns into a festering wound, it's time to worry.
There are two strains of leishmaniasis. One multiplies in and feeds on the internal organs, and is frequently fatal. The other is less dangerous but far more grotesque. Its preferred food is skin and soft tissue. It likes gums, lips, noses... you get the idea. Over the course of weeks or months, it will eat away your face.
No all parasites wreak havoc by feeding on their hosts. Some prefer to feed their hosts to other animals. These little flatworms have a strange side effect - they make the fish they infect better looking. They also take control of their brains.
Killifish are prized by aquarium keepers because of their bright colours. These probably evolved for sexual displays, but they can also attract predators, so the fish are wary about getting too close to the surface. Once infected with euhaplorchis, however, they find their colours getting brighter. As the flatworms lay eggs in their brains they literally lose their minds, and under the control of their new masters they not only swim to the surface but flip about on their sides. This attracts the attention of birds. When the fish get eaten, the flatworms are able to move into the hosts they need for the next stage of their life cycle.
Sometimes the simplest tricks are the most horrible. Screwworms are the piranhas of the parasite world. Despite the name (which comes from the shape of the larvae) they're actually a type of fly, which lays eggs in the skin of a (preferably wounded) host. Any reasonably large mammal will do, including humans. When they hatch, the young maggots begin to eat the flesh of their host, growing rapidly. As they do so, they damage the skin, making it an easy target for other screwworm flies to attack. Repeat infestations like this are common, and because they'll keep eating after their host is dead, screwworms can consume it entirely within just a couple of weeks.
If you want to help fight parasitic infestations in humans, check out the work of Developing World Health.