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The crustacean Cymothoa exigua is the first known parasite to functionally replace an entire organ of an animal. (Matthew R. Gilligan, Savannah State University)

Top 10 Real-Life Body Snatchers

Parasites and zombies are not science fiction; they infest rats, crickets, ants, moths and other creatures, sucking the life out of them

To ensure their own survival, parasites alter the appearance and behavior of their hosts in the creepiest ways. For instance, rats carrying the parasitic protozoan Toxoplasma gondii, which reproduces inside the gut of a cat, no longer fear the smell of cat urine. In fact, they are sexually attracted to the scent, according to a recent study. This way, infected rats walk right into the grips of a feline.

Here are ten other parasites whose sophisticated manipulations of animals are more horrifying than fiction.

1. Paragordius tricuspidatus
Exactly how a hairworm parasitizes a cricket is unknown. Scientists suspect that the insect ingests either an infected mosquito or water containing hairworm larvae. But once inside, the hairworm grows three to four times as long as the arthropod, filling all parts of its body except the head and legs.

What happens next is even more bizarre. The parasite, Paragordius tricuspidatus, produces proteins that hijack the cricket’s central nervous system, making it attracted to areas brighter than its shaded forest home. The cricket, Nemobius sylvestris, heads then to an exposed pond or river and dives in, at which point the hairworm emerges from its host’s rear end. In an aquatic environment, the worm can find a mate and reproduce.

For some crickets, it’s a suicide leap. But others lucky enough not to have drowned have lived for several months after the parasite removes itself. In fact, the crickets’ strange attraction to light subsides as little as 20 hours later.

2. Hymenoepimecis argyraphaga
One of the most complex manipulations of a host by a parasite happens in Costa Rica. A female parasitic wasp of the species Hymenoepimecis argyraphaga stings the spider Plesiometa argyra and paralyzes it. In the 10 to 15 minutes that the spider is immobilized, the wasp lays an egg and affixes it to the spider’s abdomen. For a week or two, the spider proceeds living as normal. Then, the egg hatches. The larva pierces the spider’s tough skin and sucks its blood for sustenance. On the night it plans to kill its host, the wasp larva injects a chemical into the spider that drugs it into spinning a web unlike any it would normally make. Basically, the spider repeats one stitch in its web-constructing repertoire over and over. The wasp larva then kills and eats the spider, spins a cocoon from the sturdy web and, a week and a half later, transforms into a wasp.

3. Glyptapanteles sp.
Little do caterpillars of the moth Thyrinteina leucocerae know, but as they feed on guava and eucalyptus trees in Brazil, the larvae of parasitic wasps of the genus Glyptapanteles may very well be feeding on them. The wasp deposits up to 80 eggs in the caterpillar. When the eggs hatch, the larvae bulk up by eating the host’s innards. At full size, all but a few squeeze through holes in the caterpillar’s skin and spin a cocoon on a nearby twig or leaf.

The larvae that stay behind begin to pull the puppet strings, so to speak. Within a day, the caterpillar stops eating and starts exhibiting a strange behavior—what scientists call “violent head-swings.” Like a bouncer at a bar, it swings at any predators that approach the cocoon, either knocking them down or causing them to back away. Once the wasps emerge, the caterpillar dies, having served its purpose.

4. Sacculina carcini
A parasitic barnacle, Sacculina carcini invades crabs and turns them into surrogate mothers. In the larval stage, female Sacculina swimming in seawater are able to sniff out crabs. They tend to latch onto European green crabs, an invasive species native to the northeast Atlantic. Once the parasite lands on a crab, it makes its way to a joint in the crustacean’s exoskeleton. The barnacle sheds a good portion of its body and, slim as a slug, slips into the hole at the base of one of the crab’s hairs. The parasite travels to the tail end of the crab, where it camps out. The Sacculina grows tendrils that wrap like vines around the inside of the crab, and it pilfers nutrients from the crab’s blood. If a male barnacle locates the bulge on the crab’s underside where the female resides, he too squeezes in and fertilizes the female’s eggs.

Crabs infected with Sacculina are essentially sterilized by it. But since the parasite’s eggs sit in the same place where the crab would carry an egg pouch, the crab cares for them as if they were its own. Even if the crab is a male, it takes on the maternal role. When the larvae have developed enough to exist on their own, the crab goes to a high rock, where it bobs up and down as it pushes the Sacculina larvae out. The crab flails its claws in the water to spread the parasite, as it would its own young.

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