5. Polymorphus paradoxus
Pond- and river-dwelling crustaceans called Gammarus lacustris typically dart deep into the water, away from light, when ducks are at the surface. But when the crustaceans are infected with Polymorphus paradoxus, a type of thorny-headed worm, they practically throw themselves at their predators. Oddly attracted to light, the parasitized crustacean swims to the surface and clings to a rock or plant. There, fully exposed, the crustacean is more likely to get eaten by a duck. Inside a duck is exactly where the parasite needs to be to reach adulthood. The clinging position of the crab on the rock is the same one that the male crustacean takes while copulating. Scientists speculate that the parasite increases serotonin levels in the crustacean, perhaps making it think it is having sex.
6. Dinocampus coccinellae
A female parasitic wasp of the species Dinocampus coccinellae surreptitiously lays one egg in the abdomen of the ladybug Coleomegilla maculata. Like many parasites, the wasp larva munches on the ladybug’s tissues. Once it reaches a particular stage in its growth, the maggot-like larva inches its way out between segments of the ladybug’s underside. As it does, scientists suspect the larva leaves venoms behind that subsequently alter the ladybug’s behavior. Stunned into a zombie-like state, the ladybug provides cover for a cocoon that the larva spins between the bug’s legs.
After the adult wasp emerges from the cocoon, about 25 percent of ladybugs actually recover from the trauma. Scientists were surprised to find that there is a cost to the parasite: The longer the ladybug guards the cocoon, the less fertile the emerging wasp will be.
7. Dicrocoelium dendriticum
One of the most active hitchhikers has to be a lancet fluke, or flatworm, called Dicrocoelium dendriticum. During its life cycle, the parasite lives in three hosts. First, a snail eats cow dung rife with the worm’s eggs. The eggs hatch inside the snail, and in defense, the snail produces a slime that entraps the larvae. Eventually, the snail hocks a slimy, larvae-filled loogie. Then, an ant comes along and slurps up the slime. The parasites set up two outposts, one around the nerves that control the ant’s mandibles and another in its head. Here is where it gets tricky. The parasite needs to spend its adulthood in the liver of a cow, so it has to get a cow—an herbivore—to eat the ant it has infected. With a little mind control, the parasite gets the ant to crawl up to the top of a blade of grass each night and bite down to stay in place. This way, a cow is more likely to chomp on it while grazing. In the cow’s liver, adult worms reproduce, and the cow later defecates the eggs. And so, the cycle, cleverly illustrated by Matthew Inman on his website The Oatmeal, continues.
8. Leucochloridium paradoxum
Another parasitic flatworm, Leucochloridium paradoxum, infects a snail and then somehow has to get from a snail to a bird, its next and final host. One problem: Birds do not normally snack on snails. Undeterred, the parasite packs itself into the snail’s translucent eyestalks. The green and brown-striped worms make the eyestalks, at least to a bird, look like juicy, quivering caterpillars. Infected snails also make themselves more visible to birds because they do not shy away from light as healthy ones do.
9. Myrmeconema neotropicum
In 2005, scientists at the Smithsonian Tropical Research Institute in Panama studying Cephalotes atratus, a species of tropical ant found in tree canopies, discovered a few ants with bright red gasters. A “gaster” is the last bulb of an insect’s abdomen. Could it be a new species? At least one biologist thought so—enough to bet on it over beers. But when they dissected the ants, the researchers found that the gasters were filled with eggs, each containing a tiny nematode worm.
Foraging ants unknowingly feed the parasite, named Myrmeconema neotropicum, to ant larva via bird feces. The worms then mate inside the grown ant’s abdomen, which fills with yellow eggs. The parasite thins the ant’s exoskeleton, and the eggs change the gaster’s color from black to red.
Mistaking it for a berry, a fruit-eating bird swoops down and plucks the ant’s egg-filled abdomen, which is ripe for the picking. The parasite weakens the exoskeleton between an ant’s gaster and postpetiole, another of its body regions, making ie easier for the bird to detach it. The bird then spreads the worm eggs in its droppings.
10. Cymothoa exigua
The crustacean Cymothoa exigua does not alter the behavior of its host, a snapper of the species Lutjanus guttatus, but it is the first known parasite to functionally replace an entire organ of an animal. The louse is a tongue snatcher. Found primarily in the Gulf of California, the parasite invades a snapper through its gills and hooks seven pairs of claws at the base of the fish’s tongue. It sucks blood from the tongue, and as the parasite grows, the tongue atrophies. By the time the host’s tongue is a nub the parasite becomes a stand-in. The snapper can carry on eating, and the crustacean is there to catch the crumbs.