Some species of viruses, fungi, protozoans, wasps, and tapeworms take over the brains and actions of the animals and insects they infect — making them do whatever their new master wants.A recent issue of the Journal of Experimental Biology was completely dedicated to these parasites, which turn their hosts into zombies.
This is usually controlled by special biochemistry. The parasites release chemicals or manipulate the genes of their host, changing their behaviours. Most them them do this by changing the brain’s chemicals, or by inducing “sickness behaviours” by messing with the host’s immune system.
“The fact that parasites can so efficiently alter host behaviour is fascinating,” guest editor of the journal, Michael Dickinson, from the University of Washington, said in an article in the issue. “There is something horrifying and wondrous about a tiny ‘implant’ being able to control such a large animal machine.”
Hymenoepimecis argyraphaga – These wasps live in the spider Plesiometa argyra. The wasp injects chemicals into the spider that make it abandon its web and build a completely different one, with a thick sheet to protect the wasp's offspring's cocoons, which they build after chewing threw the spider's body.
Baculovirus – This virus infects Gypsy moth caterpillars and releases an enzyme that stops their natural molting cycle, makes them eat continually, and forces them to climb to the top of trees and clamp onto the leaves until it dies and liquefies, raining down viruses across the forest to infect other caterpillars.
Toxoplasma gondii – Picked up from cat poop, this tiny (one-celled) parasite infects the brains of rats, causing them to make extra dopamine and testosterone, which makes them fearless. When they aren't afraid of cat smells, they venture out into the animals' presence, and get eaten, spreading the parasite to the cat. They also mate more, spreading the parasite to other rats.
Ophiocordyceps unilateralis – This fungus takes over the brain of ants like Camponotus leonardi, making it climb up to 25 centimeters above the ground, grasp onto a blade of grass or leaf, and die. Soon, the fungus clamps the ant to the plant and grows a spore stalk out of its head, which helps spread its spores.
Myrmeconema neotropicum – These tiny worms make the ant Cephalotes atratus scamper up a tree, clamp down on a twig, and turn its abdomen red like a berry ripe for the picking by a hungry bird. The worm then lives in the bird.
Phorid flies – Adult phorid flies lay their eggs inside fire ants. The eggs grow and develop inside the ant's head and when they grow big enough, end up decapitating them. Before then, the brainless ants wander around aimlessly. Because they attack fire ants, these flies are used to control the population in some areas.
Ampulex compressa – This wasp turns off the escape reflex of cockroaches with one bite, injecting venom into the brain. They also paralyze the insect's front legs. The wasp uses the roach's antenna to steer the animal home where it uses it as an egg incubator.
Dicroelium dentriticum – This flatworm infects ants, making them crawl to the top of grasses during the evening and cool mornings, where it will be eaten by a sheep or cow.
Dinocampus coccinellae – This parasitic wasp infects the spotted lady beetle Coleomegilla maculate. First, its eggs grow inside the beetle, until they paralyze her and bust out, forming a cocoon on her leg. The ladybeetle's bright shell and occasional twitching help protect the cocoon from predators.
Thorny-headed worms – These worms infect gammarids, a shrimp-like organism that lives in ponds. The worm messes with the animals' normal darkness-seeking mechanism, sending it to the top of the pond, where a bird scoops it up. This bird also gets infected with the worm, completing its lifecycle.
Leucochloridium paradoxum – Amber snails eat the eggs of this flatworm and live in its tentacles, causing blindness and making the tentacles pulsate and seek light. The light seeking brings the snails out of the dark, the blindness makes them not know when to hide from predators, and the pulsating attracts birds, who come and gobble up the parasite.
Euhaplorchis californiensis – This worm infects the brains of the killfish causing them to go belly up. Ok, not literally. In reality, it just swims up to the top of the lake and wiggles around, where it attracts the attention of birds.
Spinochordodes tellinii – This hairworm infects grasshoppers and crickets, and produces proteins that make it commit suicide by jumping into the water, where the worm wants to be so it can find a mate.
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