The New York Times has an interesting article today documenting some of the interesting ways parasites take over their hosts in order to stay alive and proliferate. Take as an example the Costa Rican spider:

In the case of the Costa Rican spider, the new web is splendidly suited to its wasp invader. Unlike the spider’s normal web, mostly a tangle of threads, this one has a platform topped by a thick sheet that protects it from the rain. The wasp larva crawls to the edge of the platform and spins a cocoon that hangs down through an opening that the spider has kindly provided for the parasite.

To manipulate the spiders, the wasp must have genes that produce proteins that alter spider behavior, and in some species, scientists are now pinpointing this type of gene. Such is the case with the baculovirus, a virus sprinkled liberally on leaves in forests and gardens. (The cabbage in a serving of coleslaw carries 100 million baculoviruses.)

Human diners need not worry, because the virus is harmful only to caterpillars of insect species, like gypsy moths. When a caterpillar bites a baculovirus-laden leaf, the parasite invades its cells and begins to replicate, sending the command “climb high.” The hosts end up high in trees, which has earned this infection the name treetop disease. The bodies of the caterpillars then dissolve, releasing a rain of viruses on unsuspecting hosts below.

David P. Hughes of Penn State University and his colleagues have found that a single gene, known as egt, is responsible for driving the caterpillars up trees. The gene encodes an enzyme. When the enzyme is released inside the caterpillar, it destroys a hormone that signals a caterpillar to stop feeding and molt.

And also the thorny head worm:

Their host is a shrimplike crustacean called a gammarid. Gammarids, which live in ponds, typically respond to disturbances by diving down into the mud. An infected gammarid, by contrast, races up to the surface of the pond. It then scoots across the water until it finds a stem, a rock or some other object it can cling to.

The gammarid’s odd swimming behavior allows the parasite to take the next step in its life cycle. Unlike baculoviruses, which go from caterpillar to caterpillar, thorny-headed worms need to live in two species: a gammarid and then a bird. Hiding in the pond mud keeps a gammarid safe from predators. By forcing it to swim to the surface, the thorny-headed worm makes it an easy target.

Simone Helluy of Wellesley College studies this suicidal reversal. Her research indicates that the parasites manipulate the gammarid’s brain through its immune system.

The invader provokes a strong response from the gammarid’s immune cells, which unleash chemicals to kill the parasite. But the parasite fends off these attacks, and the host’s immune system instead produces an inflammation that infiltrates its own brain. There, it disrupts the brain’s chemistry — in particular, causing it to produce copious amounts of the neurotransmitter serotonin.