Once upon a time, parasites were thought to live relatively simple lives: They hitched a ride on a host, sapping nutrients and energy but otherwise leaving it alone. But that was only part of the story. Many parasites actually take control, causing their hosts to act in self-destructive ways that further their invaders' interests. The Lymantria dispar baculovirus, for example, causes caterpillars to climb into treetops rather than hiding in bark. When those that go uneaten by birds finally die and decompose (as pictured above), viral particles rain onto foliage below, infecting a new generation of caterpillars. "I think the reason people are a little creeped out by seeing pathogens control behavior is that we have examples of it around us all the time," said chemical ecologist Kelli Hoover of Pennsylvania State University, who describes L. dispar's gene target in a Sept. 9 Science study. The following pages show more examples of parasites that spread by controlling their hosts. Above: Dissolving, Baculovirus-Infected Monarch Caterpillar Image: Michael Grove

Ant Berries When the roundworm Myrmeconema neotropicum infects giant Cephalotes atracus ants, the ants' abdomens swell and turn bright red, causing them to resemble berries. Because the South and Central American forest homes of Cephalotes abound with berry-loving birds, it's bad news for the ants -- but looking like a berry is just the beginning. The worms also make the ants sluggish and prone to waving their abdomens in the air, practically inviting birds to eat them. Afterwards, M. neotropicum spreads in the birds' droppings. Image: Steve Yanoviak, University of Arkansas at Little Rock

Zombie Ants The Ophiocordyceps fungus releases a chemical related to LSD, causing infected worker ants to leave their nests and find a leaf about 10 inches off the ground. Once there, the ants attach themselves to the leaf's side, then die. Over the next year, Ophiocordyceps consumes their bodies and uses them as a launching site for spores. Image: Ophiocordyceps grow from a dead ant's head. (David Hughes, Pennsylvania State University)

Suicidal Grasshoppers Once established inside a grasshopper, Spinochordodes tellinii hairworms produce chemicals that disrupt their host's central nervous system, causing it to plunge into water. The worms emerge to complete their life cycle. The grasshopper drowns. Image: After a grasshopper jumps into a pool, a hairworm leaves its body. (VB Films/CNRS Images Media)

Webs for Wasps Wasps are famous for laying eggs in insects that become living meals for their ravenous larvae. Hymenoepimecis wasps take it one step further: On the very evening that the larval wasp will kill its orb-spider host, it causes the spider to make a new type of web. Instead of spinning an elegant skein of concentric circles, the spider makes a structure capable of supporting the cocoon that the larva will build a few hours later, after killing and eating the spider. Image: At left, a regular orb spider web; at right, a wasp-influenced web. (William Eberhard/Smithsonian Institution)

Risk-Taking Fish Commonly found in North American lakes and streams, Clinostomum marginatum -- better known as yellow grubs -- lay their eggs inside fish-eating birds. The eggs are deposited via feces into water and picked up by fish, in whom the parasites mature. When enough grubs have occupied a fish, they make it swim closer to the water's surface. There it's more likely to be eaten, allowing the cycle to continue. Image: U.S. Fish and Wildlife Service