This region, which contains octopamine-secreting nerve cells, controls the ability to start walking. The venom interferes with the release of octopamine, they found.
The researchers then reversed the process: they injected an octopamine-like substance directly into the protocerebrum of cockroaches that had already been turned into zombies by wasp stings.
The result was significant recovery and restoration of the cockroach's free will.
(Related news: "Robo-Roaches Can Control Insect Groups" [November 15, 2007].)
"This helps us understand how movement is initiated in animals," Libersat said. "We know how movement itself is generated, but to understand what makes an animal decide to move or not to move is a different issue."
The jewel wasp is the only parasite known to inject its venom directly into its host's brain.
But other parasites also control the behavior of their hosts, said David Richman, curator of the Arthropod Museum at New Mexico State University in Las Cruces, who was not involved in the new study.
"This is not uncommon. There are a tremendous number of parasites, and they all have different strategies for survival and for propagation of their species," Richman said.
The behaviors of land snails, grasshoppers, and types of ants, for example, can all be affected by parasites.
"Not only that," Richman added, "[some parasites] can take over certain aspects of the host's biology, particularly as you get into microorganisms."
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