When it is fully grown, it faces a difficult problem, that of returning to water. So it has evolved a clever way of influencing its host to deliver just one further service -- the stricken grasshopper looks for water and dives in.

The suicidal behavior of the infected grasshoppers has been studied by a team of biologists from the French National Center for Scientific Research in Montpellier, France, led by Frédéric Thomas and David Biron.

They did their fieldwork around a swimming pool on the border of a forest near Avène les Bains in southern France. Hordes of infected grasshoppers -- more than 100 a night -- arrive at the pool during summer nights at the behest of the parasites.

The biologists captured grasshoppers before their suicidal plunge and removed the worms.

The worms grow to several times the length of the grasshopper's body before they emerge. Because of their unusual size, it is easy to extract and analyze the different sets of proteins that they produce before, during and after they compel their hosts to drown themselves.

"We found the parasite produces and injects proteins into the brain of its host," Dr. Thomas said.

Two of the proteins belonged to a well-known family of signaling agents known as the Wnt family that are deployed in developing the cells of the nervous system.