The lancet fluke, Dicrocoelium dendriticum, is perhaps the best-known example of parasite manipulation of host behavior, which is manifested by a radically changed behavior that leaves infected ants attached to vegetation at times when transmission to an herbivore host is optimal. Despite the publicity surrounding this parasite, curiously little is known about factors inducing and maintaining behavioral changes in its ant intermediate host. This study examined the importance of 3 environmental factors on the clinging behavior of red wood ants, Formica polyctena , infected with D. dendriticum . This behavior, hypothesized to involve cramping of the mandibular muscles in a state of tetany, was observed in naturally infected F. polyctena under controlled temperature, light, and humidity conditions. We found that low temperature significantly stimulated and maintained tetany in infected ants while light, humidity, ant size, and infection intensity had no influence on this behavior. Under none of the experimental conditions did uninfected ants attach to vegetation, demonstrating that tetany was induced by D. dendriticum . Temperature likely has a direct impact on the initiation of clinging behavior, but it may also serve as a simple but reliable indicator of the encounter rate between infected ants and ruminant definitive hosts. In addition, temperature-sensitive behavior manipulation may protect infected ants from exposure to temperatures in the upper thermal range of the host.