Parasites manipulate the host behavior to ensure the accomplishment of their life cycles. This extended phenotype raises the interest in the proximate mechanisms. The intense water-seeking behavior has been noted in horsehair worm-infected insects. Previous studies have demonstrated that proteins were expressed differently in the host brain when the host increased their activity levels and positive phototactic behavior. However, no study has tested whether gene expression in the horsehair worm directly causes the production of those proteins. In this study, the gene expressions in the mantid (Tenodera angustipennis) and the horsehair worm (Chordodes fukuii) were examined. The genes expressed differently only during the host-manipulation were considered as candidate genes that linked to the host behavioral changes. Along with these criteria, eight transcripts were detected in mantid brain. The known functions of these genes related to insect energy generation and vitellogenesis are in accordance with the current understanding of the increased host activity and castration in the infected hosts. In the horsehair worms, 1585 up-regulated and 2351 down-regulated transcripts were detected. The large scale of change in gene expression suggests a complex behavioral manipulation launched by the parasite which might be costly while the temporary deactivation of tons of genes.