The sense of smell is the main sensory modality for ants, and odors play a major role in the social organization of ant colonies. Social odors (pheromones) are used e.g. for recruiting and division of labor. In the leaf-cutting ant (Atta vollenweideri), workers are polymorphic, with specialized workers for specific tasks. Differential gene expression during development results in postembryonic polymorphism. Caste-specific variations extend not only to body size and morphology, but also to the olfactory sense. Based on neuroanatomical differences of the olfactory system workers can be divided into subcastes. Distinct odor-guided behavior, polymorphism and phenotypic plasticity of the brain represent a unique substrate to investigate causal relationships between odor detection, olfactory information processing and behaviour. In an integrative approach, we aim to understand the genetic and neuronal basis of odorguided social behavior in leaf-cutting ants. Following the identification of olfactory receptor (OR) genes, we investigate differential gene expression in castes and subcastes. Using In situ hybridization, we will identify caste-specific sensilla. Ca-imaging of selected antennal lobe neurons in vivo and neuroanatomical techniques will allow us to analyze the functional significance of OR gene expression. Integration of accumulated data will give insight into the evolution of the complex pheromone communication system in ants. By combining modern techniques in an integrative approach, the project will yield important insights into odor information processing and the evolutionary origin of a complex olfactory system.