Sex differences are basic for reproduction, parenting and other social interactions. Pheromone secretions that are differentially perceived by males and females, release stereotypical behaviours in many species. I will study how simple connectivity switches in a Drosophila sexually-dimorphic neuronal circuit are assembled into complex networks, from sensory processing to behavioural control. 11-cis-Vaccenyl-acetate (cVA) is a male-pheromone eliciting sex-specific responses: attracts females and repels males. Sex-specific wiring of olfactory neurons reroutes cVA information, forming a developmental switch in information flow. Central aSP-g neurons receive cVA innervation in females but not males, while this cluster is implicated in male-male aggressive behaviour. The role of aSP-g in social interactions was not compared between sexes, and that is my first aim. Next, I will find input and output neurons of aSP-g neurons in both sexes, by combining state-of-the-art anatomical, physiological and behavioural methodology: in-silico circuit-tracing methods to find neurons with overlapping innervations to aSP-g; and a unique electron-microscopy volume scan of a female brain to reconstruct aSP-g neurons and their synaptic partners. I will validate functional connectivity using photoactivation of output neurons while calcium-imaging target neurons. I aim to discover how sexually-specific wiring differences in homologous circuits regulate sexually-dimorphic social behaviours. These basic neuronal connectivity motifs may be conserved beyond flies.