"Acoustic signals are used to communicate in many species. It is found from insects to humans to, for example, locate preys, in courtship behaviour or to transmit more complex information to other individuals. To use auditory cues efficiently animals must be able to recognise species-specific sounds. This means that the auditory system is tuned to these sound patterns. Current models assume that sound recognition occurs through matched sensory filters along the auditory pathways, but the identification of neurons with these properties has proved challenging. A characterisation of the auditory system, from sensory input to behavioural output, is currently missing in any animal.Here we proposed to study these mechanisms in simple model systems: the brain of Drosophila fruit flies and honeybees. We will first take advantage of the small brain and of the powerful genetic tools in Drosophila to identify single neurons in the auditory brain centres. Drosophila use auditory signals during their very robust courtship behaviour. We will first record in vivo from neurons and characterise their auditory responses using whole-cell patch clamp recordings while playing natural courtship sounds. With these experiments we will identify neurons responding to species-specific sound features. Second, to identify new neurons in the auditory centres of Drosophila we will use a new tracing methods based on photoactivatable green fluorescent proteins. Here we will discover the higher order targets of auditory neurons in the brain. Finally, we will use intracellular and optical imaging methods to identify neurons in the auditory centres of the honeybee. We will take advantage of studying brain structures involved in a sophisticated communication mode (waggle dance) but in a simple organism. Our results will shed light on circuits underlying sound processing in the brain and will contribute to the understanding of fundamental principles of communication."