The structural relationship between the afferent projection and the dendrites of the interneurones was examined in the cercal-to-giant interneurone system of the cricket using intracellular recording and dye injection techniques. The physiology of the sensory neurones beneath the cercal filiform hairs was investigated by placing a recording pipette over the end of a cut hair and using movements of the pipette to characterize the directionality of the receptor. Most of the filiform receptors could be classified as belonging to one of four major types. Each type is sensitive to a different wind direction and is confined to particular regions of the cercus. The location of the terminal arborizations of each type of sensory cell was revealed by staining with cobalt chloride. Single cells were stained reliably by placing a dye-filled pipette over a cut hair. Each physiological receptor type arborizes in a different region of the central nervous system. Therefore the neuropile is functionally divided according to wind direction. The dendrites of three identified interneurones were examined in the context of this afferent projection. It was found that each of these neurones has dendrites in regions of neuropile corresponding to different wind directions. By searching for unitary synaptic potentials in identified interneurones, it was possible to show a strong correlation between anatomical overlap of primary afferent and interneurone and the existence of a synaptic connexion. Further, when there was no overlap, no synaptic potentials were seen. Therefore the over-all excitatory receptive field of an interneurone could be predicted by examining its dendritic structure. Each of the three identified interneurones examined in this study was found to have a directional response that matched the response predicted on the basis of its anatomy.