In the Drosophila olfactory system, axons of the olfactory receptor neurons (ORNs), each of which typically expresses a single specific olfactory receptor, converge onto specific glomeruli in the antennal lobe to create a spatial odour map. Olfactory information is relayed from glomeruli to higher brain centres by projection neurons (PNs), each of which forms synaptic connections with a single class of ORNs. To investigate how this wiring is established, Jefferis et al. have used `mosaic analysis with a repressible cell marker', MARCM, to study PN development at single-cell resolution (see p. 117). Surprisingly,given that previous work suggested that ORNs are central to olfactory map formation, PN dendrites established a prototypic neural map in the antennal lobe before ORN arrival. The researchers propose that this coarse PN dendritic map interacts with a coarse map formed by developing ORNs to generate the mature glomerular organisation.