Spike initiation in invertebrate neurons is quite different from vertebrate neurons. In the typical vertebrate neuron, synaptic currents from the dendrites are gathered at the soma, and spikes are initiated in the axon, which starts from the soma. In the typical invertebrate neuron, as the one studied here (Drosophila central neuron), a neurite emerges from the soma, then bifurcates into a dendritic tree and an axon. There is immunochemical evidence of an initial segment-like structure in Drosophila neurons near the bifurcation point (Trunova et al. 2011). This study confirms it with electrophysiological evidence and modeling. Morphologies are reconstructed, and passive responses to currents are measured at the soma. Optimization finds values for the passive properties – there are significant sources of uncertainty, but these are well addressed in the paper. Then they show that spikes in the soma are small, implying that the initiation zone is distal, and they use the model plus recordings of larger action potentials in other types of Drosophila neurons to get an estimate of the spike initiation site, which is found to be near the axon-dendrite bifurcation. Finally, they show that the resting potential is due mainly to Na+ and K+, as in other invertebrate neurons (Marmor, 1975).