Homozygotes have defects in optic lobe organisation. Adjacent synaptic neuropil boundaries are fused in characteristic locations (for example the posterolateral junction of the medulla and lobula plate). Ectopic fibre bundles cross regions of synaptic neuropil. The lobula complex and proximal medulla are more often affected than the lamina and distal medulla. Optic lobe position is normal. The penetrance and expressivity of this phenotype increases with rearing temperature.