(A) Overview of the developing Drosophila visual system at 24 hr APF. Arrows indicate the developmental gradient of photoreceptor differentiation in the retina and corresponding axonal targeting in the medulla neuropile. (B) Model of initial innervation of R cell axons and growth cone segregation in the medulla. C–E’’. R7/R8 axon innervation in the medulla target field at different developmental stages. (C, D, E) R8 growth cones labelled with UAS-mCD8-GFP expressed under sens-Gal4. C’, D’, E’. R7 growth cones labelled with UAS-mCD8-GFP expressed under sev-Gal4. C, C’. At 6 hr APF, 18 R8 (C arrowhead) and 15 R7 (C’ arrowhead) axons innervate the medulla. D, D’. At 12 hr APF, 24 R8 and 21 R7 axons innervate the medulla. In addition to 21 R7 axons that are already present in the medulla field, 22nd R7 axon can be seen entering at the anterior medulla (D’ arrowhead). E, E’. At 24 hr APF, 32 R8 and 29 R7 axons have innervated the medulla field. Scale bar shown in all images is 20μm. All photoreceptor axons are visualized using 24B10 antibody (Fujita et al., 1982 in red), R8 (in A, C, D, E) and R7 growth cones in (C’, D’, E’) are stained with anti-GFP antibody (in green) and medulla neuropile is stained using anti-N-Cadherin antibody (in Blue). C’’, D’’, E’’. Quantification of the sequential innervation of R7/R8 axons in the medulla field. Error bars indicate Standard Deviation.