Optic lobe neuroblasts derive from the neuroepithelium in a medial transition zone. (a-c) Single FLP-out clones expressing nuclear β-gal (red) in the optic lobe at late third instar (96 hours ALH). Dpn is in green, Scrib in blue. (a) An epi only clone containing Dpn negative epithelial cells (marked with β-gal in red, open arrowhead) but no Dpn positive neuroblasts (green, arrowhead). (b) An epi/NBs/progeny clone containing Dpn negative epithelial cells (open arrowhead), Dpn positive neuroblasts (arrowhead, yellow) and progeny cells (arrow). (c) A NBs/progeny clone containing Dpn positive neuroblasts (arrowheads, yellow) and progeny cells (arrow).