Bolwig's nerve and the eyelet tract both have a close spatial relationship to the lateral neurons in the developing optic lobe. Shown are confocal images of the right optic lobe in frontal view. Green channel, Photoreceptors (gmr-driven expression of GFP inA–F; antibody 24B10 in G–I);red channel, lateral neurons (anti-PDH). Some ommatidia of the developing retina are visible in D andE; other images depict the terminals of photoreceptors R1–R6 in the lamina (La) and R7, R8 in the second neuropil, the medulla (Me). In larvae (A) and P + 20% pupae (B), Bolwig's nerve (BN) traverses the medulla and terminates directly in the larval optic neuropil (arrowhead), which transforms into the accessory medulla during metamorphosis. In the larval optic neuropil the terminals overlap putative dendrites from the small and large lateral neurons (s-LN_v and l-LN_v). Additional fibers parallel to Bolwig's nerve (small green arrows) are possibly neurites of tangential neurons. Expression in the latter persists in early pupae (B) when expression in Bolwig's nerve starts to disappear (C). At approximately P + 50% (D) faint labeling first appears in the eyelet tract (Ey-Tr) together with the first labeling in l-LN_v. At P + 80% (E) the labeling pattern mainly resembles that of adult flies (F). The tract from eyelet now shows a ventral extension overlapping arborizations of the l-LN_v(arrowheads). Fibers arising from l-LN_v form a network over the surface of the medulla in close vicinity to the tract from eyelet (arrow in F). Bolwig's tract (G) and eyelet (H) are present in eyeless sine oculis (so¹) mutants contacting the LN_v. so¹ flies with tiny eye remnants possess, in addition, a well established axonal connection of their ommatidial photoreceptors to the medulla (I). Scale bar: (in I)A–I, 20 μm.