Fig. 1. (A) Frontal section of the optic lobe of Musca domestica. re, retina; la, lamina; me, medulla; lo, lobula. Arrows indicate the site of lesioning. Scale bar, 100 µm. (B) Light micrographs of the proximal lamina at the level of the external chiasma (ch), showing cross- sectioned photoreceptor terminals at proximal (p) and distal (d) depths in the lamina neuropile. Scale bar, 20 µm. (C) A single cartridge of the lamina comprises photoreceptors (R) and five monopolar cells. Two large monopolar cells, L1 and L2, are located at the axis of every cartridge. Scale bar, 1 µm. (D) The optic lobe of the housefly after lesioning stained with anti-PDF serum. The large PDF cell bodies (asterisk) and processes in the medulla are present in a cut-off part of the optic lobe. Arrows indicate the site of lesioning. Scale bar, 100 µm.