Lack of the blue-sensitive cones in the terrestrial salamander. Aquatic (a–c) and terrestrial (d–f) salamander retinae were double-labeled with antibodies specific for Blue-N (green color), which labels the green rods, blue-sensitive cones and UV-sensitive cones (a, d); and UV-N (blue color), which labels the UV-sensitive cones (b, e). (c, f) Superimposed images of (a) and (b), or (d) and (e), respectively. (a–c) The retina from an aquatic salamander shows that there are blue-sensitive cones expressing only the SWS2 opsin, but not the SWS1 opsin in the retina. (d–f) In the retina from the terrestrial phase salamander, every cone expressing the SWS2 opsin also expresses the SWS1 opsin, indicating that they are UV-sensitive cones. The white arrows indicate green rods (bar = 16 μm).