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Fig. 1. (A) Simple 4-stage model of colour discrimination with two spectral types of receptors. At receptor stage 1, the signals r_1a, r_1b and r_2a, r_2b arise when the animal looks at the two colours a and b. At the subsequent neural stage 2, two neural interactions are possible: summation of the receptor signals, resulting in the achromatic signals S_aa and S_ab, and subtraction (or comparison), resulting in the chromatic signals S_cha and S_chb. At stage 3, the signal arising from the two colours a and b are compared, and finally, a behaviour will occur with probability P. (Adapted from Kelber et al., 2003b.) (B) Natural light levels and limits of colour vision in different animals. Humans lose their colour vision ability in dim moonlight and so do diurnal honeybees Apis mellifera (Menzel, 1981). Nocturnal hawkmoths (Deilephila elpenor, Hyles lineata and H. gallii) can still see colour at dim starlight levels. Nocturnal geckos (Tarentola chezaliae) were tested at dim moonlight levels.

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