Fig. 2. Spectral sensitivity of the photoreceptors calculated from the sensitivity maxima given in Struwe (Struwe, 1972) for H. erato (solid lines) and Briscoe et al. (Briscoe et al., 2003) for V. cardui (broken lines) using the template from Stavenga et al. (Stavenga et al., 1993). Peak sensitivity values for the short, medium and long wavelength visual pigments are identical between V. cardui and V. atalanta (not shown; G. Bernard, personal communication). Observe that both species have the medium wavelength photoreceptor peaking at 470 nm therefore the curves for both species coincide. The vertical dotted lines correspond to the wavelengths of the stimuli used.