Fig. 5. Percentage, P, detected as water (polarotactic detectability) by an imaginary polarotactic water insect for the black (squares) and grey (dots) water-dummies (A, C, E), and difference, P, between the black and grey water-dummies (diamonds; B, D, F) as a function of the solar elevation, _s, in the blue, green and red parts of the spectrum. P gives the proportion of the black areas in column 4 of Figs 2 and 3 relative to the entire area of the region appropriate for comparative analyses (non-chequered regions in Figs 2, 3). Data points measured in the morning and afternoon are depicted as empty and filled squares/dots/diamonds, respectively. The black continuous curves (polinomials) are fitted to these data points by the method of least squares. The dashed/dashed-dotted P(_s) curves are computed for the full area of a perfectly black glass (index of refraction = 1.5) and water (1.33) reflector absorbing all penetrating light. Triangles show the P-values calculated for the perfectly black glass and water reflectors within the regions of the field of view appropriate for comparative analyses (non-chequered regions in the d- and -patterns of Figs 2, 3).