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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).
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