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First published online January 27, 2004
Journal of Experimental Biology 207, 755-765 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00810

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Why is it worth flying at dusk for aquatic insects? Polarotactic water detection is easiest at low solar elevations

Balázs Bernáth^1,2, József Gál³ and Gábor Horváth^1,*

¹ Biooptics Laboratory, Department of Biological Physics, Eötvös University, H-1117 Budapest, Pázmány sétány 1, Hungary,
² Plant Protection Institute of the Hungarian Academy of Sciences, Department of Zoology, H-1525 Budapest, P. O. B. 102, Hungary
³ International University Bremen IUB, School of Engineering and Science, P. O. B. 750561, D-28725 Bremen-Grohn, Germany

View larger version (33K): [in a new window]   Fig. 1. (A) Experimental arrangement of the 180° field-of-view imaging polarimetric measurement of the reflection-polarizational characteristics of horizontal water-dummies. (B) Relative reflectivities of the matt black and matt grey cloths (used as substrata of the glass panes) as well as the black and grey water-dummies. (C) The mirror image of the apparent celestial path of the sun during the measurements on 18 July 2002 under clear, cloudless skies at the Hungarian Kunfehértó (46°23' N, 19°24' E) in a system of polar coordinates, where the solar azimuth angle (s) is measured clockwise from the magnetic north, and the solar elevation (s) is measured radially from the horizon. Dots show the solar positions when the measurements were performed. Black dots represent the solar positions when the patterns in Figs 2 and 3 were measured.

View larger version (61K): [in a new window]   Fig. 2. Colour photographs (without polarizers) of the mirror image of the clear sky reflected from the grey water-dummy (glass pane underlaid by matt light grey cloth), patterns of the degree (d) and angle (; measured from the local mirror meridian) of linear polarization of reflected skylight, and the area detected polarotactically as water versus the solar elevation (s) and time (local solar time = UTC + 2 h). The polarization patterns are measured by 180° field-of-view imaging polarimetry in the blue part of the spectrum. Chequered areas show those regions that are inappropriate for comparative analyses due to unwanted overexposure, shadows and mirror images of the polarimeter, its holder and remote cord. In column 4, regions are shaded by black where d>dtr=5% and 85°95°. An imaginary polarotactic water insect is assumed to consider a surface as water if these two conditions are satisfied for the partially linearly polarized reflected light. In column 4, the regions where these criteria are not satisfied remain blank. The positions of the mirror image of the sun are shown by dots, and the Brewster angle (56° from the nadir for glass with an index of refraction of 1.5) is represented by an inner circle within the circular patterns. Because of disturbance by early morning dewfall, reflection-polarization patterns at low solar elevations are presented here only for the sunset and dusk period.

View larger version (65K): [in a new window]   Fig. 3. Colour photographs (without polarizers) of the mirror image of the clear sky reflected from the black water-dummy (glass pane underlaid by matt black cloth), patterns of the degree (d) and angle (; measured from the local mirror meridian) of linear polarization of reflected skylight, and the area detected polarotactically as water versus the solar elevation (s) and time (local solar time = UTC + 2 h). See Fig. 2 for further details.

View larger version (46K): [in a new window]   Fig. 4. Patterns of the degree (d) and angle (; measured from the local mirror meridian) of linear polarization of reflected skylight, and the area detected polarotactically as water versus the solar elevation (s) for a perfectly black glass (with an index of refraction of 1.5) reflector - which absorbs all penetrating light - calculated for incident single-scattered Rayleigh skylight with the use of the Fresnel formulae. The Brewster angle (56° from the nadir for glass with an index of refraction of 1.5) is represented by a circle within the circular patterns.

View larger version (35K): [in a new window]   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).