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First published online August 31, 2007
Journal of Experimental Biology 210, 3266-3276 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.007831

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Haze, clouds and limited sky visibility: polarotactic orientation of crickets under difficult stimulus conditions

Miriam J. Henze^* and Thomas Labhart

Department for Neurobiology, Zoological Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

View larger version (17K): [in this window] [in a new window]   Fig. 1. Data recording and evaluation. (A) Top view of a cricket walking on a StyrofoamTM ball under a slowly rotating, polarized stimulus. The animal is kept on the spot by a balanced arm (not shown). Its walking movements are transferred to the ball and registered by detecting the moving dots on the surface of the ball. (B) Rotational movements of the cricket recorded during two full revolutions of the stimulus (4x180°). Abscissa: walking direction (rotational component of the run) given by the number of dots that passed the detector; positive and negative values indicate right and left turns, respectively. Ordinate: stimulus orientation. Provided that the translation (forward movement) of the cricket was constant, the resulting curve also reflects the virtual walking path. Note the bias in walking direction caused by the inherent turning tendency of the animal. (C) Fourier spectrum of turning speed per degree. Data shown in B were differentiated to remove the bias and then analyzed by a fast Fourier transform (FFT). Abscissa: period of modulation of walking direction. Ordinate: amplitude of FFT signal. Because of the 180° periodicity of the polarized signal, the amplitude at 180° (S) was taken as a measure of the strength of the polarotactic response.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Optical elements and their combinations for generating the visual stimuli. To produce polarized stimuli for tests and motivation controls, we used a linear polarizer overlaid with a diffuser, a combination termed `polarization screen'. For the zero controls, the polarization screen was inverted, thus resulting in an unpolarized stimulus. Depending on the experiment, the polarization screen was combined with an additional diffuser, with an optical retarder or annuli consisting of opaque or diffusing material (for details see text). Note that the maximal diameter of the stimulus (not shown) was 92° for Stimulus size and Clouds experiments but 25° for the Haze experiment due to technical reasons. Elements marked with an asterisk were used under specific experimental conditions only. The resulting degree of polarization (d or ) is indicated at the bottom of each table cell.

View larger version (30K): [in this window] [in a new window]   Fig. 3. Stimulus size experiment. (A) 180° fisheye view of the celestial hemisphere taken by a camera positioned in a meadow. A considerable part of the sky is obstructed by vegetation. (B–D) Polarotactic response as a function of stimulus size. The radius (r) of a zenithal stimulus was reduced from 2r=92° to 1° with a degree of polarization (d) of either 100% or 0%. Tests (2r=1° to 48°, d=100%) are indicated by black, motivation controls (2r=92°, d=100%) by gray, and zero controls (2r=1° or 92°, d=0%) by white (16 series of 11 individuals). (B) Survey of results. The relative strength of the polarotactic response (S/Smot, mean ± s.d.) is plotted against stimulus size. (C,D) Comparison between the largest (92°, top row) and the smallest (1°, bottom row) stimulus. (C) Distribution of S-values. (D) Walking direction of the crickets given by the number of dots that passed the detector (mean ± s.d.; positive and negative values indicate right and left turns, respectively) plotted versus stimulus orientation. Prior to averaging, data were standardized, i.e. the runs were phase-adjusted and corrected for an overall deviation from a straight walking path by subtraction of the inherent turning tendency. Note: a reduction in stimulus size to a diameter as low as 1° did not impair the polarotactic response.

View larger version (32K): [in this window] [in a new window]   Fig. 4. Haze experiment. (A) 180° fisheye view of the celestial hemisphere on a hazy morning. Compared with clear atmospheric conditions, the degree of polarization across the whole sky is reduced. (B,C) Polarotactic response as a function of the degree of polarization for a uniform stimulus. The effective degree of linear polarization (d) of a medium-sized (25°) zenithal stimulus was reduced from d=100% to 0% by changing the ellipticity of light (see Materials and methods). Test data (d=1% to 53%) are indicated by black, motivation controls (d=100%) by gray, and zero controls (d=0%) by white (24 series of 17 individuals). (B) Survey of results. Relative strength of the polarotactic response (S/Smot; mean ± s.d.) plotted against the effective degree of linear polarization. (C) Distribution of S-values and (D) modulation of walking direction with stimulus orientation for some of the polarization levels tested (see polarization ellipses to the left). Data in D are normalized and plotted as described in Fig. 3D. Note: a reduction in polarization to d=53% did not impair the polarotactic response. With lower d-levels, the response strength decreased. However, there was a statistically significant orientation to polarized light at least down to a d-level of 7% (P<0.01).

View larger version (10K): [in this window] [in a new window]   Fig. 5. Polarotactic behavior of an especially sensitive cricket under uniform stimuli of different degrees of polarization (d). (A–C) Rotational movements with d=100%, 3% and 0%, respectively (see polarization ellipses in the diagrams). Abscissa: walking direction (rotational component of the run) given by the number of dots on the ball that passed the detector; positive and negative values indicate right and left turns, respectively. Ordinate: stimulus orientation. Note that the periodic modulation of walking direction is almost as strong for 3% as for 100% polarization.

View larger version (22K): [in this window] [in a new window]   Fig. 6. Clouds experiment. (A) 180° fisheye view of the celestial hemisphere on a cloudy day. The mean degree of polarization is reduced since polarized light from patches of blue sky mixes with partly or totally unpolarized light from clouded sky regions. (B) Polarotactic response as a function of the mean degree of polarization () for a compound stimulus. For a wide (92°) zenithal stimulus, was reduced from 100% to 0% by changing the proportion of polarized to unpolarized light. Data are from 12 individuals. Tests are indicated by black symbols (=1% to 74%, N=17–19), motivation controls by gray symbols (=100%, N=162) and zero controls by white symbols (=0%, N=17). Note: a reduction in polarization to =49% did not impair the polarotactic response. With lower degrees of polarization, response values declined, but the orientation to polarized light was statistically significant at least down to a -level of 10% (P<0.01).

View larger version (6K): [in this window] [in a new window]   Fig. 7. Comparison between the Haze and Clouds experiments. The relative strength of the polarotactic response (S/Smot; mean ± s.d.) is plotted against the degree of linear polarization (d or ) for a uniform (black diamonds) and a compound stimulus (white squares). Note: the results are basically the same under both stimulus conditions.