Fig. 5. The insect’s polarization channel for the e-vector compass. Hypothetical scheme based on neurophysiological data. (A) The e-vector pattern in the sky. The orientation and size of the blue bars indicate the angle and degree of polarization, respectively. 0°, azimuthal position of the sun; open disc, zenith. In the particular case shown here, the elevation of the sun (yellow disc) is 60°. (B) Array of polarization detectors (L and R, left and right visual field, respectively). The e-vector tuning axes of only a few of the total of 55–75 polarization (POL) detectors per eye (in Cataglyphis bicolor) are shown. Each detector consists of a pair of orthogonally arranged analyzers (photoreceptors), which interact antagonistically. The dashed line depicts the animal’s longitudinal body axis. To simplify matters, the array of detectors shown here is symmetrical with respect not only to the animal’s longitudinal but also to its transverse (L–R) body axis and, thus, introduces a 180° ambiguity in the compass responses. The latter symmetry does not hold in the animal. (C) Response ratios of three large-field POL neurons. The response ratios are schematically translated into false colours. If the animal rotates relative to the skylight pattern (see filled arrow in B), different false colours show up (see white arrow in C). (D) Circular array of hypothetical compass neurons. Each fine-tuned compass neuron encodes a particular response ratio of the broadly tuned POL neurons. The compass neuron marked by the filled red circle is maximally excited when the animal faces the solar azimuth. (Owing to the 180° ambiguity mentioned above, in this artificial case the 180° circle should be coloured red, too).