1) POL-neurons in the optic lobe

In the cricket Gryllus campestris several morphological classes of POL-neuron have been described. The data presented here refer to the best-studied POL1 class of neurons.

For more information see references 5,7.

E-vector response: When a POL-neuron is stimulated with light passed through a rotating polarizer, the spike frequency depends on the actual e-vector orientation (Fig. 2). Spike activity is a sinusoidal function of e-vector orientation with alternating portions of excitation and inhibition and with the maxima and the minima separated by 90° (Fig. 3). Thus, POL-neurons are polarization-opponent neurons receiving input from two analyzer channels with orthogonal orientations of maximal sensitivity. The two analyzer channels are represented by the two sets of photoreceptors with orthogonally-arranged microvilli that are present in each dorsal rim ommatidium (see Fig. 1) . The polarization-antagonism makes the POL-neurons insensitive against changes in light intensity and enhances polarization sensitivity.

For more information see references 2, 3, 5, 7.

E-vector tuning: There are three types of POL-neuron that exhibit maximal spike rates at different e-vector orientations, i.e. about 10°, 60° and 130° relative to the long axis of the head (Fig. 4). Each of these tuning types receives input from a large number of dorsal rim ommatidia of appropriate orientations (see Fig. 1) . The three signals from the three POL-neuron types may code the insect's orientation with respect to the celestial polarization pattern (see Models and robots).

For more information see references 6, 7.

Visual fields: The visual fields of the POL-neurons are > 60° wide and are directed to the upper part of the sky (Fig. 5). The large size of the visual fields results from (1) optical integration due to the large acceptance angles of the dorsal rim ommatidia (see Sensory input to insect POL-neurons) and (2) neural integration, since each POL-neuron receives input from a large number of dorsal rim ommatidia. Integrating polarization over a large area of the sky makes the POL-neurons insensitive to local obstructions of the polarization pattern due to clouds or tree canopies.

For more information see references 1, 4, 6, 7.

Morphological properties of cricket POL-neurons

In the POL1 neuron, a dense array of dendritic branches occupies the dorsalmost part of the medulla, where it receives sensory input from the POL-area of the eye (Fig. 6). The photoreceptor axons probably do not contact the POL-neurons directly but via yet unknown interneurons. The axon runs all the way through the brain and branches in the contralateral medulla. This indicates that the e-vector detection systems of both sides interact which each other.

For more information see references 6, 7.

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