Many migrating animals employ a celestial compass mechanism for spatial navigation. Behavioral experimentsin bees and ants have shown that sun compass navigation may rely on the spectral gradient in the sky as well as onthe pattern of sky polarization. While polarized-light sensitive interneurons (POL neurons) have been identifiedin the brain of several insect species, there are at present no data on the neural basis of coding the spectral gradientof the sky. In the present study we have analyzed the chromatic properties of two identified POL neurons in thebrain of the desert locust. Both neurons, termed TuTu1 and LoTu1, arborize in the anterior optic tubercle andrespond to unpolarized light as well as to polarized light. We show here that the polarized-light response of both types of neuron relies on blue-sensitive photoreceptors. Responses to unpolarized light depended on stimulus position and wavelength. Dorsal unpolarized blue light inhibited the neurons, while stimulation from the ipsilateral side resulted in opponent responses to UV light and green light. While LoTu1 was inhibited by UV light and was excited by green light, one subtype of TuTu1 was excited by UV and inhibited by green light. In LoTu1 the sensitivity to polarized light was at least 2 log units higher than the response to unpolarized light stimuli. Taken together, the spatial and chromatic properties of the neurons may be suited to signal azimuthal directions based on a combination of the spectral gradient and thepolarization pattern of the sky.

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