Sky compass signaling of central-complex neurons in locusts exposed to the natural sky

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Many insects, like vertebrates, can exploit celestial cues for the coding of compass orientation. Studies in honeybees, desert ants, beetles, and monarch butterflies showed that the position of the sun, the spectral gradient across the sky, and the polarization pattern of the blue sky can serve as compass cues. Analysis of polarization-sensitive neurons, especially in the desert locust, but also in beetles, the monarch butterfly, and the field cricket, showed that the central complex in the brain plays a key role in sky compass orientation. Unfortunately, the complex pattern of sky polarization, as well as the chromatic and intensity gradient in the sky cannot be mimicked faithfully in the laboratory. Therefore, we will study the activity of polarization-sensitive neurons of the central complex under the open sky. Toward this goal we will perform extracellular recordings via wire electrodes implanted in the brain of desert locusts. Activity of a neuron will be monitored first in the laboratory during rotation of the animal below stationary polarized light provided from the zenith and a dorso-lateral light source mimicking the sun. The animal will then be transferred to an observation platform on the rooftop of the institute. Under the natural sky the activity of the same neuron will be studied again during rotation of the animal around its vertical axis. Various manipulations such as painting over parts of the eyes, covering the eyes with a diffuser or studying neural activity before and after sunset will address the question of which parameters of the sky and which eye regions contribute to neural compass coding. Finally, the contribution of landmarks to direction coding in the central complex will be addressed. The study is the first to investigate sky compass neurons under natural conditions in any animal and will allow us to critically evaluate the validity of data obtained in laboratory settings.