Polarization Sensitivity in Desert Locusts, Schistocerca gregaria

Desert locusts are long-range seasonally migrating insects that rely on celestial compass signals for spatial orientation and navigation. The processing of sky polarization signals in the compound eye and brain of the locust has been studied in detail. Photoreceptors in the dorsal rim area (DRA) of the eye are highly specialized in the perception of the angle of polarization (AoP) of skylight. Signals from both eyes converge in a midline brain area, the central complex (CX). Neurons of the central complex encode the position of the sun by matched-filter coding of the polarization pattern across the entire sky, are sensitive to the extremely weak degrees of linear polarization corresponding to a fully overcast sky, and combine signals from the celestial polarization pattern with those from direct sunlight as a coherent compass signal. The azimuthal space around the insect is encoded by columnar neurons of the central complex in a topographic compass-like manner. The strength of neural responses to linearly polarized light depends on the sensory and behavioural context. Modulatory inputs, together with internal network properties, ensure shifts of activity peaks within the central complex during turns of the locust. In addition, the low polarization sensitivity (PS) of photoreceptors of the main eye might, likewise, contribute to navigational decisions. Polarization signals from the main eye are processed in brain areas that are different from sky compass pathways and include rather direct contacts to descending pathways as well as neurons of the central complex that are not directly involved in sky compass coding. The data suggest extensive networks around and within the central complex using polarization signals for navigational and goal-directed decision-making. The earlier literature about the polarization vision of desert locusts has been partly reviewed by Horváth and Varjú (Polarized light in animal vision – polarization patterns in nature. Springer, Heidelberg/Berlin/New York, 2004) and Heinze (Chapter 4: Polarized-light processing in insect brains: recent insights from the desert locust, the Monarch butterfly, the cricket, and the fruit fly. In: Horváth G (ed) Polarized light and polarization vision in animal sciences. Springer, Heidelberg/Berlin/New York, pp 61–112, 2014).