Polarization Sensitivity in Butterflies

Butterflies can use reflected linearly polarized light from the wing surface of a conspecific to track down a mating partner or from the surface of leaves to find specific plants while foraging or ovipositing. During all these behavioural tasks, linearly polarized light is detected by the abundant polarization-sensitive photoreceptors in the butterfly’s main retina of the compound eyes. At the same time, these photoreceptors mediate other visual qualities, such as colour or brightness contrasts, which results in a polarization signal for contrast enhancement and intraspecific communication that is not disentangled from colour or brightness information. Our understanding of how non-celestial polarization signals are integrated with other visual sub-modalities is still speculative and requires a deeper investigation of the underlying neural substrate. In contrast, the neural substrate underlying the processing of celestial polarized light is rather well investigated in butterflies. Much of our understanding derives from research on the Monarch butterfly, Danaus plexippus, which uses the celestial polarization pattern as a navigational cue. To specifically extract the angle of polarization of skylight, the Monarch eye is equipped with a region termed the ‘dorsal rim area’ (DRA). In combination with other celestial cues, such as the sun, polarization information from the DRA is used to maintain a specific flight heading during long-distance migration. In this chapter, we summarize our current knowledge of the polarization detectors that allow butterflies to produce a variety of different behaviours based on polarization information. Thereby, we focus on the analysers for non-celestial polarization sensitivity (PS) in the main retina and how their information might be processed in the first integration centres in the butterfly’s optic lobe. In addition, we discuss how the ommatidia in the DRA are designed to detect the angle of polarization and how this information is processed in the brain to encode a heading direction during navigation. The earlier literature about butterfly polarization vision 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. pp. 61–111. In: Horváth G (editor) Polarized light and polarization vision in animal sciences. Springer: Heidelberg, Berlin, New York).