Spectral organization of the compound eye of a migrating nymphalid, the Chestnut tiger butterfly, Parantica sita

Abstract

Several butterflies of family Nymphalidae perform long-distance migration. Orientation during migration has been extensively studied in the monarch, Danaus plexippus. Extensive studies on D. plexippus have revealed that vision plays a crucial role in orientation during migration. Differences in the migratory patterns of butterflies suggest that not all species are exposed to the same visual conditions and yet, little is known about the visual system across migratory species. Here we used intracellular electrophysiology, dye injection and electron microscopy to assess the spectral and polarization properties of the photoreceptors of a migrating nymphalid, Parantica sita. Our findings reveal three spectral classes of photoreceptors including ultraviolet, blue and green receptors. The green receptor class contains three subclasses, which are broad, narrow, and double-peaking green receptors. Ultraviolet and blue receptors are sensitive to polarized light parallel to the dorso-ventral axis of the animal, while the variety of green receptors are sensitive to light polarized at 45°, 90° and 135° away from the dorso-ventral axis. The polarization sensitivity ratio is constant across spectral receptor classes at around 1.8. Although P. sita has a typical nymphalid eye with three classes of spectral receptors, subtle differences exist among the eye of different nymphalids, which may be genus-specific.