Detectors for polarized skylight in insects: a survey of ommatidial specializations in the dorsal rim area of the compound eye. 
Apart from the sun, the polarization pattern of the sky offers insects a reference for visual compass orientation.  Using behavioral experiments, it has been shown in a few insect species (field crickets, honey bees, desert ants, and house flies) that the detection of the oscillation plane of polarized skylight is mediated exclusively by a group of specialized ommatidia situated at the dorsal rim of the compound eye (dorsal rim area).  The dorsal rim ommatidia of these species share a number physiological properties that make them especially suitable for polarization vision: each ommatidium contains two sets of homochromatic, strongly polarization-sensitive photoreceptors with orthogonally-arranged analyzer orientations.  The physiological specialization of the dorsal rim area goes along with characteristic changes in ommatidial structure, providing actual anatomical hallmarks of polarized skylight detection, that are readily detectable in histological sections of compound eyes.  The presence of anatomically specialized dorsal rim ommatidia in many other insect species belonging to a wide range of different orders indicates that polarized skylight detection is a common visual function in insects.  However, fine-structural disparities in the design of dorsal rim ommatidia of different insect groups indicate that polarization vision arose polyphyletically in the insects.