Many insects possess the ability to detect the directional component of light, what we term its polarisation properties. The dorsal rim area, in particular, is strongly sensitive to the direction of polarisation (the phase) of incoming light, and is thought to be used for navigational purposes.
The compound eye of an insect is made up of many ommatidia, which include a lens, cornea, and photoreceptor cells. Each ommatidium has its own 'preferred' orientation - this is the direction of polarisation which it responds most strongly to. By comparing signals coming from ommatidia with different preferred orientations, but which view the same area of the sky, the insect can create a 'map' of the polarisation properties of the incoming light.
When light originating from the sun hits our atmosphere, it results in a distinctive 'pattern' of different polarisation orientations and magnitudes. This pattern can be used as a sun-compass, giving us the ability to detect the position of the sun even on overcast days, or where the sun is invisible due to environmental features, eg. under a forest canopy. Other environment features, like water, or particular kinds of vegetation, can also be distinguished by their polarisation properties.
Insects have often been the inspiration for engineered solutions to common problems. They have evolved highly parsimonious yet sophisticated solutions to biomechanical problems, like long-distance navigation, flight stabilisation, or motility over highly variable terrain. We are developing lightweight visual sensors which mimic features of the insect compound eye, in order to understand and imitate their extraordinary capabilities.
Related downloads: some useful Matlab functions for analysing the polarisation properties of natural scenes.