# A Computer Vision Milky Way Compass
### Insect Vision under Starlight  
The method we present is based on observed insect behaviour; thus, it is important to understand what they can actually sense and the limitations of their sensing, since their visual perception is substantially different from our own. Insects have evolved compound eyes that have extraordinary adaptations and specialisations [6], and the tiny optics and eyes of insects are challenged by low light levels. There are two main types of compound eyes, as shown in Figure 3, which are exposition eyes and superposition eyes. In an exposition eye, each ommatidium is sleeved by light-absorbing screening pigment, preventing light reaching the photoreceptors from neighbouring ommatidia. This means that a photoreceptor only receives light from a single lens. Apposition compound eyes are typical of diurnal insects that have evolved to achieve the highest possible angular resolution, such as dragonflies [7] and honey bees [8]. In superposition compound eyes, however, there is a wide optically transparent region, the clear zone (marked cz in Figure 3), between the lenses and the retina. The superposition arrangement allows light from possibly hundreds of lenses to be focused onto photoreceptors in the retina, which improves light sensitivity at the expense of angular resolution. This eye type is more suited to nocturnal insects that are active in dim light [2;9]; such insects include night-active beetles and moths. It is apparent that at the lowest level of illumination, substantial pooling is usually being undertaken by the visual system of night-active insects both through optical means and through neural/computational interconnections [10]. Spatial pooling ensures that the effective angular resolution of the eye will be lower than that indicated by the density of the optical elements. Given the already limited resolution of insect eyes compared to our own experience, the biological findings indicate that the MW may be a useful cue even with comparatively low-resolution vision systems. Appendix A discusses the extreme limits of low light vision that have been found in insect visual systems, allowing vision well below starlight conditions, as shown in Appendix A Figure A2. Daytime celestial navigation has been observed and studied extensively in many species. In some insect species such as dragonflies, it has been found anatomically that there is polarisation sensitivity of the dorsal rim area of their compound eyes, suggesting that they are sensitive to the orientation of skylight polarization [12;13]. This characteristic allows them to perceive a clear blue sky as a large polarisation pattern aligned to the angular position of the sun. This perception is useful to a flying insect, since otherwise the sky might provide negligible heading information, either absolute or relative, particularly if the sun is near apex, near the horizon, or obscured by clouds. A comparison between the characteristics of the MW and the sky polarisation pattern can be made. Although the brightness level is quite different, they are both large, low contrast structures that dominate the sky if they are visible. Figure 3: Simplified depiction of apposition (Left) and superposition (Right) compound eyes. The clear zone (cz) found in superposition eyes is labelled. Illustration adapted from [11]. Moreover, it has been found that the desert ant _Cataglyplis_[14] and the field cricket _Gryllus campestris_[15] have the ability to use celestial cues to maintain their heading direction while walking, even when their locomotion is disturbed about the yaw axis. Compared with daytime movement, it is a challenging task to establish and maintain an inertial or celestial frame heading at right. A moonless clear night sky is much dimmer than full daylight, with a change in light intensity from 0.0001 lux under such conditions to 10,000 lux in daylight [16]. A nocturnal dung beetle in Southern Africa, _Scarabaens satyrus_, has been found to orient in very straight lines, transporting its dung balls away from competitors.