Limitations to resolution in superposition eyes

Limitations to spatial resolution in the ventral refracting superposition eye of the crepuscular dung beetle, Onitis alexis, were investigated using intracellular electrophysiological recordings and optical modelling. The measured angular-sensitivity functions for Onitis are narrow compared to many superposition eyes, but display considerable off-axis flanks, or flare. These flanks are reduced as the eye becomes light adapted, but the acceptance angle remains constant at about 4 degrees, both during the day and at night. At night, in the dark-adapted state, primary screening pigment is retracted distally to expose the full diameter of the crystalline cone tip. As the eye light adapts, the pigment migrates proximally around the crystalline-cone tip until, if the adapting light is sufficiently intense, the tip is completely occluded. During the daytime, regardless of the state of adaptation, the diameter of the area of crystalline-cone tip exposed never exceeds 5 micrometer, indicating the presence of a circadian rhythm in pigment position. Resolution in the superposition eye of Onitis alexis is not limited by diffraction, but most likely by spherical aberration of the eye and the inability to contain rays in the target rhabdom (ray cross-over). Spatial convolution models of angular sensitivity do not account for this ray cross-over. A model is developed which does, and which provides good agreement with the experimental angular-sensitivity functions. The model predicts no change in acceptance angle with the state of adaptation, as found experimentally, but does not predict the reduction in the flanks of the angular-sensitivity, function as the eye becomes light adapted. Limitations to resolution in Onitis are compared with those in the nocturnal/diurnal superposition eve of the Christmas beetle Anoplognathus pallidicollis and the strictly diurnal superposition eye of the day moth Phalaenoides tristifica. The nocturnal eye of Anoplognathus oversamples the retinal image, whilst the eye of Phalaenoides undersamples. In Onitis the spacing of the photoreceptors in the retina appears to be matched to the quality of the retinal image.