Photoreceptor design and optical properties affecting polarization sensitivity in ants and crickets

Abstract

Anatomically and physiologically specialized ommatidia at the dorsal rim of insect compound eyes play a key role in polarization vision. In this eye region the polarization sensitivity (PS) of photoreceptors is significantly higher than elsewhere in the eye. We have compared the optical properties of the dorsal rim area and normal eye region of desert ants,Cataglyphis bicolor, and field crickets,Gryllus campestris. The objective was to find the cause of the enhanced PS in the dorsal rim area of eyes where the situation is not complicated by rhabdom twist. Three pieces of information were derived: 1. From qualitative optical experiments we conclude that rhabdom waveguide effects do not enhance or reduce PS. Quantitative measurements of depolarization in the retina demonstrate unambiguously that all parts of the eye retain the state of polarization very well through the full retinal depth. The factor limiting inherent PS of receptor cells must thus be the dichroic absorption of the rhabdomeres, which is determined by the dichroic ratio of microvilli and the degree of microvillar alignment. 2. A theoretical model of light propagating in a dichroic rhabdom reveals a strong influence of random microvillar misalignment on PS. Using measured values of misalignment, we predict differences in PS between dorsal rim and unspecialized retina that explain previous electrophysiological results. 3. Theoretical modelling also demonstrates the advantage of having a predominant microvillar direction in a rhabdom (as is the case in theCataglyphis dorsal rim).