Polarization Sensitivity in Spiders and Scorpions

Abstract

Finding the way back to their web or burrow or shelter, or orienting within the web are complex tasks for which spiders also rely on their visual system. Spiders have two sets of simple eyes, a pair of anterior-median principal eyes directed forward, and three pairs of secondary eyes usually with a reflecting tapetum lining the back of the eye (Fig. 24.1A). These eyes are specialized for different tasks. All spider eyes possess microvillar photoreceptors, in certain species with orthogonally arranged microvilli (e.g. Schroer 1974; Dacke et al. 1999). Kovoor et al. (1993) studied the anatomy of the anterior median eyes and its possible relation to polarization sensitivity (PS) in Lycosa tarentula. They suggested that PS in Lycosa tarentula is mediated by the ventral part of the retina, where the photoreceptors bear rhabdomeres aligned in parallel series and successive lines of rhabdoms are orthogonal to each other. They also hypothesized that the analysis of polarization may be a successive process using a twisting of the retina due to the action of two muscles: the alternating contraction of these muscles can generate rotation and, to some extent, up and down movements of the retinal cup. The E-vector analysis by such a successive mechanism in spiders was first proposed by Schroer (1974) for Agelena gracilens.