Polarized light detection in spiders

We describe here the detection of polarized light by the simple eyes of spi ders. Using behavioural, morphological, electrophysiological and optical st udies, we show that spiders have evolved two different mechanisms to resolv e the e-vector of light. Wolf spiders (Lycosidae), are able to turn in resp onse to rotation of a polarized pattern at the zenith of their visual field s, and we also describe a strip in the ventral retina of the principal (ant erio-median) eyes that views this location and has receptors tiered into tw o layers. This provides each pair of receptors with a similar optical solut ion to that provided by the 'dorsal rim area' of the insect compound eye. I n contrast, gnaphosid spiders have evolved a pair of lensless secondary eye s for the detection of polarized light. These two eyes, each sensitive to o rthogonal directions of polarization, are perfectly designed to integrate s ignals from the larger part of the sky and cooperate to analyse the polariz ation of light. Built-in polarizers help to improve signal purity. Similar organisation in the eyes of several other spider families suggests that the se two mechanisms are not restricted to only a few families.