Some insects and vertebrates use the pattern of polarized light in the sky
as an optical compass(1-5). Only a small section of clear sky needs to be v
isible for bees and ants to obtain a compass bearing for accurate navigatio
n(5,6). The receptors involved in the polarization compass are confined to
a small part of the retina, and the eyes are built predominantly for other
visual tasks(7). Here we report the discovery of a unique compass organ in
the spider Drassodes cupreus, where a pair of specialized secondary eyes co
operate to analyse skylight polarization. These eyes do not form images, bu
t use a built-in polarization filter to determine precisely the direction o
f polarization. Measurements using a model eye indicate that the compass or
gan is best suited for navigation at dusk and dawn. Behavioural experiments
show that the spiders are primarily active after sunset and that they use
polarization cues to find their way back to the nest after foraging trips.
A similar organization of the secondary eyes in several spider families ind
icates that such compass organs may not be an isolated phenomenon.