BBC News 30th September 1999
Spiders light the way
If you've ever wondered how spiders find their way into your bathtub, Swedish researchers think they have found the answer.Marie Dacke and colleagues, of the University of Lund, have discovered that at least one species of spider, Drassodes cupreus, has a compass with which to navigate.
The compass is actually a secondary pair of eyes that sit just behind the arachnid's principal eyes on the cephalothorax, the combined head and thorax body segment.
D. cupreus spiders, which are found in many parts of Northern Europe, use their specialised secondary eyes to analyse the polarisation of light in the sky.
These eyes do not form images - they have no lens with which to focus the light - but use a unique built-in filter to determine precisely the direction in which the light falling upon them is polarised.
Dusk and dawn
Because the orientation of light in the sky changes according to the position of the Sun, the eyes effectively provide bearings for navigation.
The researchers have shown that this "compass organ" works best at dusk and dawn, and that spiders of this species are most active after sunset when they rely on their interpretations of polarised light to guide them back to the nest after foraging trips.
The team actually set up a series of tests in which spiders had to find their way around an arena containing several nests. Under normal conditions, the spiders had little difficulty finding their way home after looking for food. But when the test spiders had their secondary eyes covered up very few could find their way back to the nest.
Evidence of what appear to be similar organs in several other spider species indicates that D. cupreus may not be the only arachnid with an integral compass.
The ability to use polarised light for navigation has been well
demonstrated in other creatures such as bees and ants.