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An eight-legged boy scout

Spiders have a special sense organ which acts as their inbuilt compass, reports Aisling Irwin

THEIR eyes locked across a cluttered laboratory bench - the eyes of Swedish PhD student Marie Dacke, and the vivid, unnatural blue eyes of the Blackwall spider.

All seeing eyes: scientists have discovered that some spiders use a secondary eye to concentrate polarised light signals to pinpoint their whereabouts

It was only an ordinary grey-brown northern European arachnid, Drassodes cupreus. But its eyes shone vividly back at her, recalling those of a cat in the dark.

"The eyes were fairly strange," says Ms Dacke, who works at the University of Lund in Sweden. Though she is not a spider-lover, she decided to investigate the mystery.

The result, published in the journal Nature, was the discovery of a new sense organ, an inbuilt compass which the spider uses for navigation on its foraging trips.

The eyes in question are the spider's secondary eyes, shaped in rough ovals and lying close together, at right angles to each other, on the upper surface of the cephalothorax - its front end. They are positioned just behind the principal eyes.

Ms Dacke and her supervisor were studying the spider in an attempt to understand its miraculous powers of navigation: how it manages to wander around the open habitats where it lives, foraging for food, and yet return and correctly identify its own silk nest, spun under one of a multitude of rocks. The first thing that fascinated them about the eyes was that neither had a lens, although a lens is essential for focusing light and therefore for seeing.

"We looked right into the eye and saw the retina," says Ms Dracke. "It puzzled us."

So they sliced the eyes into tiny pieces and encased them in plastic for viewing through the microscope. Under magnification, the eyes displayed layers of ordinary photoreceptors. Behind them, lining the eye, was a reflective layer known as a tapetum.

This is a piece of wizardry because of its response to polarised light. It not only reflects it but polarises it further, concentrating the signal received by the photoreceptors. These eyes are organs extremely sensitive to polarised light.

This polarising capability is what makes the tapetum the vivid blue which startled Ms Dacke. The Blackwall spider is the first creature - as far as the scientists know - with optics that can polarise light.

Ordinary light possesses electric and magnetic fields which vibrate in all directions perpendicular to the direction in which the ray of light is travelling. But if such light strikes a polished surface, or passes through a Polaroid filter, some of these vibrations are filtered out, and the vibrations become confined to one plane - the light is said to be polarised.

Particles in the atmosphere can exert the same effect on sunlight as can polished surfaces - some of our daylight is polarised. The light which is most affected is at 90 degrees to the sun.

If you were lucky enough to have an eye which could detect polarised light, you might be able to work out your position relative to the sun by shifting around to see how the intensity of the signal varies.

The Blackwall spider's secondary eyes turn out to be adapted for just such a purpose. Each takes the form of a long, V-shaped groove, best able to detect the polarised light which hits it along its longitudinal axis.

Because the two eyes are at right angles to each other, the spider can work out, from the relative stimulation of each eye, its orientation. For example, if both eyes are stimulated equally along their long axes, the spider knows that the polarised light is hitting both these axes at the same angle - and can tell the direction from which the light is coming. So, polarised light travels into the eye, through a layer of photoreceptors and on until it hits the tapetum. There, its polarity is boosted and it is reflected back where it is detected by the photoreceptors for a second time before it travels back out of the eye.

'When you look at the spider the eye kind of shines back at you," says Ms Dacke. To explore their functioning further, the scientists built a model eye with which they measured the strength of polarisation signals from clear skies at day and night. They realised that these little compasses must perform best at dawn and dusk. At this time the light in the sky is polarised in one direction.

"Sky light is polarised tangentially to the sun and when the sun is high above the horizon, a wide-field integrating analyser would receive a mixture of different planes of polarisation," the team writes in Nature.

"This, and the input of direct unpolarised sunlight, would tend to obscure the polarisation signal.

The team wanted to test directly whether the secondary eyes are essential for spider navigation. So they placed the spiders in a large circular "arena", with four shelters arranged symmetrically around them. Above them they rigged up apparatus which shone polarised light down on to the spiders.

Of 10 spiders that made a nest under shelter during the following 24 hours, nine returned to them after the first foraging trip and seven of those managed to return without passing through any of the other shelters.

Then Ms Dacke took out her paintbrush and carefully daubed black paint over the secondary eyes of another 10 spiders. This time, only three of them returned to their original shelter. She also tested unpainted spiders who were bathed only in unpolarised light - they performed no better.

Many species of spider possess secondary eyes. The scientists now believe that they, too, may navigate with such an inbuilt compass, making spiders the boy scouts of the animal world. Bees and ants navigate in a similar way, although they do not, as far as the scientists know, have the ability to polarise light.

As for Ms Dacke, the Blackwall has not won her heart, despite the brilliance of the spider eyes: "I would like to say that I have fallen in love with spiders but I have not."

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