science

Blue makes peacock spider sexy

Why would a physicist research spiders? Isn’t that a biologist’s job? Be that as it may, Doekele Stavenga was fascinated by the dapper little spider that stands no more than four millimetres tall. He wondered: where does this peacock spider get its colours?
By Simone Harmsen / Photo Reyer Boxem / Translation Sarah van Steenderen

Through a popular internet video, professor Stavenga encountered the peacock spider: a brightly coloured creature that – standing less than four millimetres tall – tries to seduce the ladies through vehement dancing.

Stavenga is an expert in the field of colour. He had previously researched the colours of birds of paradise, butterflies, and beetles. There, Stavenga discovered that they owe their bright colours to minuscule structures on their feathers and wings.

The peacock spider owes his blue colour to special physical structures. His red, cream, and white hues can be seen from any angle, but the colour of the unpigmented blue scale can only be seen from certain angles.

This is due to interference, which is formed when light hits wafer-thin layers of transparent material. Physicists call these ‘structural’ colours.

The peacock spider’s blue scale consists of thin layers of the transparent material chitin. The structural colour is formed when light hits it.

Scales like these had been previously discovered, but the peacock spider’s beautiful blue colour is caused by the extra layers of filament that cover the scale. These filaments are smaller than the wavelength of the light, which means they can manipulate the beams of light even further.

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His PhD students pointed professor Stavenga to a popular video featuring the peacock spider. It is a brightly coloured creature that stands less than four millimetres tall and attracts females by vigorously dancing. Stavenga: ‘PhD students sometimes go online looking for fun stuff. When they found this, they said: this is totally your thing!’

That is because Stavenga is an expert in the field of colour. Previously, he researched the colours of birds of paradise, butterflies, and beetles. Stavenga: ‘The peacock spider video fascinated me mainly because his dance was reminiscent of the courtship dance displayed by birds of paradise. He doesn’t just have great colours, but his accompanying behaviour is also really funny.’

In his previous research, Stavenga discovered that the bright colours in birds of paradise, butterflies, and beetles are formed by minuscule structures on their feathers and wings. When he saw the peacock spider, he suspected it might have something new to offer. One other type of spider was already known to have a purple colour thanks to special layers in its scales. He asked the person who made the video – an Australian colleague – to send him a few spiders.

Sexy blue

The peacock spider owes his blue colour to special physical structures. The red, cream and white colours are formed by pigmented scales covering the spider’s body. These scales shine with colour from every angle. But the blue scale is unpigmented, and its colour can only be seen from certain angles. Stavenga: ‘You can compare it to a soap bubble. A soap bubble has several colours, which change depending on which angle you view the bubble from.’ The peacock spider owes this to minuscule structures (‘filaments’) on the inside of its scales. These structures can manipulate light, turning the scale a shiny blue.

Stavenga has always been interested in biology, but after he studied physics, he started his career as a biophysicist. There are pictures of butterflies in the hallway, and his office has a cabinet with tropical butterflies. He takes the wings of an azure morpho butterfly out of a CD case and puts them under the microscope. ‘Plants use pigments to colour themselves blue, but in animals, it’s almost always the result of physical structures’, he explains. ‘That has everything to do with sex. Male animals have to be attractive to the females. Thanks to these structures, animals such as the peacock spider can produce a specific, beautiful type of blue that females find very attractive.’

Animal eyes

In addition to how colours are formed, Stavenga also focuses on the way animals see colour. For instance, the female peacock spider sees the male’s colours in even more spectacular ways than we do. Stavenga: ‘Spiders, just like almost all insects and birds, can see ultraviolet. We humans sadly cannot.’

When asked about the point of this research, Stavenga has a simple answer: ‘This type of fundamental research always gives rise to the question: ‘How can it be applied?’ But we’re just trying to understand how nature works. It’ll be nice if this research leads to any technical applications, but if it doesn’t, I won’t lose a second of sleep over it. I’m just really enjoying myself.’

The peacock spider’s ingenuity

The peacock spider’s blue colour is what physicists call a ‘structural colour’. Unlike a pigment colour, a structural colour is only visible from certain angles. This is due to interference, which is formed when light hits wafer-thin layers of transparent material.

When a light wave hits one of those layers from the air, the wave is fractured and bounced back. This happens both when the wave enters and leaves the material. The two light waves that are bounced back ‘interfere’ with each other: they either amplify each other or cancel each other out. Stavenga: ‘You can compare it to a piano. When two strings are badly tuned, they also interfere. You can hear the beat between the tones.’

The peacock spider’s blue scales consist of thin layers of the transparent material chitin. When light hits it, a structural colour is formed. These types of scales were discovered previously, but the peacock spider has an added bit of ingenuity that turns it that beautiful colour blue.

The inside of the scale is covered in a layer of filaments which most closely resemble strands of spaghetti straightened out and side by side. These filaments are smaller than the wavelength of the light, which means they can manipulate the beams of light even further. The peacock spider owes its beautiful blue colour to these special structures.

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