In this series of articles in the Village NEWS, I try to describe interesting facts about flora and fauna of the Western Cape. However, in the time of COVID-19 we need a little colour in our lives, and hence have borrowed some cuties from Australia.
Adorable, colourful, fascinating – all words you wouldn’t expect to be linked to spiders. But yes, though these spiders have been around for so long, their tiny stature has made them unnoticed until recently, when they were captured on film executing a remarkable little dance to impress the ladies. They are called peacock spiders.
So far these brightly-coloured spiders (Genus Maratus) have only been found in Australia. They captivate even the most arachnophobic person. The male spiders’ flamboyant courtship displays feature technicolour bodies and dance-moves that are the envy of any ‘animé’ choreographer (search https://www.youtube.com/watch?v=d_yYC5r8xMI.)
You won’t easily see peacock spiders in nature. These miniature spiders measure less than 5mm in size – the size of a grain of rice. To the naked eye they look like little brown hopping grains of sand. Yes, they are jumping spiders, formally classified in the family Salticidae. ‘Salties’ don’t spin webs. They catch their prey by jumping on them. They are fierce and feisty for their size and can prey on insects (and other spiders) three or four times their size.
Jürgen Otto (peacockspiderman)
It was only in the last decade or so with the advent of macro photography that peacock spiders’ super cuteness and iridescent colours have fascinated the media. If one person could be dubbed the ‘father’ of peacock spider photography it is Jürgen Otto, a Sydney-based biologist working on agricultural mites for the Australian Department of Agriculture.
Mites are even smaller than peacock spiders, so the latter were relatively easy to spot in the course of his work. With a passion for macro photography, Jürgen soon spent every spare hour photographing these incredibly colourful spiders. And, in the age of social media, they soon became an internet sensation.
Jürgen was responsible for describing most of the 85 known species of peacock spiders, but other scientists are now also contributing to the increasing list of new species.
Joseph Schubert, a young scientist working at Museums Victoria, recently identified seven new species. He described them as “tiny little furry kittens, each with their own personality”. Since these spiders became famous, many citizen-scientists are sending their observations to experts like Joseph for identification. He named his personal favourite Maratus constellatus because it reminded him of van Gogh’s ‘Starry Night’.
How are the rainbow colours produced?
As is the case with bird feathers and butterfly wings, the colours are produced by iridescence – in other words, different wavelengths of light are reflected from microscopic structures.
One species of peacock spider – the rainbow peacock spider (Maratus robinsoni) – is particularly impressive. It has an intense rainbow iridescent signal in the males’ courtship displays to the females.
To figure out just how the spiders produce their incredible shimmer, a team of biologists, physicists and engineers came together to study the scales on the spider’s abdomen that produce the impressive colour. Using techniques like electron and light microscopy, imaging scatterometry and optical modelling, the team created micro-3D models of the scales to test how they worked.
They found that the rainbow colour is produced by a specialised scale shaped like an airplane wing. Parallel ridges on top of the scale act as tiny diffraction gratings, able to divide visible light into its component colours. A slight curvature of the scale allows light to pass over more ridges, separating the light into the colours of the rainbow even more effectively than if the scales were flat.
Dr Radwanul Hasan Siddique, co-author of this research, says: “As an engineer, what I found fascinating about these spiders’ structural colours is how these long-evolved complex structures can still outperform human engineering… Even with high-end fabrication techniques, we could not replicate the exact structures. I wonder how the spiders assemble these fancy structural patterns in the first place!”
Why did peacock spiders evolve such diversity of shape and colours? Most females mate with a single male, and thus they need to choose their partner very wisely. There is something about colourful, elaborate patterns that appeals to a female. If a male does not measure up to her high standards he will most likely get eaten.
Traits that evolve because they help an organism to be chosen as a mate, and thus a better chance to reproduce, is called sexual selection. This is different from natural selection, where a trait evolves because it helps an organism to survive. Natural and sexual selection are often in conflict: perhaps a colourful peacock spider has a better chance of seducing a female, but it is also more visible to its predators.
A male peacock spider would apparently rather die than be rejected by a female.
For more fascinating information, go to: https://www.youtube.com/watch?v=PQbScg3r1oQ