Wasps and flies have hidden rainbows in their wings

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The wing of a fruit fly, viewed against a white background, looks very ordinary. It is transparent, with no obvious colours except for some small brownish spots. But looks can be deceptive. If you put the wing in front of a black background, it suddenly explodes in a kaleidoscope of colour. Oranges, blues, greens, violets – virtually the entire rainbow dances across the wing, except for red.

A French scientist called Claude Charles Goureau first noticed these vivid hues back in 1843. Since then, they have languished in obscurity, “apparently unnoticed by contemporary biologists”. Whenever new species of wasps or flies are described, their discoverers almost never mention the coloured patterns of the wings. The visible pigments have even been described as “evolution in black and white”. It’s like walking through an art gallery with a blindfold.

Now, Ekaterina Shevtsova from Lund University has taken off the blind. By photographing several species against dark backgrounds, she has revealed a world of hidden colour, rivalling that of more obviously beautiful insects. “The claim that fly and wasp wing patterns are no match for the incredible diversity of colourful butterfly wing patterns is obsolete,” she says.

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Shevtsova found that different species have their own unique patterns of swirls, spots and stripes, all shining with resplendent colour. The insects probably use their wings as billboards to communicate with one another and to distinguish between different species. Insect scientists can use the patterns for the same purpose, separating species that otherwise look identical.

Shevtsova has also worked out how the patterns are created. They are “structural colours”. They exist not because of any pigment, but because of the wing’s microscopic structures. When light hits the wing, most of it passes straight through but 20% bounces back. Some reflects off the top layer, and the rest reflects off the bottom layer after passing through the thin membrane.

The two reflected beams reinforce one another to produce a strong vivid colour. The hue that you see depends on how thick the wing is at a given point. By varying the thickness of the membrane, the insect can reflect a rainbow from its wings. This phenomenon, known as “thin film interference”, also produces the colourful patterns of oil on water.

If the wing lies on top of a white surface, the colours are completely swamped by the white light reflecting off the background. This explains why the colours are seldom mentioned – insect scientists typically study their specimens against a white board. You can only appreciate their true majesty against a coloured background. Pitch black is best because it absorbs any light that makes it through the wing, leaving only the coloured reflections visible. Green will also do – after all, that’s the colour that insects most commonly find themselves in front of.

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Shevtsova calls these colours “wing interference patterns”, or WIPs, and she thinks that they’re signals aimed at other insects. For a start, these insects can’t see red and the only colour that’s not reflected by the wings just happens to be pure red. Males and females often have different patterns too, which suggests that the WIPs could be shaped by the insects’ sexual preferences.

The WIPs are also unique to different species, a fact that could be a boon to insect scientists. Small wasps and flies are hard to examine and different species often look identical. Telling them apart is a tough job and scientists need a meticulous physical check-up, a behavioural clue, or a genetic analysis. But Shevtsova thinks they could just use WIPs instead.

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By studying these patterns, she managed to tell the difference between five species of wasp, three of which were new to science. At first glance, they could all be mistaken for each other, but their colourful wing patterns gave them away.

Best of all, the patterns stand the test of time. With a black backdrop, you can still see them shining from museum specimens that have been kept in drawers for 100 years. While pigments may fade, these WIPs endure because they’re produced by the very shape of the wing. And unlike the iridescent colours of many birds and butterflies, which change depending on the angle you’re looking from, the WIPs look the same from all directions.

The WIPs cast the behaviour and evolution of insects in a new light. Flies often use their wings to signal to each other during courtships, using movements that are probably accompanied by brilliant flashes of colour. As another example, female fig wasps hold their wings upright when they arrive on a fig. As they squeeze through a narrow entrance in the fruit, the wings break off and the wasp sticks them into place with a tiny drop of glue. Viewed with WIPs in mind, the wings could act as a colourful “Do Not Disturb” sign, telling other wasps that the fruit now has a tenant.

Since the colours come from the microscopic shape of the wing, every hair, bump, ridge and vein affects the pattern of the WIPs. If an insect evolves a larger, thicker wing, its colours change. If the wing gains or loses veins, the colours change. Indeed, the unseen influence of these colours could explain why the veins of some insect wings are incredibly varied, for no obvious reason. Through these changing colours, the evolution of flies and wasps may start becoming more transparent.

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