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Wasps and flies have hidden rainbows in their wings

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claim that fly and wasp wing patterns are no match for the incredible diversity of colourful butterfly wing patterns is obsolete,” she says.

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.

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.

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.

Reference: PNAS http://dx.doi.org/10.1073/pnas.1017393108

More on animals doing cool things with colours:

If the citation link isn’t working, read why here

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12 thoughts on “Wasps and flies have hidden rainbows in their wings

  1. Funny, i’ve seen this after examining the swatted flies and wasps and whatnot. Kinda figured it’s known already.
    But not in such detail as the photos though, so well done there.

  2. Hello Ed,
    I have read in one of your excellent posts (can´t remmember which, thought) that you are not very interested in human evolution and that’s the reason why you do not post a lot about this matter. I am really interested in that subject, so, since I trust your advise, I would like to ask you, please, to recommend to me one blog as scientifically serious and well written as yours (english is not my first language as I am sure you have already noticed) but specifically dedicated to prehistory or that pays a lot of attention to it.

    Thank you very much

    [You should read Carl Zimmer’s The Loom, Brian Switek’s Laelaps, or anything that Kate Wong writes at Scientific American. – Ed]

  3. On fig wasps, in one of his books (can’t recall which one) Dawkins has a long and complicated explanation of male fig wasp competition, often involving deadly fights inside the fig. I don’t remember him mentioning the female breaking off her wings, though.
    Every time I eat a fig now, I have to try not to think of a wasp possibly being inside it.

    The female wings visible at the entrance to the fig would presumably be an invitation to the males to come and try their luck.

    Even though no pigmentation is involved, I imagine that Hox genes still have a role in constructing the wings and thus creating the colours, by influencing the thickness at various points.

  4. Ed: The slide show doesn’t show up when browsing using Google Reader. It only shows up when you click on the link to open it up in a separate tab. I was confused at first too.

  5. I wonder how huge the differences between the WIPs are of live and well dried animals (pressure in the veins influence the wing thickness..). I also wonder how the patterns are influenced by the bending/torsion of the wings in flight. There’s the potential of the final message of god being displayed on the wings of a fly during escape from a towel-swatting.

  6. The slideshow thing, which no doubt is very snazzy if you can see it, also doesn’t show up if you’re blocking Javascript by default. (or switch it off entirely)

    (The phrase you want to mention to the tech guys is “progressive enhancement”.) :)

  7. Maurizio Morabito: Since “every hair, bump, ridge and vein affects the pattern of the WIPs”, in what way are they “unique to different species”? (question)

    I think the unique or species specific WIP is formed by a combination of all microstructure features. It is like the whole complex of things : arrangement of veins, hair placement, pigments if any and microstructures of the very wing membrane including thickness profile. And all these things are not about to change (unless artificially) during life of a fully developed insect as far as we know for now. So species specific WIP is caused by the whole wing morphology which is reasonably similar in all individuals within a species (a certain level of variation is perfectly normal), and by changing (evolving differences) some of the parametres of the wing one will get a different overall pattern.

  8. Oh, fascinating post, by the way. Almost forgot!

    The whole idea that you can get all these colors from constructive / destructive interference, whether it is in wings or oil films, is one of those totally unexpected revelations that I wish I could go back and tell my younger self, it’s just so damn cool.

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