Why do some insects look metallic? © Getty Images

Why do some insects look metallic?

The shiny and colourful skin on some bugs does not come from pigment molecules but ridges, which direct light to create the effect.

Asked by: Rebecca Walker 

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Many insects, particularly scarab and jewel beetles, have vivid, metallic green, blue or gold colouration. This effect doesn’t come from pigments, but is an example of ‘structural colouration’.  

Microscopic ridges and transparent layers on the surface of a beetle’s carapace act as an array of lenses that direct different wavelengths of light in such a way that some colours cancel out and others are amplified – the rainbow effect on a DVD is similarly caused by the microscopic pits on its surface.  

This metallic sheen may have evolved because it offers bright colours that can serve as a mating signal over long distances. Alternatively, some researchers have suggested that it might mimic the appearance of raindrops on leaves, helping to camouflage the beetles.  

The ridges and layers may also take less energy to grow than metabolically expensive pigment molecules – particularly important for insects because they regularly shed and regrow their outer skin.

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