For centuries, Renaissance artists pursued the most opulent pigments for their wealthy patrons, and lapis-lazuli-based ultramarine blue was the rarest and most prized of all. Although the lack of natural blue pigments has not hampered some species from decking themselves out in opulent azure tones, the exotic blue shades sported by animals ranging from kingfishers to Morpho butterflies share more in common with the iridescent colours of an oily film than the masterpieces of Vermeer.
Marco Giraldo, from the University of Antioquia, Colombia, explains that the colours visible in thin liquid films are produced when the thickness of the fluid layer is similar to the wavelength of the colour of light that is reflected. And this is exactly how the vivid blue colour of Morpho butterfly wings is produced: microscopically thin layers of chitin on the surface of the wing scales only reflect colours where the wavelength is similar to the separation of the chitin plates. Intrigued by the differences in shade across members of the Morpho genus, Giraldo and his colleagues, Shinya Yoshioka (Tokyo University of Science, Japan) and Doekele Stavenga (University of Groningen, the Netherlands), investigated how other components of the wing structure contribute to the butterflies’ startling colours.
Assembling in Stavenga's laboratory and selecting 16 species from the 30 possible members of the Morpho genus, Giraldo and Chunzi Liu first photographed the arrangement of scales on the wings of each species. In addition, they used scanning electron microscopy to learn more about the microscopic structure of the transparent ‘cover’ scales and the more deeply buried brown-pigmented ‘ground’ scales. Next, the team painstakingly measuring the light spectra scattered from the individual scales and tiny portions of the wings before precisely recording the spectrum of light reflected from intact wings.
‘[We had] to deal with thousands of observations and try to see the big picture’ says Giraldo, recalling the challenge of collating data including the size, shape, distributions and structures of the upper and lower scales. However, the team's big breakthrough came when they organised their observations according to the number of reflecting layer structures inside the scales and the amount that that the cover scales overlapped the ground scales beneath. ‘We realised that it agreed with the phylogeny of the genus’, says Giraldo. As the younger members of the genus became increasingly evolved, the cover scales became smaller until they were barely visible. So the transparent cover scales of the most ancient member of the Morpho genus, M. marcus, completely cover the pigmented ground scales beneath, while the cover scales of the youngest member of the genus, M. aega, were so tiny that they scarcely overlap the ground scales at all.
Next the team investigated the optical mechanisms underpinning the colour of each species’ wing colour and realised that thin reflecting structures in the upper surface of the ground scales of the ancient M. marcus butterflies worked in conjunction with the reflecting structures in the transparent cover scales to produced the vivid hue. However, the striking sky-blue tone of the M. aega wings is produced exclusively by deep stacks of reflecting structures in the ground scales, coupled with ridge structures on the surface of the scale that behave like colour-selecting diffraction gratings.
‘We conclude that Morpho coloration is a subtle combination of overlapping pigmented and/or unpigmented scales, multilayer systems, optical thin films and sometimes undulated scale surfaces’, says Giraldo and colleagues, who are keen to develop novel Morpho-inspired colour technologies that never fade to brighten our lives.