Siddique RH , Vignolini S , Bartels C , Wacker I , Hölscher H .

	Figure 1. Scales of the butterfly Hypolimnas salmacis.(a) Photo of a Hypolimnas salmacis butterfly. Its dorsal wings feature three regions: blue, white and dark brown/black. (b) Optical image of the forewing showing all three regions without and with diffused light from the top. Blue coloured scales only appear under diffused light condition. A zoom shows the scales stacking on the wing. (c) By tilting of the wing, the colour of blue regions shifts to purple-violet. This effect demonstrates the iridescence of Hypolimnas salmacis butterfly wings. (d) Overlapping of single white and brown scales causes the blue appearance of the white scales and reveals the colouration mechanism by scale stacking.
	Figure 2. Micro- and nanostructure of the Hypolimnas salmacis scales.(a,b) SEM images of stacks of white cover scales on brown ground scales. (c) Detail of a single white scale. Longitudinal grating like ridges along the scale are connected with cross-ribs. The sides of the ridges are covered with small microribs. (d) Detail of single brown scale. In terms of dimension, it mimics almost exactly the white scale but the windows, created by the cross-ribs, are closed by thin pigmented membranes. (e) SEM image of a cross section of the forewing including a white and brown scale. Both scales show similar features with upper lamina of ridges and tiny microribs as well as lower lamina of thin films. Thin membranes between the ridges are visible only in the brown scale.
	Figure 3. Reflectance spectra of Hypolimnas salmacis recorded on different wing areas.Total reflectance spectra of blue, white and brown regions measured as indicated in the inset. Blue areas show a considerably low intense reflection with a broad peak at ≈430 nm. No distinct peak is observed in the visible regime (380–760 nm) of the reflection spectra of the white area. In the UV, however, there is a broad peak at ≈375 nm. Brown scales barely reflect in the UV and the visible regime but the reflection increases towards the infrared.
	Figure 4. Single scale spectroscopy of Hypolimnas salmacis.(a) Spectra of a single white scale in reflection (solid line) and transmission (dashed line). (b) Spectra of a single brown scale in reflection (solid line) and transmission (dashed line). (c) Reflection spectrum measured on stack of a white scale on a wing membrane (solid line). Individually measured reflectance and transmittance properties are used to calculate the white scale-wing membrane stack reflection (dashed line) and compared with the experimental stack measurement (solid line). A schematics of a single white scale spectrometry on a wing membrane with individual reflectance and transmittance terms is provided in the inset. (d) Reflection spectrum measured on stack of a white and brown scale (solid line) which demonstrates the blue colouration due to the peak at the 420 nm. Individually measured reflectance and transmittance properties are also used to calculate (dashed line) and confirm the experimental stack measurement (solid line). The insets show a schematic of the measurement.
	Figure 5. Light diffraction of the white scales of Hypolimnas salmacis.(a) A blue (445 nm) and (b) a red laser (635 nm) shine through the white region of a Hypolimnas salmacis wing. The resultant higher diffraction orders on the screen demonstrate the transmission grating like behaviour of the white scales. (c) K-space imaging of a single white scale in transmission mode showing high order diffraction.
	Figure 6. Simulation of the lower thin lamina of Hypolimnas salmacis white scales.(a) Thin film reflection is calculated at normal incidence considering a thin chitin film surrounded by air. In order to account for the local variations observed in the lower lamina of the white scales, the thickness of the film is modeled with a Gaussian distribution as shown in the inset with a mean thickness of 190 nm and a variance (σd) of 31 nm. The simulated mean reflectance (dash-dotted line) of the distribution shows good agreement with the experimental reflection spectrum (solid line). (b) The experimental specular reflection spectra (solid line) at oblique incident angles are compared with the developed model in unpolarised light condition (average of s- and p-polarisation) for a mean thickness of 190 nm and a variance of 31 nm. Calculated reflection spectra of the bio-inspired thin film (dash-dotted line, σd = 31 nm) encounters a red-shift of 10 nm in the peak wavelength of reflection at oblique incident angles with respect to a simple flat thin film (dashed line, σd = 0 nm).
	Figure 7. The mechanism of structural colouration by scale stacking in Hypolimnas salmacis butterfly wing.Schematics of scale stacking colouration mechanism of Hypolimnas salmacis butterfly wing demonstrating the role of a diffusive white wing membrane and an absorbing brown scale on colour appearance of a translucent white scale.

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