Structural colour, which is produced by the interaction of light with transparent microstructures, has long been known to occur in butterflies (Lepidoptera). Though much previous work considers in detail the mechanisms by which a single species produces a particular colour over a specific range of angles, little work has been done in relating the different categories of butterfly structural colour, or quantifying the diversity of optical properties within these groups. In the course of this project the structural and optical properties of 14 butterfly species, the majority of which have never been studied in this context before, are rigorously characterized. These results are then considered in the context of the evolutionary relationships between the species that possess them. This unique opportunity to experimentally study such complex systems before we have the technology to reproduce them benefits our understanding of both the optical processes involved and of their potential to be adapted for technological applications. It also allows us to begin to unravel the biological functions, evolutionary relationships and developmental limitations of these systems.

To find and calculate a first-order approximation of the reflected colour in a butterfly wing, given basic information about the lattice of photonic crystals in the wing in the form of Fourier coefficients of the refraction indices and the size of each lattice cell. This approximation will give a non-computationally-intensive, “back-of-the-envelope” method of finding the photonic bandgaps in the material with a reasonable level of accuracy.

Whilst it was technically impossible to fabricate and measure a real-sized honeycomb structure, investigation of larger analogues yielded some interesting results. Firstly, in most cases, the addition of a honeycomb actually increased the reflectance off a black surface. If this remains true in the butterfly, then the impedance-matching ridges on the top of the scale would be its primary anti-reflection structure, with the honeycomb providing other functionality. Secondly, for a hole-size around 25-30 microns, the reflectance off a honeycomb surface drops dramatically, despite Fresnel reflection. If a similar effect occurs in the smaller butterfly structure (and, as it stands, there is no apparent reason why it would not) then this would contribute to its excellent absorption properties. Understanding exactly how this drop is dependant on hole-size is potentially critical to our ability to utilise a similar structure industrially and certainly warrants further investigation.Ultimately, however, the blackness of these butterfly scales is not really magic at all, just nature refining her sleight of hand.

In summary, it has been shown that scales with ridging and honeycomb infill from 5 species of papillionidae produce extremely low reflectances (all <2% for 300-650nm) in the visible region. Data obtained on other species is unreliable, but suggests that these low reflectances are not unique to papillionidae or the honeycomb structure, and that other methods such as microribbing may produce even lower reflectances. Understanding the relationship between scale geometry and reflectance is non-trivial and requires further research. It is also not clear why these particular structures are used to produce black areas on the wings, nor why such intense black areas are needed at all.