Analysis of the optical properties of the silver spots on the wings of the Gulf Fritillary, Dione vanillae

CHISTIAN LUTZ; A. E. DOLINKO; WACKER, IRENE; VIDAL, MARIA SOL; MARINA INCHAUSSANDAGUE; DIANA C. SKIGIN; TUBARO, PABLO; BARREIRA, ANA; BORGMANN, LUISA; RONALD CURTICEAN, ERNEST; SZISCHIK, CANDELA; HÖLSCHER, HENDRIK

Cambridge

Congreso; Living Light 2020; 2020

Cambridge University

Structural colours from silvery-white to metallic have emerged in various evolutionary lineages. One strategy is to create broadband interference reflectors, occurring for example in insect cuticles. These are typically rather thick compared to butterfly wing scales of only a few micrometres in thickness. However, some butterfly species create a similar silvery coloration with remarkably thin scales. The butterfly Dione vanillae (formerly known as Agraulis vanillae) for instance is covered with bright, shiny silver spots on the ventral wings. These spots possess densely packed transparent scales with a bright white appearance. On the microscopic level these scales exhibit rows of rainbow-coloured spots. Analysis of the optical properties of the silvery wingspots and of individual scales by means of (micro-)spectroscopic measurements showed a uniform reflectance over all visible wavelengths with about 50% and 20%respectively. Electron microscopy reveals that the scales exhibit a typical ridge structure, but exceptionally have a closed andpartially smooth surface to cause reflections. Cross-sections of the scales show an air-filled cavity, enclosed by an upper and lower lamina. Both possess a thickness in the range where thin-film-interference effects are to be expected, explaining the microscopic multi-colour impression adding up to a bright silvery-white appearance. A high variation in the air-filled cavity between both laminae is observed, compared to the variation of the layer thickness. The ridges exhibit a triangular shape and the closed upper lamina forms a round groove creating a circus-tent resembling structure. To further model the optical properties of this structure a simulation approach inspired by the propagation of mechanical waves along a two-dimensional array of particles contained in the x-y-plane is used. It offers the possibility of defining the simulation domain by means of digital images or bitmaps. In a first approach a simplified structure was considered in which the upper lamina has a perfectly periodic circus-tent-structure and the lower lamina is completelyflat. The computed reflectance exhibits peaks and is varying between 20% and 40 %. As expected, the near field distribution is highly symmetric. The experimentally obtained reflectance is uniform along the spectrum, therefore the perfectly periodic model does not adequately represent the natural structure. To take the observed structural irregularities into account the microscopical cross-sectionis directly used for calculation. Here, the near field distribution is more complex, and the reflectance is quite uniform with