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

LUTZ CHRISTIAN; VIDAL MARÍA SOL; SKIGIN DIANA; BARREIRA ANA; BORGMANN LUISA; CURTICEAN ERNEST RONALD; SZISCHIK CANDELA; HOLSCHER HENDRIK; DOLINKO ANDRÉS; WACKER IRENE; INCHAUSSANDAGUE MARINA; TUBARO PABLO

Cambridge

Conferencia; Biological and bio-inspired optics Faraday Discussion - Living Light edition; 2020

The Royal Society of Chemistry

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 [1]. 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 [2]. The butterfly Dione vanillae (also known as Agraulis vanillae [3]) 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% for whole spots and about 20 % for individual scales. Electron microscopy reveals that the scales exhibit a typical ridge structure, but exceptionally have a closed and partially 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 xy-plane is used [4, 5]. 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 completely flat. The computed reflectance exhibits peaks and is varying between 20% and 40%. As expected, the near field distribution of the simplified structure 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-section is directly used for calculation. Here, the near field distribution is more complex, and the reflectance is quite uniform with approximately 20%. This agrees very well with the experimental measurements and shows that a moderate disorder is important for the uniform reflection of light.[1] Neville AC. Journal of Insect Physiology 23, 1267 (1977)[2] Vukusic P, Kelly R, Hooper I. Journal of the Royal Society Interface 6, S193 (2009)[3] Nen A. et al., Frontiers in Ecology and Evolution 8, 206 (2020)[4] Zhang J, Cong Q, Shen J, Opler PA, Grishin NV. The Taxonomic Report of the international Lepidoptera Survey 8, 1 (2019)[5] Dolinko AE. European Journal of Physics 30, 1217 (2009)[6] Dolinko AE, Skigin DC. Journal of the Optical Society of America A 30, 1746 (2013).