Cores estruturais da asa da libélula: Chalcopterix rutilans

Abstract

In dragonflies, color has many functions, the most important ones being sex recognition, courtship and territory defense behaviors. In Chalcopteryx rutilans - a dragonfly found in the Amazonian rain forest those functions are performed by displaying their strongly iridescent hind wings, whereas the hyaline forewings are used to maintain the flight. The phenomenon of iridescence results from physical optics effects such as diffraction and interference. The aim of this work is to study the structures responsible for the male wing iridescence in Chalcopteryx. Visible range reflectance was measured for each different colored region of the wings. In order to determine the internal microstructure of the region that exhibits the same color, Scanning Electron Microscopy SEM was performed in cross-sections of the wings, after cutting in situ by Focused-Ion Beam FIB. Transmission Electron Microscopy TEM images were obtained from ultrathin sections of osmium treated, resin embedded and uranyl-stained wing. The SEM and TEM images revealed that the wings have a multilayered structure alternating different electron density materials. The number and thicknesses of the layers change across the wing, correlating with the local color. The composition of the layers is considered as being of chitin with different levels of melanin pigmentation. The electron density of the SEM cross-section images was assumed as being directly proportional to the optical density, and was used to define the modulation of the refractive indexes in the multilayered structure. The optical reflectance resulting from a structure with such a modulated refractive index was calculated numerically, using the transfer matrix method. A good correlation is obtained between experiment and simulation, thus confirming that in Chalcopteryx rutilans male wings colors result from a multi-layer structure, i.e., these wings are natural one dimensional photonic crystals.