Structural color in the scales of blue-banded bees

Abstract

The iridescent colors of the blue-banded bee (Aphrodita sp.) come from the bands of scales on their backs. Observed in optical microscope, the scales are typically 100μm x 20μm, with bright greenish and dark bands arranged alternately over the whole scale. Electron microscope images of the cross-section along the axis of scale show that arrays of air tubes are embedded orderly in the chitin matrix composing each scale. The submicron air tubes, mostly aligned along one direction, are well organized in particular periodic structure, which can be modeled as two-dimensional photonic crystal. Viewing from the plane of periodicity, the air tubes are many hexagonal cells oriented in a certain direction. There are mainly two kinds of hexagonal arrangement found: Orientation 1 and 2, corresponding to ΓM and ΓK directions along normal incident wave vector. Spectroscopic measurement with spatial resolution of several micrometers is used to demonstrate the UV and visible wavelength range spectra of two independent modes of polarized light, TE and TM modes. The obtained experimental spectra are then compared with theoretical calculation based on two orientations of air tubes model, to show correspondence between two color regions and two orientations. Irregularities occurred in periodic structure are also found to affect the spectra and optical images of the bee scale. Experiments are designed to study and evaluate the contribution of photonic band gap effect and guided resonance effect to structural colors