Quantified interference and diffraction in single Morpho butterfly scales

Brilliant iridescent colouring in male butterflies enables long-range conspecific communication and it has long been accepted that microstructures, rather than pigments, are responsible for this coloration. Few studies, however, explicitly relate the intra-scale microstructures to overall butterfly visibility, both in terms of reflected and transmitted intensities and viewing angles. Using a focused-laser technique, we investigated the absolute reflectivity and transmissivity associated with the single-scale microstructures of two species of Morpho butterfly and the mechanisms behind their remarkable wide-angle visibility. Measurements indicate that certain Morpho microstructures reflect up to 75% of the incident blue light over an angle range of greater than 100° in one plane and 15° in the other. We show that incorporation of a second layer of more transparent scales, above a layer of highly iridescent scales, leads to very strong diffraction, and we suggest this effect acts to increase further the angle range over which incident light is reflected. Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single-scale microstructure to be n = (1.56 ±0.01) + (0.06 ±0.01)i. This figure is required for theoretical modelling of such microstructure systems.