School of BioSciences - Theses
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ItemLight manipulation by Christmas beetles: quantification, mechanisms and ecological relevance( 2022)The brilliant and colourful appearances of Christmas beetles (Scarabeidae - Rutelinae) are famous for decorating eucalyptus trees during the Australian summer. Millions of years of evolution perfected the design of their hardened fore wings (elytra) to manipulate light and create striking optical effects. However, little is known about the exact underlying nanostructures or the ecological variables driving their evolution. Despite the recent increase in biophotonics studies, this remains the case for many organisms with striking colourful appearances. By studying light manipulation in Christmas beetles, I aim to develop methods and concepts generalisable to other organisms. Christmas beetles produce a wide variety of optical effects including saturated colours, black, and pearlescent white. Some species look mirror-like and metallic gold or brass. Moreover, these colours change with the viewing or illumination angle. Christmas beetles can also interact with long wavelengths outside of the human-visible spectrum, in the near-infrared (NIR), and reflect circularly polarised light, which makes them appear as different colours through the right and left lenses of 3D cinema glasses. To better understand this diversity, I proposed a method for its quantification, studied some of its underlying mechanisms and tested if it can be explained by ecological differences. To characterise the beetles’ optical effects, I proposed a generalization of existing spectroscopy methods and parameters to describe reflection profiles across the visible and NIR spectrum, regardless of their underlying mechanism. Ultimately these methods break down complex optical effects into simpler traits and can facilitate comparison between studies. The proposed terminology attempts to conceptually unify disparate fields (biology and optics) and allows a clear distinction with terms used to describe colour perception. To study the optical mechanisms in Christmas beetles, I analysed the architecture of their elytra. I discovered that unlike scarab beetles studied to date, three different Christmas beetle species use multicomponent photonic systems, with an upper layer acting as a green pigment-based filter and an underlaying broadband reflector, that particularly enhances NIR reflectance. For beetles with spiral nanostructures, I found a trade-off between polarisation and NIR reflectance. This diversity of photonic structures shows that beetles are a promising model for understanding complex optical properties of natural materials. To investigate ecological correlates of the optical effects in Christmas beetles, I used a biophysical essay and a phylogenetic comparative analysis. High reflectance reduced heating of the beetle elytra under controlled conditions, but I did not find any evidence that highly reflective species occur in hotter environments. Christmas beetles do not follow a simple eco-geographical pattern, possibly because their optical effects respond to species-specific combinations of environmental challenges. I demonstrated that diversity in optical effects and photonic mechanisms is more than meets the eye, even for a small group such as Christmas beetles. My thesis highlights the value of a cross-disciplinary approach where optical methods can spark stimulating biological questions and the comparative study of phylogenetically related species can inform species selection for photonics studies. The resulting conversation between the two disciplines accelerates progress in the search for the biological function of striking optical effects.