Transparency: physical origin, adaptive functions and evolution in clearwing butterflies – CLEARWING

Although apparently simple, transparency is a highly complex coloration strategy. While abundant in water, transparency is nearly absent on land, with the exception of insect wings. Research effort has been devoted to water transparency, leaving transparency on land – a specific question since optical requirements largely differ in air and water – virtually unstudied. Rare studies have shown diverse structural solutions, poorly known physical properties, unstudied functions and evolution. Understanding transparency cannot be approached without an interdisciplinary perspective and a coherent grasp of both transparency – as a physical phenomenon – and organisms – in their eco-evolutionary context.

At the frontiers between biology and physics, this project will (i) characterize the diversity of structural solutions for transparency and their optical, thermal and hydrophobic properties, (ii) test the adaptive functions of transparency for camouflage and communication, and (iii) decipher the processes driving its evolution at large scale. We will work on Lepidoptera, insects with wings covered with scales, a key multifunctional (coloration, thermoregulation, flight, water repellency) innovation abandoned by many species which are transparent. This raises questions about how vital functions are ensured, the functions and evolution of transparency.

Objective 1. We will characterize (i) clearwing nanostructural diversity with SEM, TEM, and photonic imaging, (ii) transmission and anti-reflective properties with scatterometry and UV-visible spectrometry, (iii) thermal and hydrophobic properties with IR spectrometry and water contact angles. Physical investigation will help assess the trade-offs between properties, the robustness of global effects to variations in structures, and identify the key elements responsible for the different functions.
Objective 2. We will evaluate the roles of transparency in communication and camouflage. Contrasting with the classical view of transparency as exclusively serving camouflage, we change paradigm and hypothesize that transparency serves both camouflage and communication, through iridescence or polarization patterns which can function as private signals. (i) We will determine detectability of transparent prey by birds in experiments in controlled conditions, compare survival values of artificial prey differing in transparent/opaque patterns in field experiments, and examine cognitive aspects with computer games. (ii) With imaging, scatterometry and spectrometry, we will test in comparative analyses whether polarization and iridescence patterns serve communication in sympatric similar species.
Objective 3. Transparency has impacts at macroevolutionary level. With comparative analyses, we will (i) reconstruct the evolution of transparency at large scale and examine structure-function relationships, (ii) test whether transparency – which can produce diverse signals for communication beyond a common protective appearance – can promote species diversification and co-evolution of butterfly vision.

Bridging the gap between disciplines, this project will bring significant advances to a crucial lack of knowledge: physics will provide thorough quantitative measurements indispensable to answer biological questions; biology will offer an evolutionary framework needed to understand multifunctional photonics, to characterize solutions for invisibility and understand robustness to disorder, promising lines of research. Project feasibility is ensured by the complementary expertise of the biologists D. Gomez and M. Elias, specialists of communication, community ecology and phylogeny, and the physicists S. Berthier and C. Andraud, specialists of optics and physics. Project originality and novelty guarantees highly-valuable results we will disseminate to scientists, trainees, and the general public through a large range of actions.