By Aaron Pomerantz
How does an animal become invisible? Enter the paradox of the glasswing butterfly. As the name implies, these butterflies have transparent parts of their wings, engendering a common notion that they are “invisible” to avoid predators. However, these butterflies can also have striking orange and iridescent patterns on their wings. Numerous other species are known to mimic the glasswing butterfly’s wing patterns, highlighting the fact that these butterflies are in fact toxic, as they sequester noxious chemicals called pyrrolizidine alkaloids. Their bright colors serve as a warning signal to would-be predators such as birds.
Let’s take a step back to consider where color in butterfly wings comes from in the first place. The primary unit for color in Lepidoptera (insects that include butterflies and moths) is the wing scale cell. The underlying mechanism for a particular color is due to either pigmentation from a biochemical pathway, or to the physical architecture of scales manipulating wavelengths of light, known as structural color. To better understand processes underlying structural scale modifications, my work focuses on a unique coloration strategy: wing transparency in butterflies and moths. Numerous species of Lepidoptera develop wings that allow light to pass through so that objects behind them can be distinctly seen, which has led to the common belief that these species are “invisible” in the context of camouflage to go undetected by predators.
However, my lab and collaborators hypothesize that transparency is a much more complex coloration strategy, playing a role in visual communication through light polarization and iridescence. This form of terrestrial transparency also entails challenging optical requirements whose morphological, physiological, and genetic mechanisms remain virtually unknown.
To investigate the development of transparent species endemic to the Neotropics (the tropical terrestrial ecoregions of the Americas and the South American temperate zone), it was critical to obtain living specimens at various life stages. Furthermore, experiments in developmental biology often require access to tissue at precisely known time-points. With the support of a Tinker Foundation and CLAS-funded research grant, I turned my sights to the Smithsonian Tropical Research Institute (STRI) located in Gamboa, Panama. Nestled in a small sleepy town in the rainforest, STRI has recently been upgraded to a building with state-of-the-art molecular laboratories.
My goal at STRI was to raise glasswing butterflies, then investigate and experimentally manipulate pupal wings at various developmental stages in order to identify cellular and cytoskeletal scale modifications. I was able to collect and establish a colony of glasswing butterflies at the local insectary. Under the auspices of the laboratory at STRI, I performed dissections of pupal wings and stained wing tissue with fluorescent markers to visualize nuclei and scale cytoskeletal modifications. Additional tissue was preserved for downstream genomic and RNA experiments.
Results thus far indicate that glasswing butterflies become transparent by modifying the size and shape of their scales. This results in more light passing through to the membrane of the wing, which harbors anti-reflective nanostructures. This has been a critical step in experiments investigating the development of transparency, including gathering material for studying the expression and function of genes involved in scale development. The results from this project and future work on the established colony can now feed into comparative analyses with our physicist and evolutionary biology collaborators and provide insight into the development and evolution of terrestrial transparency.
My questions remain: Are glasswings transparent to avoid being seen? Are they bright to show off warning colors? Or perhaps a bit of both? It would be interesting if the dual nature serves to avoid a certain kind of predator under reflected light. Or do glasswings show ultraviolet colors as a warning? These would be invisible to humans, but clear as day to other animals such as birds, many of which contain opsins in their eyes that are capable of detecting UV. Either way, they are a beautiful group of butterflies, and this is a beautiful scientific mystery to (attempt to) solve.
AARON POMERANTZ is a PhD Candidate in the Integrative Biology department at UC Berkeley. Aaron holds an MS in Molecular Biology from the University of Florida and a BS in Entomology from UC Riverside. He is interested in how butterflies are able to produce such an incredible array of colors through the use of both pigments and nanostructure formations in their scales.