Glasswing Butterfly

Greta oto is a small butterfly of the Central and northern South American cloud forest, with a wingspan of just 5–6 cm and almost completely transparent wings. The transparency is the result of two structural adaptations. First, the wing membrane between the supportive veins lacks the colored or pigmented scales that cover other butterfly wings — leaving only a thin chitinous panel. Second, the few remaining scales are sparsely arranged and the chitin surface is covered in microscopic, randomly-arranged nano-pillars about 100 nm tall. These pillars create a graded refractive-index surface that drastically reduces light reflection at every viewing angle: measurements show under 2% reflectance, beating most commercial anti-glare optical coatings. The biological function is camouflage — predators have a much harder time seeing the butterfly against any background. Glasswings are also chemically defended: caterpillars feed on toxic Cestrum plants, sequester pyrrolizidine alkaloids, and the toxins persist into adulthood. Materials scientists studying glasswing nano-pillars have generated patents for anti-reflective coatings on camera lenses, smartphone screens, solar panels, and biomedical implants.