Most of the glasswing's wing surface lacks the dense overlapping scales that colour other butterflies. Where scales remain they are sparse, hair-like and upright, leaving the chitin membrane exposed. That membrane would ordinarily reflect several per cent of incident light at each interface, producing a visible sheen from certain angles.
Measurements of Greta oto wings found the clear regions covered in nanoscale pillars of waxy material with irregular heights and spacing. The irregularity matters: it creates a gradual change in refractive index between air and chitin rather than an abrupt step, and because the pillars are not uniformly sized the suppression works across a broad range of wavelengths and viewing angles. Reflectance in the clear zones is roughly an order of magnitude lower than from a comparable smooth surface.
The conventional reading is that this is anti-predator camouflage in a cluttered forest understorey, and experiments with transparent artificial prey support reduced detection. Whether the wax layer also serves water repellency or thermal functions has been raised but is less firmly established.