Ultraviolet reflectance accompanies classical aposematic palette in Delias butterflies (family: Pieridae)

Ultraviolet (UV) reflectance has been widely documented in butterflies, primarily in the context of sexual signaling. Here, we document, for the first time, the presence of UV-reflective pattern elements in multiple species of the aposematic pierid genus Delias. Importantly, these UV elements occur on the ventral wing surface and are spatially integrated within classical aposematic color patterns. We conducted a comparative survey across ∼28 species and combined reflectance spectroscopy, chemical depigmentation of pterin scales, absorbance measurements, and microscopy to investigate the structural basis of this signal. Experimental removal of pterin pigments increased short-wavelength reflectance and produced UV reflectance visually similar to naturally occurring UV pattern elements. Microscopy revealed reduced pigment granulation in UV-reflective scales but did not detect highly ordered multilayer ridge architectures typical of scales reflecting iridescent UV in pierids. These findings indicate that UV reflectance in Delias is consistent with pigment-mediated modulation of broadband scattering of epicuticular compounds rather than previously described iridescent multilayer interference. The experimental manipulation of pigment occurrence demonstrates how modification of scale chemistry alters visual sensory output, highlighting a previously unrecognized component of aposematic coloration in this speciose genus.