Aposematic butterflies exhibit ultraviolet reflection on their wings

Researchers have identified ultraviolet reflection integrated into the aposematic patterns on the ventral surface of butterfly wings of the genus Delias (family Pieridae). The survey evaluated 29 species and found UV signals in different color patterns. The study records this visual component in the genus for the first time.

Reflection occurred primarily on the ventral surface, sometimes overlapping with visibly white or yellow regions. Species such as Delias dixeyi, Delias bakeri, Delias arabuana, and Delias albertisi showed more defined signals. On the hindwing of Delias dixeyi, the UV regions reflected sharply between 300 and 400 nanometers. This reflection is part of a broadband profile, which also extends to longer wavelengths.

The researchers combined reflectance spectroscopy, chemical pigment removal, absorbance measurements, and microscopy. The application of ammonium hydroxide removed pterin pigments and increased reflection at short wavelengths. The depigmented areas then exhibited a UV response similar to that observed in naturally reflective regions.

Microscopy revealed a lower presence of pigment granules in the scales with UV reflection. At the examined magnification of 10.000 times, the analyses did not detect highly ordered multilayered structures. In other pierids, these structures are associated with iridescent ultraviolet reflection.

The results indicate a relationship between the absence or reduction of pterins and the reflection of the epicuticle of the scales. The researchers have not yet determined whether this signal participates in aposematic defense against predators. The work points to this function as a hypothesis for future studies. The authors also consider the possibility that UV light is merely a byproduct of selection on pigmentation, without its own function (doi 10.1093/jisesa/ieag061).