UV-reflectivity of parafocal eyespot elements on butterfly wings in normal and abnormal specimens

An unusual specimen of Aglais urticae, lacking characteristic UV-reflecting parafocal eyespot elements along the margins of both fore and hind wings, is compared with normal, wild-type specimens. Wing scales, responsible for generating structural coloration, aremissing in the abnormal individual and have been replaced with a type that is typical of pigment-based colours. Other modifications seen in the abnormal specimen include firstly, a distal expansion of a uniformly brown region, that otherwise occupies a proximal position on the hind wings of the wild type, and secondly, the lack of a characteristic orange cross-vein band that runs proximal to the parafocal eyespot elements on the hindwing. The differences in coloration between abnormal and wild type are seen as evidence of a proximal-distal developmental axis (originally proposed by Nijhout 1991) and support a view recently aired by Beldade and Brakefield (2003). It is now clear that studies on butterfly eyespot development must consider not only pigmentcontaining scales, but also the structurallymodified scales responsible for physical colours, i.e. UV reflectivity.