Morphological and Spatial Diversity of the Discal Spot on the Hindwings of Nymphalid Butterflies: Revision of the Nymphalid Groundplan.

Diverse butterfly wing color patterns are understood through the nymphalid groundplan, which mainly consists of central, border, and basal symmetry systems and a discal spot. However, the status of the discal spot remains unexplored. Here, the morphological and spatial diversity of the discal spot was studied in nymphalid hindwings. The discal spot is expressed as a small or narrow spot, a pair of parallel bands, a diamond or oval structure, a large dark spot, a few fragmented spots, or a white structure. In some cases, the discal spot is morphologically similar to and integrated with the central symmetry system (CSS). The discal spot is always located in a distal portion of the discal cell defined by the wing veins, which is sandwiched by the distal and proximal bands of the CSS (dBC and pBC) and is rarely occupied by border ocelli. The CSS occasionally has the central band (cBC), which differs from the discal spot. These results suggest that the discal spot is an independent and diverse miniature symmetry system nested within the CSS and that the locations of the discal spot and the CSS are determined by the wing veins at the early stage of wing development.