Subjective visual evaluation vs. traditional and geometric morphometrics in species delimitation: a comparison of moth genitalia

Species-level taxonomic studies usually include detailed morphological descriptions of taxa. Traditionally, species descriptions have been based on nonmetric, subjective, visual comparisons of morphological traits regardless of whether diagnostic characters between species are quantitative or qualitative in nature. In difficult cases, traditional morphometrics (morphometrics based on linear measurements or ratios) and appropriate multivariate statistics have been applied to validate species delimitations. Modern geometric morphometrics, a new powerful method to quantify shapes, is increasingly being used in taxonomy and systematics. We compared the results from the three morphology-based methods (subjective visual differentiation and differentiation by traditional morphometrics and geometric morphometrics) using male genital morphology for this purpose. Five species complexes of Lepidoptera Tortricidae and Geometridae were analysed, each having two to four species, and each species being readily identifiable by their wing patterns. The between-species differences were visualized with thin-plate spline deformation grids using average landmark configurations of each species. The results from exploratory and confirmatory statistics of geometric data in a taxonomic context were compared. Morphometrics provided more accurate identification than subjective visual differentiation and the best result was achieved by combined size and shape data. Furthermore, species delimitation using exploratory statistics of landmark data is often problematic because of overlap in scatters. We conclude that geometric morphometrics provides a powerful way to search for differences between taxa and serves as an objective, useful and novel way to visualize morphological variation in shape in insect taxonomy. We recommend more extensive use of geometric morphometric tools in taxonomy.