Head geometric morphometrics as a reliable method to discriminate sexes and species of Megalopta, a nocturnal bee genus (Hymenoptera, Apoidea)

Geometric morphometrics is an approach widely used in biological research. For bees, wing landmarks are applied to investigate several questions, such as species identification and population dynamics, yet other morphological structures remain understudied. Megalopta Smith reunite species that forage at dim-light conditions having heads with modifications on shape and size associated with specialized compound eyes and ocelli. In this study, we selected both sexes of 14 species to test if head landmarks can successfully differentiate Megalopta sexes, species, and taxonomic groups. We found that head and eye centroid size and Mahalanobis and Procrustes distances were consistently different between males and females. Male and female differed on lower head landmarks. When contrasting both species and taxonomic groups, canonical variate analysis could differentiate species pairs for most comparisons, while principal component and cluster analysis did not recover such taxonomic groups. Species differences were linked to variation in upper eye landmarks. We conclude that head and wing geometric morphometrics have similar potential and constraints, and response subjects should be selected based on underlying biological questions rather than convenience alone.