Association of Mandible Shape and Nesting Material in <I>Osmia</I> Panzer (Hymenoptera: Megachilidae): A Morphometric Analysis

Abstract

We combined methods of geometric morphometrics and phylogenetically controlled comparisons to examine the relationship between the shape of mandibles and material used to construct the nest for 5 Osmia Panzer species. We measured leaf- and mud-using species from 2 subgenera: in the subgenus Osmia, O. lignaria Cresson (mud-user), O. cornuta Latreille (mud-user) and O. ribifloris Cockerell (leaf-user), and in the subgenus Centrosmia, O. tanneri Sandhouse (mud-user), and O. bucephela Cresson (leaf-user). Landmark-based geometric morphometrics and multivariate statistics provided a way to compare mandible shapes, distill major shape differences and visualize mandible regions that differed most among bee species. Mandible shape differed significantly among all species based on paired comparisons of Mahalanobis distances. Principal component analysis of the shape variables (relative warps analysis) revealed greater similarity between the 2 leaf-using bees, than among the mud-users. There were however, no consistent differences between these mud- and leaf-using species. Testing with two-way multivariate analysis of variance showed that the differences in shape between the leaf- and mud-using species depended on the subgenus. A broad proximate bite surface characterized leaf-using species. No such trait was common to mud-using species. Mandibles of leaf-using species may vary less because of selection on shape, or because of functional constraints for shearing and macerating leaf material; both constraints are absent in mud-using species. Mandibles of mud-using bees may respond more to selection imposed by other aspects of bee biology.