Comparative Skeletal Morphology of the Bees (Hymenoptera: Apoidea): A Phylogenetic Perspective

Abstract

Advances in bee systematics have enhanced our understanding of the phylogenetic relationships among families, subfamilies, and tribes. However, a comprehensive morphological treatment was still lacking in light of current phylogenetic knowledge. In this work, we present the most extensive comparative analysis of the adult external and internal skeletal anatomy of bees, resulting in the largest morphological phylogenetic character matrix to date. We sampled 116 bee species representing all families and subfamilies, 64 tribes (89%), and 111 (19%) genera, along with eight species of apoid wasp outgroups. Our comparative study of bee skeletal anatomy includes 394 characters—11% of which are entirely new and 68% reinterpreted from previous literature, covering both external (73% of characters) and internal (27%) anatomy. We analyzed this phylogenetic dataset using Bayesian and implied weights parsimony inferences, partitioning the data to account for among-character rate heterogeneity. While confirming the monophyly of all seven bee families, our results reveal exceptionally high evolutionary rates in the mouthparts during the transition to long-tongued bees and in the mesosoma of corbiculate lineages, providing new insights into the evolutionary dynamics of the bee phenome. We also revisited a persistent conflict regarding the earliest divergences among extant lineages, in which morphological evidence supports the Colletidae + Stenotritidae clade as the sister of all remaining bees rather than the Melittidae. By interpreting character-state transformations across the entire skeletal anatomy, we offer an essential, updated reference of synapomorphies for the seven families and include a detailed discussion of major patterns in morphological evolution.