Comparative anatomy of the thoracic muscles of bees (Hymenoptera: Apoidea)

Bees exhibit a remarkable anatomical diversity, with phenotypic traits that reflect broad evolutionary patterns and specific adaptations. Understanding these patterns requires examining key anatomical features, such as thoracic musculature, which drives morpho-functional variation and underscores their extensive phenotypic diversity. The thorax (or 'mesosoma,' as it can be referred to in the context of bees and other apocritan Hymenoptera) serves as a power core, housing muscles responsible for leg, wing, and also head and metasomal articulation movements. Despite the role of the thoracic musculature in the flight mechanics of bees, detailed studies are limited to accounts of individual species or small subsets of muscles, with truly comparative analyses being scarce, leaving gaps in understanding muscular variation and phylogenetic significance. To address this, we conducted detailed dissections of 13 species, representing six bee families (Andrenidae, Apidae, Colletidae, Halictidae, Megachilidae, and Melittidae) and three additional apoid wasp taxa (Bembicidae, Crabronidae, Philanthidae), selected to capture a broad range of morphological and phylogenetic diversity. Our results revealed high conservation in mesosomal musculature, with only 16 of 58 muscle groups showing significant variation, primarily in origin points, suggesting a balance between functional constraints and evolutionary flexibility in muscle attachment. Phylogenetically relevant changes were investigated by coding 17 morphological characters, revealing potential synapomorphies for bees or certain lineages. These include the dorsomedial origin of Idlm1 (M. prophragma-occipitalis) in Meliponini, as evident in species such as Melipona quadrifasciata and Tetragonisca fiebrigi, suggesting a shared derived trait for this tribe. Additionally, the extended origin of IIIscm2 is observed in Andrenidae, Colletidae, and Halictidae, indicating closer evolutionary relationships among these families. Bee-specific modifications, including the non-separation of IItpm7b and IItpm7c by the mesepisternal ridge, distinguished bees from most apoid wasps, interpreted here as a potential synapomorphy for bees. Additional variations, such as the ventral origin of Ivlm3 in select lineages and the branched morphology of IIpcm4, suggest independent evolutionary shifts potentially linked to biomechanical demands. These findings underscore the evolutionary stability and phylogenetic value of bee mesosomal musculature, revealing a conserved framework punctuated by lineage-specific adaptations that may correlate with ecological traits.