Evolutionary associations among pollen scopa structure, floral breadth and eusociality in Halictidae (Hymenoptera: Apoidea).

The biological traits associated with the origin of social behaviour remain a fundamental question in evolutionary biology. Among bees, the family Halictidae provides a particularly valuable system for addressing this question, as it represents the only bee family outside Apidae in which eusociality has evolved. Yet, the ecological and morphological factors underlying these transitions remain poorly understood. To investigate the evolutionary relationships among scopa type, floral breadth and eusociality, we conducted a comparative analysis using a multilocus phylogeny reconstructed from 207 species. Ancestral state reconstructions indicate that the posteriorly caged scopa evolved multiple times from an ancestral straight form, most prominently within Halictini and closely related tribes. Comparative analyses indicate a consistently positive but transformation- and clade-sensitive association between posteriorly caged scopa and recorded floral breadth, whereas eusocial taxa show a more robust association with broader recorded floral breadth. Together, these findings suggest non-random phylogenetic associations among pollen-transport morphology, floral breadth and sociality in Halictidae, while indicating that these relationships do not follow a single causal pathway.