Phylogeny of halictid bees used in this study. Halictids encompass the full range of social behavior, from solitary to eusocial. Social behavior in this group has originated at least twice independently (indicated with asterisks). Solitary species are depicted in orange, social species in blue; and the size of the circle is relative to the mean sensilla density for each species. PGLS analysis revealed a strong trend of correlated evolution in antennal traits, independent of the species’ shared phylogenetic history (R2 = 0.2125), F(2, 33) = 4.45, P = 0.019. This difference is driven primarily by the decrease in sensilla density in secondarily solitary species that have reverted to a solitary life history compared with their eusocial ancestors (P = 0.0096). Ancestrally solitary species have densities intermediate to eusocial and secondarily solitary forms, which is perhaps driven by oligolecty in the ancestral, solitary lineages.