Worker-queen dimorphism and queen winglessness in Megalomyrmex Forel, 1885

A central point regarding the evolutionary success of social insects is the division of labor between reproductive and non-reproductive castes. For most ant species, the winged queens are related to colony reproduction. However, for some species, ergatoid queens (wingless queens)—with intermediate morphologies between winged queens and workers—have replaced winged queens. In the ant genus Megalomyrmex, 16 of the 44 species described have recorded ergatoid queens with variable morphology which can be similar to the conspecific workers or distinct. As a consequence of flight loss, wingless queens are dependent colony foundresses that disperse on foot with a subset of their sisters from their natal nest (i.e., budding or fission). In Megalomyrmex, queen winglessness (1) has been gained and lost multiple times throughout its evolutionary history, (2) occurs simultaneously with the winged queen strategy in the same species, and (3) does not occur in the most derived clade that includes social parasites. Using extensive morphological measurements, we examine worker-queen dimorphism and thoracic simplification of wingless queens in Megalomyrmex. We explore how these traits have evolved within the genus and how it relates to other life history traits like social parasitism and species distribution. Our study contributes to the growing body of knowledge on ergatoid queen evolution, as well as the evolutionary relationship between morphology, natural history, and colony reproduction.