Scaling relationships in Formica ants with continuous worker size variation

Social insects exhibit highly variable body plans at multiple scales: within colonies, between conspecific colonies, and across different species. The interspecific variation in the existence and prevalence of morphologically discrete worker subcastes in social insects raises questions about the ontogeny and functional importance of alternative worker body plans. Here, we examine the allometry of four Formica species. Formica are among the most common ants in the northern hemisphere temperate zone, and species vary greatly in the degree of worker size variation. However, no Formica species exhibit obvious worker subcastes. By carefully measuring head width, head height, scape length, thorax length, hind femur length, and hind tibia length in 180 individuals, we confirm that Formica workers exhibit continuous linear scaling, meaning that they lack discrete morphological subcastes. Most measurements scale allometrically. Different colonies of the same species are generally consistent in the slope of these relationships, and we detect unexpected similarities in scaling relationships among the four Formica species as well. Some scaling relationships, including a proportionally shorter scape and larger head in large-bodied workers, were also previously found in fire ants. Identifying worker size and shape distributions among different species is a vital step in understanding the selection pressures shaping division of labor in insect societies.