Evolution of allometries in the worker caste of Dorylus army ants

The worker caste of polymorphic ant species consists of individuals of strikingly different morphologies. Most studies of polymorphic species have focused on intracolonial allometries and their association with division of labour and task performance of workers of different physical subcastes. However, the factors driving the evolution of these allometries are poorly understood. Here, we analyse the importance of life-style (hypogaeic vs intermediate vs epigaeic) as a factor in the evolution of allometries of functionally important body traits in Dorylus army ants in the context of an evolutionary scenario according to which a hypogaeic life-style is the ancestral state in this group. To this end, we conducted a detailed comparative analysis of the allometries of nine characters for ten species belonging to all six currently recognized subgenera and showing different life-styles. Eight of the nine traits under consideration show a clear increase in relative size from hypogaeic to intermediate to epigaeic life-style. These results strongly suggest that shifts in ecological niche are linked with adaptations in these traits. The degree of overall differentiation among species is more pronounced in larger than in smaller workers. The pattern of division of labour in the epigaeic D. molestus indicates that two factors that may have caused this phenomenon are new food habits and an increased need for colony defence.