Size and shape in the evolution of the worker head in Pheidole ants (Hymenoptera: Formicidae)

The division of labor into sterile and reproductive castes in social insects is often reflected in marked morphological differences, which might have played an important role in the remarkable adaptive success of these organisms. Some ant lineages have undergone further morphological differentiation, with the evolution of differences within the worker caste. In this study, we characterize morphological diversity in the head of Pheidole ants by comparing differences in size and shape among species and between minor and major worker subcastes. To this end, we integrate data from high-resolution images, geometric morphometrics, and phylogenetic comparative methods. Our results indicated differences in morphological variation of each subcaste with respect to their geographical distribution, with distinct morphological patterns and evolutionary routes related to head shape. Allometry was shown to be a crucial element for the differentiation within and between each subcaste, corroborating the role of size in their morphological evolution. Additionally, we observed that closely related species often diverge considerably in morphospace, whereas convergence in their morphospace occupation characterizes some West and East Hemisphere species. Finally, although multiple shifts in the rate of morphological evolution occurred during the Miocene, the timing and position of these shifts were independent of size and shape, suggesting that their evolution has been decoupled throughout Pheidole evolution