Ants use their mandibles for a wide variety of tasks related to substrate manipulation, brood transport, food processing, and colony defence. Due to constraints involved in colony upkeep, ants evolved a remarkable diversity of mandibular forms, often related to specific roles such as specialized hunting and seed milling. Considering these varied functional demands, we focused on understanding how the mandible and head shape vary within and between Pheidole sub-castes. Using x-ray microtomography and 3D geometric morphometrics, we tested if these structures are integrated and modular, and how ecological predictors influenced these features. Our results showed that mandible and head shape of majors and minor workers tend to vary from robust to slender, with some more complex changes related to the mandibular base. Additionally, we found that head and mandible shapes are characterized by a high degree of integration, but with little correlation with feeding and nesting habits. Our results suggest that a combination of structural (allometric) constraints and the behavioural flexibility conferred by caste dimorphism might largely buffer selective pressures that would otherwise lead to a fine-tuning between ecological conditions and morphological adaptation.