The Pronotum of Worker of Camponotus borellii Emery (Hymenoptera: Formicidae): How Can It Affect Performance of the Head, Work Division, and Development of the Worker Caste?

In polymorphic ants, whose workers display continuous size distribution, each subcaste occupies a phenotypic space, usually with diffuse morphological boundaries. These morphological differences are closely associated to size by allometry although the environment also plays a key role that affects the fitness of the species. In Camponotus borellii Emery, the species selected as a study model, workers exhibit a continuous increase in size; geometric morphometric (GM) was used over four morphological traits: head capsule, clypeus, pronotum, and mesosoma, in order to assess (1) changes in shape, among the worker caste; (2) the influence of allometry on such changes; and (3) pronotum shape in respect to the head so as to infer which factors may influence the polymorphic development of the worker caste. The results indicated that the pronotum is organized into two highly integrated functional modules (neck and shield), corresponding to one developmental module. GM shows a similar pattern to that obtained for linear morphometry, though the worker ratio was different along continuous size distribution due to shape changes in two traits, with are also useful for delimiting modular units: (1) rounded shape of the posterior region of the head in minor workers; (2) shape of the pronotum, especially its anterior region, henceforth, neck, which widens as a consequence of the higher development of its central region, henceforth, shield, in major workers. The relevance of these results is discussed regarding functional morphology (pronotum in relation to the head), work division, and development of the worker caste.