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Army ants are well known for their destructive raids of other ant colonies. Some known defensive strategies include nest evacuation, modification of nest architecture, blockade of nest entrances using rocks or debris, and direct combat outside the nest. Since army ants highly prefer  Pheidole  ants as prey in desert habitats, there may be strong selective pressure on  Pheidole  to evolve defensive strategies to better survive raids. In the case of  P. obtusospinosa  Pergande (Hymenoptera: Formicidae), the worker caste system includes super majors in addition to smaller majors and minor workers. Interestingly,  P. obtusospinosa  and the six other New World  Pheidole  species described to have polymorphic major workers are all found in the desert southwest and adjacent regions of Mexico, all co-occurring with various species of  Neivamyrmex  army ants.  Pheidole obtusospinosa  used a multi-phase defensive strategy against army ant raids that involved their largest major workers. During army ant attacks, these super majors were involved in blocking the nest entrance with their enlarged heads. This is the first description of defensive head-blocking by an ant species that lacks highly modified head morphology, such as a truncated or disc-shaped head.  P. obtusospinosa  super majors switched effectively between passive headblocking at the nest entrance and aggressive combat outside the nest. If this multi-phase strategy is found to be used by other  Pheidole  species with polymorphic majors in future studies, it is possible that selective pressure by army ant raids may have been partially responsible for the convergent evolution of this extra worker caste.

Multi-Phase Defense by the Big-Headed Ant,Pheidole obtusospinosa, Against Raiding Army Ants