Worker-dimorphism
Guides
Pheidole megacephala
big-headed ant, coastal brown ant, African big-headed ant
Pheidole megacephala is a highly successful invasive ant species native to tropical Africa, considered among the world's worst invasive organisms. It exhibits true worker dimorphism with distinct major and minor worker castes, the former bearing disproportionately large heads used for crushing food. The species forms expansive supercolonies through budding reproduction, enabling rapid territorial expansion. It has spread globally to tropical and subtropical regions, where it aggressively displaces native ant faunas and disrupts ecosystem function through generalist predation and mutualistic associations with sap-sucking insects.
Pheidole obscurithorax
Large Imported Big-headed Ant
Pheidole obscurithorax is a large, dark, dimorphic ant native to northern Argentina and Paraguay. It was introduced to Mobile, Alabama in the early 1950s and has since spread along an 80-km-wide coastal band between Alabama and Tallahassee, Florida. The species is characterized by worker dimorphism with enlarged-headed majors and has demonstrated rapid population growth in invaded areas, with nest density increasing 6.4-fold over two years in Tallahassee. It coexists with the red imported fire ant (Solenopsis invicta) and appears to have minimal negative impact on native ant communities, being part of a largely exotic ant assemblage adapted to disturbed habitats.