Pheidole obtusospinosa supermajor and minor workers.

Ants of the genus Pheidole have a dimorphic worker caste system. There are minor (small) workers, and majors (large) workers, each type being well-suited for specific tasks within the colony.

Pheidole obtusospinosa is remarkable for being one of only six New World Pheidole species to posses a third worker caste; the 'supermajor'. Researchers have documented P. obtusospinosa supermajors using their heads to block the nest entrance against raiding army ants. We have observed them playing a critical defensive role during skirmishes with neighboring Pheidole colonies, as well. [see images in accompanying Pheidole gallery].

This ant is found at elevations of 300 to 1900 m / 984 to 6234 ft in the southern Arizona mountains (USA), and in parts of Mexico. The colonies we observed (Nov. 2010) were found in juniper/oak woodlands, with nest structures mostly obscured by leaves and other plant debris. A very small crater of fine sand was seen around one nest entrance.

Pheidole obtuscospinosa is part of the 'pilifera group', which consists of 48 species found in the US and Mexico. A high percentage (85%) of Pheidole species in this group are known to harvest seeds, and it is likely that most also scavenge dead insects and other arthropods. We have observed Pheidole obtusospinosa minors bringing dead insects back to nest, but have not yet seen this species harvesting seeds or other plant parts.

[IMAGE: Pheidole obtusospinosa supermajor (left) and minor worker (right) in southern Arizona, USA] [scroll down for additional notes/references]


·Huang, M. 2010. Multi-Phase Defense by the Big-Headed Ant, Pheidole obtusospinosa, Against Raiding Army Ants. Journal of Insect Science, 10(1):1-10. 2010.
·Johnson, R.A. 2000. Seed Harvesting Ants (Hymenoptera: Formicidae) of North America: An Overview of Ecology and Biogeography. Sociobiology Vol.36, No. 1, 2000
·Wilson, E.O. 2003. Pheidole in the New World: A Dominant Hyperdiverse Ant Genus. Harvard University Press, Cambridge, Massachusetts