Desert ants exhibit remarkable adaptations to harsh environments, including dietary specializations that allow them to survive in arid regions. My dissertation focuses on the evolution of granivory (seed-feeding) in the harvester ant genus Pogonomyrmex, a rare diet among ants. Understanding the environmental pressures and morphological traits associated with this diet sheds light on broader patterns in ant evolution and insect dietary adaptations. In Chapter 1, I used ultra-conserved elements (UCEs) to construct a robust phylogeny of Pogonomyrmex, including ~80% of described species and several undescribed ones. This new phylogeny reveals that the genus originated ~42 million years ago in mesic regions of northern or central South America. Pogonomyrmex later expanded into arid areas of South America, North America (first ~34 Mya, again ~24 Mya), and Hispaniola (~10 Mya). These results provide important context for the genus's adaptation to xeric habitats. In Chapter 2, I examined the link between head morphology and granivory. I found that large head size is strongly associated with seed-feeding and that Pogonomyrmex underwent two major dietary shifts: an early transition from generalist omnivory to granivory, and a more recent reversal to omnivory and elaiosome-feeding in a small South American clade. Additionally, head size negatively correlates with precipitation, suggesting that larger heads may have been selected for in dry environments where seeds are a reliable food source. In Chapter 3, I analyzed host-associated bacterial communities across Pogonomyrmex species to determine whether microbial composition reflects evolutionary history or diet. I identified 2,926 unique bacterial ASVs from 20 phyla and 559 genera, with 28 core ASVs consistent across species. While bacterial communities were largely conserved, precipitation showed a marginal influence on beta diversity, hinting at an environmental effect. Together, my research reveals how ecological, morphological, and microbial traits have interacted to shape the success of Pogonomyrmex ants in arid ecosystems and highlights the complex pathways through which insects adapt to extreme environments.