This chapter examines the older literature in the context of how the environment regulates development of worker castes in ants, where environmental and social regulation of imaginal disc and larval development has facilitated the evolution of novel morphological castes. Social insects are an ecologically dominant and evolutionarily successful lineage within the holometabolous insects. The high level of modularity in holometabolous insect ontogeny when compared with hemimetabolous insects is in part due to cells that are set aside during development called imaginal discs. Extrinsic regulation of imaginal disc growth is also observed when a disc is damaged or ectopically introduced in a larval host, which shows that other discs adjust their growth to synchronize with the affected organ. Pioneers in insect developmental biology showed that when imaginal discs are transplanted into an adult fly’s abdomen, they grow reliably to the final size typically observed in the donor larvae.