Differential specification of dorsal flight appendages, wing and haltere, in Drosophila provides an excellent model system to address a number of important questions in developmental biology at the levels of molecules, pathways, tissues, organs, organisms and evolution. Here we discuss the mechanism by which the Hox protein Ubx recognizes and regulates its downstream targets, implications of the same in growth control at cellular and organ level and finally the evolution of haltere from ancestral hindwings in other holometabolous insects. © 2020 UPV/EHU Press.