The Iroquois complex (Iro-C) homeodomain proteins allow cells at the proximal part of the Drosophila imaginal wing disc to form mesothoracic body wall (notum). Cells lacking these proteins form wing hinge structures instead (tegula and axillary sclerites). Moreover, the mutant cells impose on neighboring wild-type cells more distal developmental fates, like lateral notum or wing hinge. These findings support a tergal phylogenetic origin for the most proximal part of the wing and provide evidence for a novel pattern organizing center at the border between the apposed notum (Iro-C-expressing) and hinge (Iro-C-nonexpressing) cells. This border is not a cell lineage restriction boundary.