Diffusible factors of several protein families control appendage outgrowth and patterning in both insects and vertebrates. In Drosophila wing development, the gene decapentaplegic (dpp) is expressed along the anteroposterior compartment boundary. Early wingless (wg) expression is involved in setting up the dorsoventral boundary. Interaction between dpp- and wg-expressing cells promotes appendage outgrowth. Here, it is shown that optomotor-blind (omb) expression is required for distal wing development and is controlled by both dpp and wg. Ectopic omb expression can lead to the growth of additional wings. Thus, omb is essential for wing development and is controlled by two signaling pathways.