Figure 19.20 A Mutation in a Hox Gene Changed the Number of Legs in Insects In the insect lineage (blue box) of the arthropods, a change to the Ubx gene resulted in a protein that inhibits the Dll gene, which is required for legs to form. Because insects express this modified Ubx gene in their abdominal segments, no legs grow from these segments. Other arthropods, such as centipedes, produce an unmodified Ubx protein and do grow legs from their abdominal segments.