Differential enlargement of hind (T3) legs represents one of the hallmarks of insect evolution. However, the actual mechanism(s) responsible are yet to be determined. To address this issue, we have now studied the molecular basis of T3 leg enlargement in Oncopeltus fasciatus (milkweed bug) and Acheta domesticus (house cricket). In Oncopeltus, the T3 tibia displays a moderate increase in size, whereas in Acheta, the T3 femur, tibia, and tarsus are all greatly enlarged. Here, we show that the hox gene Ultrabithorax (Ubx) is expressed in the enlarged segments of hind legs. Furthermore, we demonstrate that depletion of Ubx during embryogenesis has a primary effect in T3 legs and causes shortening of leg segments that are enlarged in a wild type. This result shows that Ubx is regulating the differential growth and enlargement of T3 legs in both Oncopeltus and Acheta. The emerging view suggests that Ubx was co-opted for a novel role in regulating leg growth and that the transcriptional modification of its expression may be a universal mechanism for the evolutionary diversification of insect hind legs.