Although the expression of the POU homeodomain gene nubbin (nub) has been examined in several arthropod species, its function has been studied only in Drosophila. Here, we provide the first insight into functional roles of this gene in a hemimetabolous insect species, Oncopeltus fasciatus. The analysis of its function using RNAi resulted in the altered morphology of antennae and labial tubes in the head, legs in the thorax, and, most notably, the growth of ectopic appendages originating from abdominal segments A2-A6. This change in the morphology of the abdomen can largely be attributed to the altered expression patterns of two hox genes, Ubx and abd-A, in RNAinub embryos. First, abd-A expression is completely abolished in A3-A6. Second, weak Ubx expression expands posteriorly to encompass novel domains in A2 and A3. Concomitant with these changes, limbs on A2 and A3 are small and less developed, whereas limbs on A4-A6 are large thoracic-like legs. These results show that nub function is necessary for normal abd-A expression and thus plays a critical role in suppressing leg formation on the abdomen. The loss of this regulation leads to upregulation of Distal-less, and subsequent development of appendages. In Drosophila, however, abd-A expression is unaffected in a nub-depleted background, indicating that no such regulatory relationship exists between these two genes in the fruit fly. These differences reveal that variation exists in the genetic mechanisms that maintain an ancient insect feature, the limbless abdomen.