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In insects, forewings and hindwings usually have different shapes, sizes, and color patterns. A variety of RNAi experiments across insect species have shown that the hox gene Ultrabithorax ( Ubx ) is necessary to promote hindwing identity. However, it remains unclear whether Ubx is sufficient to confer hindwing fate to forewings across insects. Here, we address this question by over-expressing Ubx in the butterfly Bicyclus anynana using a heat-shock promoter. Ubx whole-body over-expression during embryonic and larvae development led to body plan changes in larvae but to mere quantitative changes to adult morphology, respectively. Embryonic heat-shocks led to fused segments, loss of thoracic and abdominal limbs, and transformation of head limbs to larger appendages. Larval heat-shocks led to reduced eyespot size in the expected homeotic direction, but neither additional eyespots nor wing shape changes were observed in forewings as expected of a homeotic transformation. Interestingly, Ubx was found to be expressed in a novel, non-characteristic domain – in the hindwing eyespot centers. Furthermore, ectopic expression of Ubx on the pupal wing activated the eyespot-associated genes spalt and Distal-less , known to be directly repressed by Ubx in the fly׳s haltere and leg primordia, respectively, and led to the differentiation of black wing scales. These results suggest that Ubx has been co-opted into a novel eyespot gene regulatory network, and that it is capable of activating black pigmentation in butterflies. [ABSTRACT FROM AUTHOR]