In insects, there is a considerable diversity in leg distribution on the body, including number, segmental arrangement, morphological identity and consequent function, but the genetic basis for these differences is not well understood. Here by positional cloning, we showed that a ~355 kb region, including Bombyx mori Ultrabithorax (BmUbx) and abdominal-A (Bmabd-A), was responsible for the silkworm mutant Kh-extra-crescents-like (E-l) that displayed additional thoracic limb-like legs on the first abdominal segment (A1) and occasionally on the second abdominal segment (A2). We found that BmUbx gene was downregulated at both messenger RNA level and protein level in E-l embryo, while its expression domain in the E-l embryo was almost the same as that in the wild type. Whereas Bmabd-A was upregulated at both levels and was ectopically overexpressed on the supernumerary leg-bearing segments in E-l. Compared with the previously reported E-l mutant in which increased expression of both BmUbx and Bmabd-A gave rise to ectopic proleg-like appendages on the same segments, we propose that overexpressed Bmabd-A gene is capable to promote the outgrowth of extra leg appendages on A1 and A2 segments, whereas BmUbx gene is required to specify accurate morphologies of the ectopic legs in a dosage-dependent manner in silkworm. These results provide insights into how these hox genes regulate the leg morphologic diversity on the same segments.