In insects,there are many sets of serially homologous appendages.Nevertheless,they show great divergences in phenotype,function,identity and so on.The gain-of-function mutation of Hox gene Antenpedia (Antp) transformed the antenna into second thoracic legs in Drosophila melanogaster.However,it is still unknown which factors determine the differences between the thoracic and abdominal prolegs.Here we show that a ~355kb region including Bombyx mori Ultrabithorax (BmUbx) and abdominal-A (Bmabd-A) was responsible for the silkworm mutant Kh-extra-crescents (EKh),which displayed additional thoracic leg-like prolegs and extra markings on the ventral and dorsal side of the first abdominal segment (A1),respectively.On one hand,we found that BmUbx gene was down-regulated not only at the level of mRNA,but also at the protein level.Furthermore,the BmUbx protein expression was reduced on the A1 segment probably correlated with the phenotype of the extra thoracic leg-like prolegs.On the other hand,Bmabd-A was up-regulated at both levels and ectopically over-expressed on the segments of the supernumerary prolegs,A1 and A2.In conclusion,our results suggest that the Bmabd-A gene promotes the development of abdominal limbs,while BmUbx gene modifies the identity of the thoracic legs.