Abstract An arthropod leg represents a protuberance of the body segmental integument which bears distinctive markers in both the mediolateral and the anteroposterior axes. To clarify the biaxial organization of the body segmental morphogenetic field, and to study the relation among the whole-limb, limb segmental, and body segmental fields previously recognized in arthropods, we have grafted a proximal leg segment into the ventral midline in crayfish. After this operation the majority of animals regenerated a mirror-symmetric pair of supernumerary legs at the host site. Some of these legs had the most proximal segment, the coxa, partially fused to the adjacent body surface. Minority patterns of regeneration included one midline leg with a gill, three midline legs with a gill, and two normal legs with a third double-half leg. These results are compatible with the principle that intercalary regeneration restores the continuity of positional information.