Naturally occurring veinless specimen of the swallowtail Papilio xuthus show an extremely aberrant colour pattern. In spite of the fact that we have no breeding data, these veinless specimen are provisionally called veins-reduced mutant. In these mutants seven longitudinal veins of the fore wing and five of the hind wing are absent. The absence of wing veins is associated with a loss of the broad black venous stripes that normally are present along the proximal portion of the veins. In addition, missing veins cause a loss of the dislocation of black bands in adjacent wing cells, so that what are discrete black segments in normal wings become continuous bands in the veinless wing. Computer simulations show that the morphology of the striped patterns on both the veinless and veined wing can be explained if the wing margin acts as an inductive source of pattern formation and the veins act simply as boundaries to the propagation of the signal from the wing margin. The vein-dependent patterns by contrast, require that the veins act as inductive sources, at least along their proximal portion. This dual role of wing veins is consistent with prior observations on the biology of colour pattern formation. The unique veinless colour pattern strongly supports the hypothesis that the wing margin is the dominant organiser of colour pattern in this species, and possibly in other Papilionidae.