We have utilized enhancer trap markers to follow the development of ectopic sensillar precursors in the wings of Drosophila induced by the mutations hairy and Hairy wing. Normal sensilla are still present in these mutations, and can be distinguished from ectopic sensilla based upon both the position and the timing of their development. This correlates well with the development of ectopic achaete expression in these mutations: such staining is detected only after the appearance of normal staining. Thus, neither mutation appears to alter the specification of proneural clusters in the wing, as identified with anti-achaete, or the specification of sensillar precursors within these clusters. Rather, both act to induce the formation of a temporally and spatially distinct phase of sensillar development.