winglessactivity is required for the patterning of tergites and sternites in the adult abdomen ofDrosophila melanogaster.In the absence ofwg,tergite and sternite cuticular differentiation is replaced by that of pleura. Temperature shift analysis of awgtemperature-sensitive allele has shown that sensory bristles and tergite/sternite histotype are determined independently and thatwgis required between 15 and 20 hr after pupariation for bristle formation. The determination of sensory mother cells at this stage of development was confirmed by expression of theneuralizedgene in a subset of the proliferating histoblasts. Ectopic expression ofwgleads to the appearance of ectopic bristles and expanded tergite and sternite, indicating thatwgexpression is sufficient to promote both bristle formation and tergite/sternite differentiation.wgis expressed in the dividing and spreading histoblast population in a restricted pattern which may determine the spatial arrangement of cuticular elements.