The differentiation of copper cells, which secrete stomach acid in Drosophila larvae, has been shown previously to be sensitive to the labialk3 mutation. Here we found that stomach acid secretion in adults was insensitive to labk3. The basis for this stage-specific effect was elucidated by characterizing the development, structure, and function of the adult midgut. First, we demonstrated by copper-dependent fluorescence and morphology that copper cells were present in the adult stomach. Fine-structure analysis of adult copper cells led to the identification of a previously unrecognized plasma membrane domain: apicolateral contacts between copper cells and their neighbors consisted of smooth septate junctions that were enriched in alphabeta-spectrin and ankyrin. Second, we demonstrated that adult copper cells were present in labk3/labvd1 (conditional/null) adults. The labial protein was expressed in adult labk3/labvd1 copper cells, but not in larvae. Thus the labk3 mutation had a stage-specific effect on midgut labial expression, but did not appear to affect protein function. Surprisingly, stomach acidification was dispensable during larval development, since labk3/labvd1 mutant larvae that lacked midgut acidification developed into fertile adults.