The 5000 bristles that protrude from the cuticle of a Drosophila adult function as either mechanosensors or chemosensors, and they are arranged in surprisingly intricate patterns. Development of the patterns appears to involve five stages: (1) establishment of a coordinate system of 'positional information'; (2) partitioning of the epidermis into areas where bristles either can or cannot originate; (3) selection of one or more bristle mother cells within each permissible area; (4) suppression of bristle development in the neighborhood of each mother cell; and (5) differentiation of the mother cell to produce four or more descendant cells, each of which forms part of the bristle apparatus. Some of the genes that control these events participate in more than one stage, and others play key roles in seemingly unrelated developmental pathways, including embryonic neurogenesis, body segmentation, and sex determination.