We describe cell lineage of the tarsus of wild-type Drosophila. Large Minute+ clones were made to map the position of the antero-posterior compartment boundary in all three tarsi. The tarsus is mirror symmetric, but the compartment boundary does not coincide with the mirror plane. This boundary runs along the dorsal and the ventral rows of bristles which are immediately posterior to the mirror plane; elements in these rows being made by both anterior and posterior polyclones. The provenance of bristles and bracts suggests that the bristle cells move into their final positions. The homoeotic mutation engrailed affects only the posterior compartments of all three tarsi. The mutations bithorax and postbithorax affect only the anterior and posterior compartments of the third legs, respectively, transforming them into homologous compartments of the second leg. These results support the selector gene model of development (Garcia-Bellido, 1975) and emphasize that collaboration between polyclones is important in pattern formation.