Macrochaetes (large bristles) are arranged on the drosophila head and notum in a specific bristle pattern. The number and positions of the macrochaetes forming the pattern are important species-specific characteristics, which are determined by a strict positioning of the proneural clusters in the in the imaginal disc ectoderm in the third instar larvae and prepupae. In turn, the positioning of proneural clusters depends on the distribution of the so-called prepattern factors, responsible for the bristle prepatterning. The current concept identifies the prepattern factors with the transcription factors that initiate the local expression of the achaete-scute complex (AS-C) genes. Expression of these genes confined to certain regions of the ectoderm is the particular factor that determines the macrochaete pattern on the adult fly body. The review considers and systematizes the data on establishment of the prepatterning as the final stage in the functioning of hierarchically organized molecular genetic system resulting in the local expression of AS-C genes in the ectoderm of imaginal discs.