Patterns of expression of the reporter gene, lac-Z, were analyzed in the central nervous system (CNS) of third instar larvae from a total of 181 fertile and 129 lethal enhancer-trap P element insertion strains of the fruit fly, Drosophila melanogaster. Reporter gene expression in the CNS was obtained in >95% of the strains, and showed diverse patterns: of the fertile and lethal strains, regional staining patterns were obtained in 69% and 56%, respectively, staining of neuron groups in 32% and 11%, respectively; of the fertile strains, staining of homologous neurons in different segments and staining of a few neurons were obtained in 12% and 2%, respectively, but not of the lethal strains. These lac-Z expression patterns reveal that the CNS is organized of three longitudinally arranged units: brain, thoracic ganglia and abdominal ganglia; each unit was divided into subunits, possibly segmental ganglia; the units were also arranged laterally as an array of homologous neurons spread over the longitudinal unit(s). These results suggest that the identified neurons of insects can be specified by at least two differently arranged system that are specified by independent genes.