When Dscam^Scer\UAS.cWa is driven in the midline by Scer\GAL4^sim.PS, neither the morphology of the midline cells nor guidance of the interneurons across it are disrupted. When Dscam^Scer\UAS.cWa is driven in the interneurons by Scer\GAL4^eg-Mz360, in about 59% of the segments, posterior commissure (PC) interneurons do not cross the midline. These axons are often visible as short stumps, or as longer processes that invade the midline but fail to fasciculate with axons of PC interneurons from the contralateral side. The arrangement of PC cell bodies is also abnormal, rather than forming a tight cluster they remain loosely associated or in some cases completely separate from one another. When Dscam^Scer\UAS.cWa is driven in both the interneurons and the midline by Scer\GAL4^eg-Mz360 and Scer\GAL4^sim.PS, only 3.5% of the PC axons cross the midline. The anterior commissure interneurons are not affected.