Due to its intermediate complexity and its sophisticated genetic tools, the larval brain of Drosophila is a useful experimental system to study the mechanisms that control the generation of cell diversity in the CNS. In order to gain insight into the neuronal and glial lineage specificity of neural progenitor cells during postembryonic brain development, we have carried an extensive mosaic analysis throughout larval brain development. In contrast to embryonic CNS development, we have found that most postembryonic neurons and glial cells of the optic lobe and central brain originate from segregated progenitors. Our analysis also provides relevant information about the origin and proliferation patterns of several postembryonic lineages such as the superficial glia and the medial-anterior Medulla neuropile glia. Additionally, we have studied the spatio-temporal relationship between gcm expression and gliogenesis. We found that gcm expression is restricted to the post-mitotic cells of a few neuronal and glial lineages and it is mostly absent from postembryonic progenitors. Thus, in contrast to its major gliogenic role in the embryo, the function of gcm during postembryonic brain development seems to have evolved to the specification and differentiation of certain neuronal and glial lineages.