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During larval life most of the thoracic neuroblasts (NBs) in Drosophila undergo a second phase of neurogenesis to generate adult-specific neurons that remain in an immature, developmentally stalled state until pupation. Using a combination of MARCM and immunostaining with a neurotactin antibody, Truman et al. (2004; Development 131:5167-5184) identified 24 adult-specific NB lineages within each thoracic hemineuromere of the larval ventral nervous system (VNS), but because of the neurotactin labeling of lineage tracts disappearing early in metamorphosis, they were unable extend the identification of these lineages into the adult. Here we show that immunostaining with an antibody against the cell adhesion molecule neuroglian reveals the same larval secondary lineage projections through metamorphosis and bfy identifying each neuroglian-positive tract at selected stages we have traced the larval hemilineage tracts for all three thoracic neuromeres through metamorphosis into the adult. To validate tract identifications we used the genetic toolkit developed by Harris et al. (2015; Elife 4) to preserve hemilineage-specific GAL4 expression patterns from larval into the adult stage. The immortalized expression proved a powerful confirmation of the analysis of the neuroglian scaffold. This work has enabled us to directly link the secondary, larval NB lineages to their adult counterparts. The data provide an anatomical framework that 1) makes it possible to assign most neurons to their parent lineage and 2) allows more precise definitions of the neuronal organization of the adult VNS based in developmental units/rules. J. Comp. Neurol. 524:2677-2695, 2016. © 2016 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.

The legs and wings of insects are borne on the middle body segments, which make up the thorax. The nervous system inside of the thorax is part of the insect equivalent of the spinal cord and contains clusters of interneurons that relay signals between the sensory nerves, the brain and the muscles. This enables the insect to perform complex actions such as walking and flying. The thoracic interneurons are produced by a fixed set of stem cells. Each stem cell makes neurons in a pair-wise fashion by producing a sequence of neural progenitor cells, each of which then divides to produce two different types of daughter neurons. All of the daughter neurons of the same type are said to belong to the same hemilineage, and in the fruit fly Drosophila, the majority of the interneurons in the thorax are from one of 33 hemilineages. Each interneuron cluster in the insect thorax is made up of cells from a single hemilineage. Harris et al. developed genetic tools that allow the different hemilineages in the Drosophila thorax to be labeled, and used this to create a set of flies that allows the role of the different clusters to be investigated. Each fly type was modified so that increasing the temperature activated a heat-sensitive channel in the neurons of a single hemilineage, and Harris et al. recorded the behavioral response this produced. Each hemilineage caused the fly to move in a distinctive way when stimulated, and many of these movements were unique to a single cluster. Furthermore, the hemilineages can be divided into different groups based on their complexity. Activating the simplest group of hemilineage clusters produces simple movements such as leg twitches and stretches. Another group of hemilineages are then able to organize these movements into more complicated behaviors, such as walking. The third, most complex, hemilineages can coordinate several complex actions to enable the flies to perform very complicated tasks, like take off for flight. These findings suggest that hemilineages act as the basic modules of the nervous system in the fly thorax. Furthermore, the flies and techniques developed by Harris et al. will provide valuable resources for future studies into the organization and function of the nervous system.