Abstract The embryonic development of the principal tracts, commissures and neuropils in the thoracic ganglia of the locust Schistocerca gregaria are described. We show that the major tracts and commissures are generated during the earliest stages of axon outgrowth. Some longitudinal tracts can be identified as early as 42 % of embryonic development and by 55 % all tracts except the dorsal median tract (DMT) and median dorsal tract (MDT) can be recognized. DMT and MDT cannot be reliably identified until 65% . The major neuropilar regions, in contrast, are identifiable relatively late in embryogenesis. They are first evident at 65-70 %, but do not become fully distinct until 70-75 %. This coincides with the developmental timing of synaptogenesis. Onto this developmental groundplan we have mapped the growth of an identified group of local interneurons. The early growth of these interneurons (50-65% ) is characterized by slow and directed axon outgrowth which assembles the basic skeletal structure of the interneurons without aberrant growth. This is followed by a period of extensive growth (65-80% ) during which the basic scaffold is elaborated. Finally there is a m aturation phase during which branches are pruned away to produce the mature interneuron structure. We show that, despite initial extensive overgrowth of branches, there is no branching into inappropriate neuropil regions in the embryo. The development of arborizations within specific neuropils appears to be tightly controlled. By using this information on interneuron growth and neuropil development it is now possible to begin to understand the developmental mechanisms that shape the neuronal architecture of the locust central nervous system.