Anatomical studies of the insect central nervous system: A ground‐plan of the midbrain and an introduction to the central complex in the locust, <i>Schistocerca gregaria</i> (Orthoptera)

The supra‐oesophageal ganglion (brain) of Schistocerca gregaria Forskál has been studied at the light‐microscopical level using Wigglesworth's osmic acid‐ethyl gallate method. Particular attention was paid to the midbrain. A structural ground‐plan is described which incorporates the ocellar nerve roots, the antenno‐glomerular bundles, the corpora cardiaca nerves I roots, the antennal lobes, the corpora pedunculata and the central complex. The “undifferentiated” midbrain is described in terms of the distribution of the main features of 15 unique pairs of large neurones all of which have their cell bodies in the brain and which project, either ipsilaterally or contralaterally, to the circumoesophageal connectives. In addition a single unique neurone with bilateral distribution restricted to the brain is described. All these neurones provide a conspicuous and constant framework for further investigation. A ground‐plan based on the distribution of 64 individual neurones (central complex system I) is presented for the central complex. This system indicates that the central complex is fundamentally organized on the basis of 16 repetitive groups of neurones.