The Central Nervous Organization of the Motor Neurones to a Steering Muscle In Locusts

1. The pleuroaxillary or pleuroalar muscles of the locust (M85, Ml 14) are located in the meso- and metathoracic segments only. Each extends from the posterior face of the pleural ridge and runs dorsally and obliquely, inserting on the third axillary sclerite of the wing hinge. Each muscle consists of two distinct parts, a and b (Fig. 1). 2. Each pleuroaxillary muscle is innervated by two motor neurones which give rise to a small and a large muscle potential in electromyogram recordings (Fig. 6E). The cell body of each neurone lies posteriorly in the ganglion and the axon runs out in nerve 4 (Figs 3–6). 3. The two motor neurones of a particular muscle share many common morphological features (Figs 3–6). There is also clear segmental homology between the motor neurones supplying the meso- and metathoracic muscles (Fig. 3). 4. Serial transverse sections of the motor neurones show that their arborization is confined mainly to a dorsal region of the neuropile. Some of the collaterals encompass, and terminate in, dorsal longitudinal tracts. Branching extends far anteriorly. Posteriorly, one secondary neurite runs ventrally (Figs 7, 8). A few secondary and tertiary neurites of the metathoracic pleuroaxillary motor neurones terminate within the neuropile of the first abdominal neuromere (Figs 6, 8). Additional features which distinguish these neurones from other flight motor neurones are discussed.