Motoneurons, DUM cells, and sensory neurons in an insect thoracic ganglion: a tracing study in the stick insect Carausius morosus.

Anatomical features of leg motoneurons, dorsal unpaired median (DUM) cells, and sensory neurons in stick insect mesothoracic ganglia were examined using fluorescent dye backfills of lateral nerves. Structures were analyzed in whole-mounts of ganglia and transverse sections. Numbers of motoneurons and details of their structure by far exceed previously published data. The general neuroanatomical layout of motoneurons matches the general orthopteran pattern. Cell bodies of excitatory motoneurons form clusters in the lateral cortex, dendrites branch mainly in the dorsal neuropil. We identified nine DUM cells, six of which have axons in nerve nl5. Most sensory fibers terminate in the ventral association center (VAC). Twenty-three small cell bodies located close to the soma of the fast extensor tibiae motoneuron likely belong to strand receptors. Labeled structures are compared with previously published data from stick insects and other orthopterous insects.