A somatotopic organization of groups of afferents in insect peripheral nerves

Sensory axon projections in the main leg nerves of two orthopteran insects, the cockroach Periplaneta americana and the grasshopper Melanoplus bivittatus, were studied by observing patterns of axonal degeneration after ablation of different leg segments. The patterns of degeneration seen in transverse sections of the leg nerves, close to their entrance to the central nervous system (thoracic ganglion) show that sensory axons occur in constant positions in the leg nerves. When distal leg segments are removed, a discrete area of degeneration is found in the leg nerve along its posterior edge. More proximal ablations produce larger areas of degeneration that progressively extend into the anterior half of the nerve. A comparison of the patterns of degeneration produced by different leg ablations shows a posterior to anterior laminar arrangement of groups of sensory axons that corresponds to a distal to proximal map of the leg. This mapping has been confirmed by localized ablations of small groups of leg sensory receptors.