Leg proprioceptors of the tobacco hornworm, Manduca sexta: Organization of central projections at larval and adult stages

Organization of the central neuropil of the insect ganglion is characterized in part by a modality-specific layering of afferent projections. This organization has been particularly well described for the central projections of thoracic leg sensory neurons of adult locusts, crickets, and flies. Tactile sensory neurons project into a ventral layer of neuropil, while proprioceptive sensory neurons project into an intermediate layer of neuropil. In order to determine whether a modality-specific layering exists in the CNS of larval Manduca sexta, we have examined the projections of sensory neurons innervating one class of putative proprioceptors, the campaniform sensilla, of the larval metathoracic legs. We find that campaniform sensory neurons of the larval legs have central projection patterns that generally distinguish them from each other and from the tactile sensory neurons. The campaniform projections, however, are not completely segregated from tactile projections in ventral layers of neuropil, as has been described in other insects. By contrast, the projections of campaniform sensory neurons from the adult legs are more extensive and elaborate than their larval counterparts and dramatically different from projections of nearby adult tactile hairs, having extensive arborizations in more dorsal regions of neuropil while those of tactile sensory neurons are restricted to very ventral layers of neuropil. This difference in organization of the afferent projections in larval and adult ganglia may reflect different functions of the leg sensilla and different functions of the legs at the two stages.