Central projections of leg sense organs inCarausius morosus (Insecta, Phasmida)

SummaryThe present study describes the central projections of leg proprioceptors important in resistance reflexes and in the control of leg movement in the stick insect. The following proprioceptors were studied: the femoral chordotonal organ and the campaniform sensilla on the proximal femur, the hair plate, the hair field and three groups of campaniform sensilla on the trochanter, and the two hair plates and four hair rows on the coxa. For comparison, single tactile hairs on the sternum, coxa, trochanter, and femur were also investigated. Afferent fibers were backfilled with cobalt and the central projections were studied in wholemounts and in sections. Results are compared with those from other insects and arthropods. Results for all three thoracic ganglia are similar. Projections of all the proprioceptors are confined to the ipsilateral half of the segmental ganglion. They all terminate in four common target areas — two each in the lateral and in the intermediate part of the hemiganglion. The two lateral areas lie rostrally and caudally in dorsal neuropile occupied by motoneuron processes. The two intermediate areas lie rostrally and caudally in midventral neuropile lateral to the ventral intermediate tract (VIT). These intermediate areas include part of the ventral coarse-grained neuropile (vcN). Target areas of different proprioceptors overlap considerably, but the intermediate projections of the campaniform sensilla lie slightly closer together than those of the other organs. In addition to these four areas, the afferent fibers of the femoral chordotonal organ (fCO) project to two medial target areas extending into neuropile medial to the VIT. Afferent fibers from the various sense organs reach these common target areas using different pathways, but these pathways share some common elements. Afferent fibers from one organ can follow several alternative pathways to the common target areas. The intermediate areas are reached by projections which form a rostral and a caudal prong extending medially in midventral neuropile. Fibers enter these rostral and caudal prongs either along the lateral margin of the ganglion (a path referred to as a lateral longitudinal bundle) or by crossing from one to the other through coarse-grained neuropile occupying the central core of each hemiganglion (a path referred to as an intermediate longitudinal bundle). Collaterals entering the dorsolateral target areas rise either directly along the margin of the neuropile or from the intermediate longitudinal bundle. The medial target areas of the fCO projections are reached by a third branch which proceeds from the margin of the ganglion medioventrally between the anterior and posterior prongs and then bifurcates. Fibers from different organs follow different routes to reach common target areas. In addition, fibers from the same organ vary in their distribution among alternative pathways. Projections from tactile hairs on the sternum, coxa, trochanter, and femur are quite different from those of the proprioceptive hairs. They proceed medially along the ventral margin of the ganglion to terminate in a loose plexus within medial parts of the ventral association center.