Remodeling of the femoral chordotonal organ during metamorphosis of the hawkmoth, Manduca sexta

Abstract

During metamorphosis of the moth, Manduca sexta, the larval legs degenerate and are replaced by adult legs with a diverse array of new sensory organs. The majority of the larval sensory neurons degenerate but some hair sensilla and chordotonal organ sensory neurons survive metamorphosis (Consoulas [2000] J. Comp. Neurol. 419:154-174). In the present study nerve-tracing techniques, birth-date labeling (5-bromodeoxyuridine), and electrophysiology were used to describe the remodeling of the femoral chordotonal organ (FCO) in the prothoracic legs. The larval FCO is composed of two scoloparia, which are associated with separate apodemes. At the onset of metamorphosis, some of the 13 larval neurons degenerate, together with the larval FCO apodemes. The remaining larval FCO sensory neurons persist in the imaginal leg to become the precursors of the adult femoral and tibial chordotonal organs respectively. Early in the pupal stage, 45 to 60 new sensory neurons are generated de novo and become associated with 6 persistent larval neurons in the imaginal femur to compose the adult FCO. Two clusters of persistent and new neurons are enclosed into two scoloparia with short apodemes that eventually become fused. In both larval and adult stages, the FCO contains units that respond phasically and others that respond tonically to femorotibial movements. Nerve activity from the FCO neurons can be recorded continuously during the remodeling of the organ. A persistent leg flexor motoneuron receives inputs from the FCO sensory neurons in both larval and adult stages, offering the opportunity to investigate the remodeling of the neural circuits underlying the proprioceptive control of the femorotibial joint.