A multiterminal stretch receptor, chordotonal organ, and hair plate at the wing-hinge of Manduca sexta: unravelling the mystery of the noctuid moth ear B cell.

The present study aims to shed light on the evolutionary origin of the B cell, a sensory element of unknown function in the noctuid moth ear. Peripheral projections of the metathoracic nerve IIIN1b1, homologue of the noctuid moth tympanic nerve, are described in the atympanate moth Manduca sexta on the basis of dissections with the aid of Janus Green B, and intracellular tracer dyes Lucifer yellow and cobalt lysine. A large multiterminal (Type II) neurone, attaching to membranous cuticle ventral to the hind wing axillary cord, was discovered. This cell appears to be homologous to the B cell in the noctuid moth ear. Recordings from the IIIN1b1 nerve in M. sexta reveal a continuous train of large, uniform spikes, presumed to originate from the multiterminal cell. This unit increases its rate of firing in response to hind wing elevation, suggesting that it functions as a stretch receptor monitoring wing movements during flight. Also identified in the tympanic nerve homologue, and closely associated with the multiterminal cell, were a chordotonal organ and hair plate. The chordotonal organ consists of a proximal scolopidial region and a distal strand that attaches to the sclerotized epimeron slightly medial to the multiterminal cell. This simple chordotonal organ, having three uniterminal (Type I) sensory cells, is homologous to the auditory cells of the noctuid moth ear. The significance of these receptors as proprioceptors in M. sexta, and as evolutionary precursors to the noctuid moth ear, is discussed.