Mechanoreception

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) neuron, 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 which 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.

NEED FIGURES

Fig. 1a-c Diagrams illustrating the location of the metathoracic wing-hinge sensory complex in Manduca sexta. a: Left lateral view of M. Sexta. An arrow marks the general location of the wing-hinge receptor complex, as illustrated in c. Scale bar: 1 cm. b: Left half of the metathorax viewed from the midline showing the principle branches of nerve root IIIN1 with surrounding nerves and musculature. The first branch of IIIN1b (IIIN1b1) takes a lateral course between the dorsoventral and dorsolongitudinal musculature where it innervates the wing-hinge sensory cells. dl, dorsolongitudinal muscle; dv, dorsoventral muscle; Sc, scutum; Scl, scutellum. Scale bar: 2 mm. c: Posterior view of the left, descaled metathorax with parts of the cuticle, musculature and tracheal tissue removed to reveal the wing-hinge receptor complex. The stretch receptor attaches to the membranous region (stippled) ventral to the axillary cord. The strand of the chordotonal organ attaches to the sclerotized cuticle of the epimeron in a slightly ventral position to the stretch receptor. AxC, axillary cord; CO, chordotonal organ; dl, dorsolongitudinal muscle; HW, hind wing; HP, hair plate; Sa, subalar sclerite; Sc, scutum; Scl, scutellum; SR, multiterminal stretch receptor. Scale bar: 500 mm

Copyright: This document is composed of unpublished material by Jayne E. Yack, James Fullard, and G.G. Scudder.

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