Organization of the auditory pathway in the thoracic ganglia of noctuid moths

Abstract

We describe the neuroarchitecture of the noctuid thoracic nerve cord and use this framework to interpret the organization of the auditory pathway responsible for escape behaviour in noctuid moths. Noctuid moths possess only two auditory receptors (A1, A2), in each ear. The axon of the A1 cell projects initially to a glomerulus located ventrally and medially in the metathoracic ganglion, where it bifurcates. One branch ascends in the ventral intermediate tract to the brain, the other descends in the ventral intermediate tract into abdominal neuromeres of the metathoracic ganglion. Both axons arborize in the median ventral and ring tracts in each neuromere. The central projections of the A2 cell remain largely within the metathoracic ganglion. The axon bifurcates at the midline and directs arborizations dorsally to the dorsal intermediate and median dorsal tracts, and ventrally into the ring tract where the arborizations overlap those of the A1 afferent. The afferent projections remain ipsilateral to the ear of origin. We describe a posterior auditory association area in the metathoracic ganglion in which the major arborizations of several identified interneurones overlap those of the A1 afferent and make monosynaptic connections with it. These interneurones all respond tonically to sound stimuli. We have also identified the projections of the A1 afferent, interneurones, and motor neurones in the segmentally equivalent anterior auditory association area of the mesothoracic ganglion. An interneurone with major arborizations in the same tracts as the A1 afferent, and receiving monosynaptic input from it, is described. The arborizations of higher order interneurones lie mainly in dorsal tracts along with those of flight motor neurones. All the interneurones in this anterior centre respond phasically or phasic/tonically to sound stimuli. The relevance of this anatomical organization for predator avoidance behaviour is considered and the organization of auditory pathways in tympanate insects compared.