ANOTHER LOOK AT INSECT AUDITION - THE TYMPANIC RECEPTORS AS AN EVOLUTIONARY SPECIALIZATION OF THE CHORDOTONAL SYSTEM

The groundplan of longitudinal fiber tracts and commissures comprising the thoracic CNS is remarkably similar throughout a diverse group of holo- and hemimetabolous insects. Among tympanate insects, the axons of tympanal (auditory) receptors project to equivalent tracts and association areas in the central nervous system, even when the ears are located on different parts of the body. In segments without tympanal receptors, mechanoreceptive pleural chordotonal afferents are found to project to these neuropilar areas. Consistent with this, the tympanal receptors of the grasshopper have been shown to be serially homologous with those of the pleural chordotonal organs arrayed along the body wall. In atympanate insects, mechanosensitive receptors project to the equivalent neuropilar areas, via the same routes, as do tympanal receptors in insects with ears. Again, the auditory receptors of tympanate insects such as the grasshopper have been shown to be homologous with the chordotonal receptors of atympanate insects such as Drosophila. Identified first-order interneurons which have their dendrites in areas of neuropil occupied by tympanal afferents have been shown to be serially homologous along the CNS of the grasshopper. Putative homologs to these interneurons have also been identified in a range of other tympanate and atympanate insects. It is proposed that the tympanal receptors are a special part of a general chordotonal system in which homologous peripheral receptors project to sets of homologous interneurons.