What determines the number of auditory sensilla in the tympanal hearing organs of Tettigoniidae? Perspectives from comparative neuroanatomy and evolutionary forces

Abstract

Insects have evolved complex receptor organs for the major sensory modalities. For the sense of hearing, the tympanal organ of Tettigoniidae (bush crickets or katydids) shows remarkable convergence to vertebrate hearing by impedance conversion and tonotopic frequency analysis. The main auditory receptors are scolopidial sensilla in the crista acustica. Morphological studies established that the numbers of auditory sensilla are species-specific. However, the factors determining the specific number of auditory sensilla are not well understood. This review provides an overview of the functional organization of the auditory organ in Tettigoniidae, including the diversification of the crista acustica sensilla, a list of species with the numbers of auditory sensilla, and a discussion of evolutionary forces affecting the number of sensilla in the crista acustica and their sensitivity. While all species of Tettigoniidae studied so far have a crista acustica, the number of sensilla varies on average from 15–116. While the relative differences or divergence in sensillum numbers may be explained by adaptive or regressive changes, it is more difficult to explain a specific number of sensilla in the crista acustica of a specific species (like for the model species Ancistrura nigrovittata, Copiphora gorgonensis, Gampsocleis gratiosa, Mecopoda elongata, Requena verticalis, or Tettigonia viridissima): sexual and natural selection as well as allometric relationships have been identified as key factors influencing the number of sensilla. Sexual selection affects the number of auditory sensilla in the crista acustica by the communication system and call patterns. Further, positive allometric relationships indicate positive selection for certain traits. Loss of selection leads to evolutionary regression of the auditory system and reduced number of auditory sensilla. This diversity in the auditory sensilla can be best addressed by comparative studies reconstructing adaptive or regressive changes in the crista acustica.