The auditory system of the Auckland tree weta (Hemideina thoracica): sound transmission through an ensiferan ear

• Main Author: Lomas, Kathryn
• Other Authors: Parsons, Stuart
• Published: ResearchSpace@Auckland 2011
• Online Access: http://hdl.handle.net/2292/8350

The aim of this thesis is to examine the structure and function of the auditory system of the Auckland tree weta Hemideina thoracica, an iconic endemic insect of New Zealand (Orthoptera, Anostostomatidae). H. thoracica hear in a narrow frequency range relevant to conspecific communication. The thresholds and firing rates of the tympanal organ receptors and auditory interneurons were measured. Results showed that males responded most strongly to frequencies relevant to intraspecific communication indicating that acoustic communication by male weta may have a more important role in intrasexual competition rather than mate choice. Weta hear with typical ensiferan prothoracic tibial ears located on each foreleg. Each ear comprises three functional parts: two equally sized tympanal membranes, an underlying system of highly modified tracheal chambers, and the auditory sensory organ, the crista acustica. Weta have the thickest and largest membranes so far described in any insect. It is expected that a membrane of this thickness would have low impedance. However, Hemideina have extremely sensitive hearing. microscanning laser Doppler vibrometry was used to determine how such a tympanal membrane vibrates in response to sound and whether the sclerotised region plays a role in hearing. The tympanum displays a single resonance at the calling frequency of the weta, an unusual example of insect tympana acting as a narrow bandpass filter. To determine what internal mechanisms in the weta are responsible for fine-scale frequency discrimination, measurements taken from high-resolution three-dimensional models of the complete weta ear, combined with vibration analysis of the tympanal deflections in response to sound waves, to construct a theoretical model of sound transmission. The combination of using advanced histological techniques and three-dimensional modelling has allowed me to uncover a new structure (Olivarus) within the auditory system of weta that has not previously been described in any other insect hearing systems. Mass spectrometry analysis of the surrounding liquid has revealed this structure sits within lipids of an unknown type and this structure may be synthesizing the lipids. Three possible theories are presented of the role in hearing played by the Olivarus and lipid and suggest iv that a simple ancient insect auditory system is possibly homologous to that of the mammalian basilar membrane.