Andrew Mason, Ph.D.
Abstract
The parasitoid fly Ormia ochracea (Diptera: Tachinidae) locates its cricket host by means of phonotactic responses to male cricket songs. The small size of the flies means that the directional cues available to them from cricket sounds are severely limited. Nevertheless, female flies are capable of surprisingly accurate localisation of cricket calls. When presented with songs from a single source, directional responses are mediated by direction-dependent interaural amplitude and phase differences in tympanal vibration that are encoded as interaural latency differences in primary auditory receptors. I have examined the effects of more complex acoustic conditions on the accuracy of phonotactic responses, however. One common context for cricket song is multi-male choruses. Although spatial separation of simultaneous sources allows the flies to recognise and respond to multiple stimuli, they fail to accurately localise sound sources under these conditions. By combining stimuli presented from different locations, I have been able to manipulate interaural differences in the amplitude and phase of tympanal vibration independently. These experiments enabled me to show that phase differences in tympanal vibration do not contribute to directional hearing.