Meeting Abstract
Ormia ochracea is a parasitoid fly endemic to the Americas. Gravid females respond phonotactically to calls of their male Gryllidae cricket hosts. Surprisingly, O. ochracea can locate their hosts with an azimuthal precision of 2°— equal to that of humans — in spite of their small size, which should prohibit this level of precision because of fundamental constraints imposed by the physics of sound propagation (Mason et al., Nature, 2001). Miles et al. demonstrated that the fly’s two tympanal membranes are mechanically coupled, which increases the interaural time delay (ITD) between the tympana, allowing the fly to resolve nanosecond time differences, and greatly increasing the precision with which she can locate her larval hosts (Miles et al., J Acoust Soc Am, 1995). Here, we present the first measurements documenting tympanal size asymmetry in O. ochracea. We measured 38 tympanal membranes in 19 specimens and found a mean asymmetry of 5.6% in tympanal area between the left and right sides. We hypothesized and then demonstrated mathematically that this slight asymmetry should provide an additional significant increase to O. ochracea’s sound localization abilities, beyond that provided by the mechanical coupling of the tympana. We introduced a tympanal size asymmetry into the mathematical model of hearing in O. ochracea provided by Miles et al. and showed that an asymmetry of just 5% can increase the ITD by an order of magnitude compared to the symmetric case, and can similarly significantly increase the interaural amplitude difference (IAD) in O. ochracea. The ITD and IAD are the two quantities used by the fly to determine its prey’s azimuthal location. Thus, the small asymmetry present in tympanal sizes in the fly may provide a significant advantage in sound localization.
