The start of phonotactic walking in the flyOrmia ochracea: a kinematic study

Left-right tympanal size asymmetry in the parasitoid fly Ormia ochracea

Ormia ochracea is a parasitoid fly notable for its impressive hearing abilities relative to its small size. Here, we use it as a model organism to investigate if minor size differences in paired sensory organs may be beneficial or neutral to an organism's perception abilities. We took high-resolution images of tympanal organs from 21 O. ochracea specimens and found a statistically significant surface area asymmetry (up to 6.88%) between the left and right membranes. Numerical experiments indicated that peak values of key sound localization variables increased with increasing tympanal asymmetry, which may explain features of the limited available physiological data.

Parasitoid-host eavesdropping reveals temperature coupling of preferences to communication signals without genetic coupling

Receivers of acoustic communication signals evaluate signal features to identify conspecifics. Changes in the ambient temperature can alter these features, rendering species recognition a challenge. To maintain effective communication, temperature coupling-changes in receiver signal preferences that parallel temperature-induced changes in signal parameters-occurs among genetically coupled signallers and receivers. Whether eavesdroppers of communication signals exhibit temperature coupling is unknown. Here, we investigate if the parasitoid fly Ormia ochracea, an eavesdropper of cricket calling songs, exhibits song pulse rate preferences that are temperature coupled. We use a high-speed treadmill system to record walking phonotaxis at three ambient temperatures (21, 25, and 30°C) in response to songs that varied in pulse rates (20 to 90 pulses per second). Total walking distance, peak steering velocity, angular heading, and the phonotaxis performance index varied with song pulse rates and ambient temperature. The peak of phonotaxis performance index preference functions became broader and shifted to higher pulse rate values at higher temperatures. Temperature-related changes in cricket songs between 21 and 30°C did not drastically affect the ability of flies to recognize cricket calling songs. These results confirm that temperature coupling can occur in eavesdroppers that are not genetically coupled with signallers.

Reconsidering tympanal-acoustic interactions leads to an improved model of auditory acuity in a parasitoid fly

Although most binaural organisms locate sound sources using neurological structures to amplify the sounds they hear, some animals use mechanically coupled hearing organs instead. One of these animals, the parasitoid flyOrmia ochracea(O. ochracea), has astoundingly accurate sound localization abilities. It can locate objects in the azimuthal plane with a precision of 2°, equal to that of humans, despite an intertympanal distance of only 0.5 mm, which is less than1/100th of the wavelength of the sound emitted by the crickets that it parasitizes.O. ochraceaaccomplishes this feat via mechanically coupled tympana that interact with incoming acoustic pressure waves to amplify differences in the signals received at the two ears. In 1995, Mileset aldeveloped a model of hearing mechanics inO. ochraceathat represents the tympana as flat, front-facing prosternal membranes, though they lie on a convex surface at an angle from the flies' frontal and transverse planes. The model works well for incoming sound angles less than±30∘but suffers from reduced accuracy (up to 60% error) at higher angles compared to response data acquired fromO. ochraceaspecimens. Despite this limitation, it has been the basis for bio-inspired microphone designs for decades. Here, we present critical improvements to this classic hearing model based on information from three-dimensional reconstructions ofO. ochracea's tympanal organ. We identified the orientation of the tympana with respect to a frontal plane and the azimuthal angle segment between the tympana as morphological features essential to the flies' auditory acuity, and hypothesized a differentiated mechanical response to incoming sound on the ipsi- and contralateral sides that depend on these features. We incorporated spatially-varying model coefficients representing this asymmetric response, making a new quasi-two-dimensional (q2D) model. The q2D model has high accuracy (average errors of under 10%) for all incoming sound angles. This improved biomechanical model may inform the design of new microscale directional microphones and other small-scale acoustic sensor systems.

Behavioral responses of a parasitoid fly to rapidly evolving host signals

Animals eavesdrop on signals and cues generated by prey, predators, hosts, parasites, competing species, and conspecifics, and the conspicuousness of sexual signals makes them particularly susceptible. Yet, when sexual signals evolve, most attention is paid to impacts on intended receivers (potential mates) rather than fitness consequences for eavesdroppers. Using the rapidly evolving interaction between the Pacific field cricket, Teleogryllus oceanicus, and the parasitoid fly, Ormia ochracea, we asked how parasitoids initially respond to novel changes in host signals. We recently discovered a novel sexual signal, purring song, in Hawaiian populations of T. oceanicus that appears to have evolved because it protects the cricket from the parasitoid while still allowing males to attract female crickets for mating. In Hawaii, there are no known alternative hosts for the parasitoid, so we would expect flies to be under selection to detect and attend to the new purring song. We used complementary field and laboratory phonotaxis experiments to test fly responses to purring songs that varied in many dimensions, as well as to ancestral song. We found that flies strongly prefer ancestral song over purring songs in both the field and the lab, but we caught more flies to purring songs in the field than reported in previous work, indicating that flies may be exerting some selective pressure on the novel song. When played at realistic amplitudes, we found no preferences-flies responded equally to all purrs that varied in frequency, broadbandedness, and temporal measures. However, our lab experiment did reveal the first evidence of preference for purring song amplitude, as flies were more attracted to purrs played at amplitudes greater than naturally occurring purring songs. As purring becomes more common throughout Hawaii, flies that can use purring song to locate hosts should be favored by selection and increase in frequency.

How spatial release from masking may fail to function in a highly directional auditory system

Spatial release from masking (SRM) occurs when spatial separation between a signal and masker decreases masked thresholds. The mechanically-coupled ears of Ormia ochracea are specialized for hyperacute directional hearing, but the possible role of SRM, or whether such specializations exhibit limitations for sound source segregation, is unknown. We recorded phonotaxis to a cricket song masked by band-limited noise. With a masker, response thresholds increased and localization was diverted away from the signal and masker. Increased separation from 6° to 90° did not decrease response thresholds or improve localization accuracy, thus SRM does not operate in this range of spatial separations. Tympanal vibrations and auditory nerve responses reveal that localization errors were consistent with changes in peripheral coding of signal location and flies localized towards the ear with better signal detection. Our results demonstrate that, in a mechanically coupled auditory system, specialization for directional hearing does not contribute to source segregation.

Parasitoid flies exploiting acoustic communication of insects-comparative aspects of independent functional adaptations

Two taxa of parasitoid Diptera have independently evolved tympanal hearing organs to locate sound producing host insects. Here we review and compare functional adaptations in both groups of parasitoids, Ormiini and Emblemasomatini. Tympanal organs in both groups originate from a common precursor organ and are somewhat similar in morphology and physiology. In terms of functional adaptations, the hearing thresholds are largely adapted to the frequency spectra of the calling song of the hosts. The large host ranges of some parasitoids indicate that their neuronal filter for the temporal patterns of the calling songs are broader than those found in intraspecific communication. For host localization the night active Ormia ochracea and the day active E. auditrix are able to locate a sound source precisely in space. For phonotaxis flight and walking phases are used, whereby O. ochracea approaches hosts during flight while E. auditrix employs intermediate landings and re-orientation, apparently separating azimuthal and vertical angles. The consequences of the parasitoid pressure are discussed for signal evolution and intraspecific communication of the host species. This natural selection pressure might have led to different avoidance strategies in the hosts: silent males in crickets, shorter signals in tettigoniids and fluctuating population abundances in cicadas.

Detailed tracking of body and leg movements of a freely walking female cricket during phonotaxis

We describe a semi-automated tracking system for insect motion based on commercially available high-speed video cameras and freely available software. We use it to collect detailed three-dimensional kinematic information from female crickets performing free walking phonotaxis towards a calling song stimulus. We mark the insect's joints with small dots of paint and record the movements from underneath with a pair of cameras following the insect as it walks on the transparent floor of an arena. Tracking is done offline, utilizing a kinematic model to constrain the processing. We can obtain the positions and angles of all joints of all legs and six additional body joints, synchronised with stance-swing transitions and the sound pattern, at a 300 Hz frame rate. This data will be used in the further development of models of neural control of phonotaxis.

Kinematics of phonotactic steering in the walking cricket Gryllus bimaculatus (de Geer)

Female crickets, Gryllus bimaculatus, are attracted by the male calling song and approach singing males; a behaviour known as phonotaxis. Even tethered females walking on a trackball steer towards a computer-generated male song presented from their left or right side. High-speed video analysis showed how this auditory-evoked steering was integrated with walking. Typically all the front and middle legs showed kinematic adjustments during steering, with the trajectories tilted towards the side of acoustic stimulation. Furthermore, the average speed of the tarsi contralateral to song increased relative to the ipsilateral tarsi. Kinematic changes of the hind legs were small and may be a consequence of the front and middle leg adjustments. Although phonotactic steering generally led to stereotyped adjustments there were differences in the specific combination of kinematic changes in leg trajectories. The most reliable kinematic steering response was by the contralateral front leg, such that, during its swing phase the tarsus moved towards the side of acoustic stimulation through an increased forward rotation of the femur and an increased extension of the tibia. Relating the changes in tarsal positioning of each leg to the steering velocity of the animal indicated that typically the front and middle legs contralateral to song generated the turning forces. Phonotactic steering was integrated into forward walking without changes to the walking motor cycle.

A precedence effect resolves phantom sound source illusions in the parasitoid fly Ormia ochracea

Localizing individual sound sources under reverberant environmental conditions can be a challenge when the original source and its acoustic reflections arrive at the ears simultaneously from different paths that convey ambiguous directional information. The acoustic parasitoid fly Ormia ochracea (Diptera: Tachinidae) relies on a pair of ears exquisitely sensitive to sound direction to localize the 5-kHz tone pulsatile calling song of their host crickets. In nature, flies are expected to encounter a complex sound field with multiple sources and their reflections from acoustic clutter potentially masking temporal information relevant to source recognition and localization. In field experiments, O. ochracea were lured onto a test arena and subjected to small random acoustic asymmetries between 2 simultaneous sources. Most flies successfully localize a single source but some localize a 'phantom' source that is a summed effect of both source locations. Such misdirected phonotaxis can be elicited reliably in laboratory experiments that present symmetric acoustic stimulation. By varying onset delay between 2 sources, we test whether hyperacute directional hearing in O. ochracea can function to exploit small time differences to determine source location. Selective localization depends on both the relative timing and location of competing sources. Flies preferred phonotaxis to a forward source. With small onset disparities within a 10-ms temporal window of attention, flies selectively localize the leading source while the lagging source has minimal influence on orientation. These results demonstrate the precedence effect as a mechanism to overcome phantom source illusions that arise from acoustic reflections or competing sources.