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Notes: Summary 1. The accuracy with whichTeleogryllus oceanicus females orientate to synthetic calling songs, containing carrier frequencies in the range 2.5 kHz to 12 kHz, has been quantified in terms of the angle of incidence of stimulation (target angle) during each pause in the response and the angle through which the female turned immediately following the pause (Figs. 1, 2). 2. Females orientated to the synthetic calls containing carrier frequencies in the range 3.5 kHz to 9 kHz. The highest proportion of turns towards the activated loudspeaker occurred for calls containing 3.5 kHz and 4.5 kHz (Table 1). No phonotactic response occurred to calls containing carrier frequencies of 2.5 kHz and 12 kHz. 3. For calls containing carrier frequencies in the range 3.5 kHz to 9 kHz females made a significant proportion of turns towards the activated loudspeaker for target angles ahead of and behind the lateral axis (Table 2), indicating that females can discriminate between the ipsilateral and contralateral sides for a wide range of target angles. 4. Females became less accurate at discriminating between the ipsilateral and contralateral sides as the frequency in the call was shifted away from 4.5 kHz. For the call containing 4.5 kHz females made a significant proportion of turns towards the activated loudspeaker for target angles greater than 12°–14° to the midline while for calls containing 5.5 kHz, 6.5 kHz and 9 kHz the minimum target angle for which significant side discrimination occurred was greater than 28° to the midline. 5. As the frequency in the call was moved away from 4.5 kHz the probability of a female turning towards the target, for target angles between 0° and 100°, decreased (Fig. 3). 6. When presented the call containing 4.5 kHz females did not scale their turns to target angles between 13° and 93°, but made an average turn of 41°±24° (Fig. 4, Table 2). To target angles greater than 90° to the body axis females made a significantly larger average turn (79°±43°) than to target angles less than 90° (Table 2). Females can, therefore, discriminate between stimulation from an anterior and posterior direction. The magnitude of the difference in the average turn angle to target angles ahead of and behind the lateral axis would suggest a separately programmed motor response for these two categories of target angle. 7. Individual females display a preference for turning left or right of the midline when presented a nondirectional stimulus (Table 3). The neurophysiological and behavioural bias cited in other studies is, therefore, expressed in the phonotactic response.

Notes: Summary 1. The gross anatomical features of the crista acustica in the tettigoniidsMygalopsis marki andPolichne sp. are compared (Fig. 1). The crista inPolichne sp. was longer and contained 12 more receptors than that inM. marki. Both organs, however, displayed a gradual and even taper from the proximal to the distal end. The size of the attachment cells and the length of the sensory dendrites displayed a similar gradual decrease in size from the proximal to the distal end of the array (Fig. 1). 2. The characeristic sound frequency and roll offs in sensitivity of ascending auditory interneurons in the oesophageal connectives ofM. marki did not alter after the leg cuticle covering the crista acustica was removed (Fig. 2). Since such interneurons receive excitatory and possible inhibitory input from several receptors in the crista this result indicated that the tuning of these receptors was not affected by removing this portion of the leg cuticle. 3. Physiological recordings were obtained from the cell bodies of individual receptors in the crista acustica ofM. marki andPolichne sp. (Fig. 3 A, B). Receptors inM. marki produced a maximum response to an 80 ms tone pulse of 16 spikes/stimulus, had a dynamic range of approximately 30 dB and a rate of increase in the spike response of 6 spikes/10 dB (Fig. 3C). The maximum response, dynamic range and slope of the intensity-response characteristics of auditory receptors inPolichne sp. were less than that for receptors inM. marki (Fig. 3 D). The slope of the intensity response characteristics, dynamic range and maximum spike response of individual auditory receptors in bothM. marki andPolichne sp. did not alter for sound frequencies above and below the characteristic frequency of the individual receptor (Fig. 3 C, D). 4. The tonotopic organisation of the crista acustica inM. marki andPolichne sp. was determined by the injection of Lucifer Yellow into the cell bodies and dendrites (Fig. 4 A) of receptors for which the frequency-threshold characteristics had been obtained (Fig. 4B). The characteristic frequencies of adjacent receptors in the crista acustica ofM. marki were seperated by irregular amounts with receptor 6 tuned to 7 kHz, receptor 7 to 14 kHz, receptor 8 to 17 kHz and receptors 10 and 11 tuned to 20 kHz (Fig. 6). Although adjacent receptors inPolichne sp. were seperated by approximately 1 kHz, the crista acustica of the species contains four receptors tuned to 20 kHz (Fig. 7). 5. The discontinuities in the tonotopic organisation of the crista acustica ofM. marki andPolichne sp. are incompatable with the notion that mechanical resonances within the receptor array could be responsible for the frequency selectivity of these auditory receptors. It is therefore proposed that these receptors are tuned as a result of electrical and/or mechanical resonance intrinsic to the individual sensilla.

Notes: Summary 1. An ascending, auditory unit has been extracellularly recorded from the cervical connective in specimens of two tettigoniid species. Spike potentials arising from unilateral afferent input were monitored in response to airborne sounds in the frequency range 6.3–40 kHz and incident from different directions. Recordings were also obtained of the massed auditory response in the tympanal nerve in a third species. 2. Frequency sensitivities of auditory organs were measured at threshold and suprathreshold levels. Optimum frequencies generally corresponded with energy peaks in relevant song spectra (Figs. 3, 4). Intensity characteristics of the ascending unit were obtained near the corresponding frequency optimum (Fig. 5). Directional sensitivities of the auditory organs near the frequency optima were measured at 15 ° intervals around the preparation at threshold and suprathreshold levels of stimulation (Fig. 6). Threshold sensitivities to ipsilateral and contralateral stimulation were compared over the range 6.3–40 kHz. 3. Directional sensitivity of the ascending unit generally increased with increasing sound frequency. Maximum differences of from 8–25 dB were observed between sensitivities to ipsilateral and contralateral sound presentation in the frequency range 6.3–40 kHz (Fig. 8). Similar results were obtained for the massed primary response. 4. The frequency sensitivity and the directional sensitivity of the auditory organs, for sound frequencies higher than 8–10 kHz (depending on species) and at threshold and suprathreshold levels, were found to be independent of pressure-gradient effects at the auditory tympana (Figs. 7, 8). 5. The gross morphology of the prothoracic spiracle and associated trachea to the level of the tympanic membranes has been examined in each species. In the thorax, the trachea abruptly reduces in cross sectional area by about a factor of five. From the coxa to the distal femur the trachea undergoes a further three to four-fold reduction in cross section (Fig. 2). 6. Physiological experiments and direct measurements using miniature probe microphones indicate that, in the frequency range 8–40 kHz (depending on species) the acoustic gain of the trachea leading from the prothoracic spiracle to the auditory tympana is 10–30 dB (Figs. 9, 10). The shape of the hearing threshold curve, however, only partially matches the gain curve of the trachea. 7. Near the frequency optimum of the auditory organ, the sound pressure measured by a probe microphone in the thoracic tracheal vesicle adjacent to the prothoracic spiracle changes by 10–16 dB as a function of the direction of the incident sound (Fig. 12 A). 8. Simultaneous measurements using two probe microphones show that relative sound pressure in the thoracic tracheal vesicle changes by 10–16 dB, whereas that in the trachea distal in the femur may change by 8–24 dB for different directions of sound (Fig. 12C). Directional sensitivities measured neurophysiologically are, therefore, fully explained by the relative sound pressure level in the fore-leg trachea leading to the auditory tympana. 9. Dissimilarities sometimes observed between the directivity patterns of the sound pressures measured in the thoracic vesicle and in the femoral trachea possibly are due to the effects of sound entering the trachea along the length of the leg. 10. These results are discussed in relation to the properties of acoustic horns and to other recent studies of auditory function in tettigoniids.

Notes: Summary 1. Single unit recording from the oesophageal connectives was used to characterise the physiological responses to monaural stimulation of ascending auditory interneurons in the tettigoniidCaedicia simplex. 2. In terms of characteristic patterns of spike discharge, ascending auditory interneurons have been classified as tonic, phasic-tonic or phasic (Fig. 1). 3. According to suprathreshold, isoresponse criteria, tonic interneurons either are broadlytuned between 10–16 kHz or are more narrowlytuned to a single sound frequency below 10 kHz (Fig. 2A). All tonic interneurons recorded have a roll-off in sensitivity of 20–40 dB/octave above and below their frequencies of maximum sensitivity. This corresponds with the roll-offs applicable to primary afferent fibres. 4. Most phasic-tonic interneurons display uniform sensitivity to one of three different frequency bands (about 3 kHz in width) in the range 10-18 kHz (Fig. 2B). These three classes of phasictonic interneuron have roll-off in sensitivity of more than 50 dB/octave for sound frequencies below their range of maximum sensitivity. 5. In contrast, a distinct class of phasic-tonic interneuron is sharply-tuned to 16 kHz, the species' call frequency (Fig. 3 A). The roll-off in sensitivity of this class of interneuron 40–50 dB/octave for frequencies above and greater than 50 dB/octave for frequencies below 16 kHz. For frequencies below 16 kHz, this roll-off in sensitivity exceeds that of primary afferent fibres by a factor of two (Fig. 3B). 6. Phasic interneurons each are tuned to a single sound frequency in the range 10–16 kHz and have roll-off's in sensitivity of 40–50 dB/octave above and 20–30 dB/octave below their frequency of maximum sensitivity (Fig. 2C). 7. For tonic interneurons, a positive, approximately linear relationship exists between sound intensity and spike response that is similar for different sound frequencies (Fig. 4 A). 8. Phasic-tonic interneurons have a complex intensity-response characteristic that is dependent on sound frequency (Fig. 4B). For frequencies within or above the range of maximum sensitivity, the response increases at a rate of 6 spikes/20 dB, saturates at 10–12 spikes/stimulus and may then decline to as little as 2–3 spikes/stimulus at the higher sound intensities. For frequencies below the band of highest sensitivity the gradient, maximum response and dynamic range of the intensity characteristic progressively decrease. 9. Phasic interneurons produce a maximum of 2–4 spikes/stimulus and, as with tonic interneurons, the intensity characteristic is independent of sound frequency (Fig. 4C). 10. Two-tone experiments demonstrate that phasic-tonic interneurons are inhibited by a sensory response to sound frequencies below their band of highest sensitivity (Fig. 5) and that the onset of this inhibition is delayed by 20 ms, relative to that of excitation (Fig. 6). Similar two-tone experiments conducted on primary afferent fibres demonstrate that no such inhibition occurs at the periphery (Fig. 7). 11. Based on idealized frequency and intensity characteristics of primary afferent fibres (Fig. 8) derived from empirical data (Oldfield 1982, 1983) and on the characteristics of monaurally-stimulated ascending interneurons, a simple model of integration is developed, in which the response of each class of ascending interneuron is attributed to the sum of the inferred excitatory and inhibitory inputs provided by an array of separately-tuned primary afferent fibres. The model accurately predicts the responses of ascending auditory interneurons inC. simplex for the monaural case (Fig. 9).

Notes: Summary 1. Electrophysiological recordings from individual auditory sensilla in the tettigoniid (Caedicia simplex) ear (Fig. 1) show that spike potentials of two distinct amplitudes occur in the sensory neuron (Fig. 2). Generally, the smaller of the two classes of spike is observed as an inflection on the rising phase of the larger spike. In cases where the larger spike fails or is blocked, the smaller spike becomes distinct. In the attachment cell of the sensillum, negative-going monophasic, or biphasic spikes are recorded (Fig. 3). In biphasic spikes, the positive phase is of smaller amplitude and always follows the negative phase. 2. Simultaneous recordings from the sensory neuron and from its associated attachment cell, using two electrodes, show that the negative-going spike recorded from the attachment cell corresponds with the smaller spike recorded from the neuron (Fig. 4). The occurrence of a large spike in the neuron causes the positive-going potential in the attachment cell, immediately following the negative spike. 3. Injection of negative current into the attachment cell via a recording electrode elicits spikes in the sensory neuron. If large spikes in the neuron fail or are blocked by hyperpolarization of that cell, the injection of negative current into the attachment cell elicits small spikes in the neuron (Fig. 4). 4. Electrical stimulation of the tympanal nerve induces retrograde spikes in the soma of the sensory neuron. Such spikes show no inflection in the rising phase, indicating the absence of the small spike as a precursor to the retrograde spike (Fig. 6). Recordings from the attachment cell, when retrograde spikes are induced, show only a positive-going potential correlated with each retrograde spike. The positive deflections recorded in the attachment cell, resulting from retrograde spikes, generally are of greater amplitude than those specifically associated with large, orthograde spikes occurring in the sensory neuron. 5. These results confirm previous suggestions that, in insect auditory, chordotonal sensilla, spikes of relatively small amplitude occur at the apex of the sensory dendrite and subsequently, trigger spikes of conventional amplitude at a site more proximal in the dendrite. The occurrence of small spikes in the neuron implies novel equilibrium potentials at the apical dendritic membrane, which may result from the scolopale lumen functioning as a receptor lymph cavity.

Notes: Summary 1. Removing the anterior tympanic membrane of the tettigoniidCaedicia simplex did not significantly alter either the tuning or the intensity-response characteristics of individual auditory fibers recorded in the leg nerve (Figs. 2 and 3). However, this operation did make the auditory sensilla in the crista acustica accessible for physiological recording. 2. The physiological responses of individual auditory receptors in the crista acustica have been recorded and the frequency-threshold characteristics of identified sensilla have been measured for sound frequencies between 1 kHz and 40 kHz. 3. In response to an acoustic stimulus a slow hyperpolarising potential (1–2 mV), lasting the duration of the tone and superimposed hyperpolarising spike potentials (2–4 mV) were recorded in the attachment cell of the sensillum (Fig. 4A). In two of the 28 sensilla from which recordings were obtained, depolarising spike potentials were recorded superimposed on the hyperpolarising potential (Fig. 4B). On rare occasions, depolarising spike potentials were recorded in the absence of any slow potential. Lucifer Yellow was successfully injected into the attachment cell of a sensillum only when the slow potential was recorded (Fig. 6). 4. Each sensillum has a typical V shaped tuning curve with the most sensitive sensilla tuned to sound frequencies between 7 kHz and 18 kHz (Figs. 7 and 8). The sensilla of the crista acustica are tonotopically organised with the more proximal receptors tuned to the lower sound frequencies (Figs. 6–8). The frequencies of optimal sensitivity of adjacent sensilla are separated by an interval of approximately 1 kHz (Fig. 8).

Notes: Summary 1. Physiological responses of individual primary auditory fibres in the tettigoniidCaedida simplex were quantified for sound frequencies between 5 kHz and 35 kHz and the anatomical organisation of the central projections of several fibres was determined by staining individual fibres with Lucifer Yellow CH. 2. The most sensitive cells recorded were tuned to sound frequencies between 9 kHz and 25 kHz. These cells had roll-offs in sensitivity of 30 to 40 dB/octave for frequencies below and 40 to 50 dB/octave for sound frequencies above their centre frequency (Fig. 1). 3. All primary auditory fibres responded to a 50 ms sound pulse with a simple tonic discharge of spikes, that increased by 5 to 7 spikes/stimulus for a 20 dB increase in the sound intensity (Fig. 2A) and saturated at 15 spikes/stimulus. For each individual primary auditory fibre, the slope and the maximum response of the intensity-response characteristic were almost constant for different sound frequencies (Fig. 2B). 4. The central projections of identified primary auditory fibres ofC. simplex display a degree of tonotopic organisation. Cells tuned to sound frequencies lower than 16 kHz (Figs. 3, 4) project to the anterio-ventral region of the prothoracic ganglion while cells tuned to frequencies at or above 16 kHz project to the medio-ventral region of the ganglion (Figs. 5, 6). 5. In the prothoracic ganglion, the fibres of cells tuned to 16 kHz, the calling song frequency ofC. simplex bifurcate immediately prior to dividing into an extensive field of terminal arborisation (Fig. 5A). The extent of this arborisation is greater than that of primary auditory fibres tuned to other frequencies. This suggests that these cells synapse on a greater number of interneurons and/or have a more potent effect on specific central neurons, than primary fibres tuned to other frequencies.

Notes: Summary Physiological recordings were obtained from identified receptors in the tympanal organ ofGryllus bimaculatus. By immersing the prothoracic leg in Ringer solution and removing the anterior tympanic membrane the auditory receptors were exposed without significantly altering the frequency response of the auditory organ (Fig. 1). Each receptor was tuned to a specific sound frequency. For sound frequencies below this characteristic frequency the roll-off in sensitivity decreased from 20–30 dB/octave to 10–15 dB/octave as the characteristic frequency of receptors increased from 3–11 kHz (Fig. 4A). For each individual receptor the slope, dynamic range and maximum spike response were similar for different sound frequencies (Fig. 9A). The receptors were tonotopically organized with the characteristic frequency of the receptors increasing from the proximal to the distal end of the array (Figs. 5, 6). Several receptors had characteristic frequencies of 5 kHz. These receptors were divided into two groups on the basis of their maximum spike response produced in response to pure tones of increasing intensity (Fig. 7). Independent of the tuning of the receptor no two-tone inhibition was observed in the periphery, thus confirming that such interactions are a property of central integration.