Auditory sensitivity and frequency selectivity in greater spear-nosed bats suggest specializations for acoustic communication

We investigated the relationship between auditory sensitivity, frequency selectivity, and the vocal repertoire of greater spear-nosed bats (Phyllostomus hastatus). P. hastatus commonly emit three types of vocalizations: group-specific foraging calls that range from 6 to 11 kHz, low amplitude echolocation calls that sweep from 80 to 40 kHz, and infant isolation calls from 15 to 100 kHz. To determine if hearing in P. hastatus is differentially sensitive or selective to frequencies in these calls, we determined absolute thresholds and masked thresholds using an operant conditioning procedure. Both absolute and masked thresholds were lowest at 15 kHz, which corresponds with the peak energy of isolation calls. Auditory and masked thresholds were higher at sound frequencies used for group-specific foraging calls and echolocation calls. Isolation calls meet the requirements of individual signatures and facilitate parent-offspring recognition. Many bat species produce isolation calls with peak energy between 10 and 25 kHz, which corresponds with the frequency region of highest sensitivity in those species for which audiogram data are available. These findings suggest that selection for accurate offspring recognition exerts a strong influence on the sensory system of P. hastatus and likely on other species of group-living bats.     

Hearing in the FM-bat Phyllostomus discolor: a behavioral audiogram

Absolute auditory thresholds of six adult lesser spear-nosed bats Phyllostomus discolor (Chiroptera, Phyllostomidae) were determined in a two-alternative forced-choice procedure. Behavioral responses to pure tone stimuli could be elicited throughout the tested frequency range of 5–142 kHz. The shape of the average audiogram is characterized by two sensitivity peaks and a pronounced increase of thresholds around 55 kHz, and towards the limits of the tested frequency range. The spectral extent of both sensitivity peaks shows a close relation to the bandwidth of two types of species-specific vocalizations. The first low threshold area (> 10 and < 55 kHz) of the audiogram seems perfectly adapted to the directive call used for intraspecific communication, whereas the second sensitivity peak, centered around 85 kHz, covers most of the bandwidth of the species' echolocation calls.Abbreviations CF constant frequency - FM frequency modulation - l left - r right - SPL Sound pressure level     

Correspondence between evoked vocal responses and auditory thresholds in Pleurodema thaul (Amphibia; Leptodactylidae)

Thresholds for evoked vocal responses and thresholds of multiunit midbrain auditory responses to pure tones and synthetic calls were investigated in males of Pleurodema thaul, as behavioral thresholds well above auditory sensitivity have been reported for other anurans. Thresholds for evoked vocal responses to synthetic advertisement calls played back at increasing intensity averaged 43 dB RMS SPL (range 31–52 dB RMS SPL), measured at the subjects’ position. Number of pulses increased with stimulus intensities, reaching a plateau at about 18–39 dB above threshold and decreased at higher intensities. Latency to call followed inverse trends relative to number of pulses. Neural audiograms yielded an average best threshold in the high frequency range of 46.6 dB RMS SPL (range 41–51 dB RMS SPL) and a center frequency of 1.9 kHz (range 1.7–2.6 kHz). Auditory thresholds for a synthetic call having a carrier frequency of 2.1 kHz averaged 44 dB RMS SPL (range 39–47 dB RMS SPL). The similarity between thresholds for advertisement calling and auditory thresholds for the advertisement call indicates that male P. thaul use the full extent of their auditory sensitivity in acoustic interactions, likely an evolutionary adaptation allowing chorusing activity in low-density aggregations.     

Low frequency hearing in cephalopods

Summary Classical conditioning was employed to test the sensitivity of cephalopods to vibrations between 1 and 100 Hz generated in a standing wave acoustic tube. The animals were trained to associate sound stimuli with a weak electric shock, and the recorded conditioned responses were changes in breathing and jetting activity. Five specimens of Sepia officinalis were tested, and all responded to these low frequency sounds. The relevant stimulus parameter was particle motion rather than sound pressure. The threshold values (measured as particle acceleration) decreased towards lower frequencies in the tested range, reaching values below 4 × 10^-3 m/s². The thresholds in the most sensitive range may have been masked by the considerable background noise at the experimental site (Naples). Two individuals of Octopus vulgaris and one Loligo vulgaris were also tested, and showed a similar sensitivity to low frequency sound.     

Response properties of primary auditory fibers in the cricketTeleogryllus oceanieus (Le Guillou)

Summary Single unit recordings of primary auditory fibers ofTeleogryllus oceanicus show responses to frequencies over the range 0.5 kHz to 42 kHz. The characteristic frequencies (ChFs) of units were distributed over much of the bandwidth investigated although few units were recorded with ChFs below 4 kHz or in the region 7 kHz to 10 kHz. Some units showed more than one peak of sensitivity and others were broad-banded with no tuning to a particular frequency. Units whose ChFs approximated to the carrier frequency (CF) of the proclamation song were the most highly tuned. The majority of units had a tonic response pattern and were not spontaneously active. The implications of these findings are discussed.Abbreviations ChF characteristic frequency - CF carrier frequency We thank Mr. P. Foster for techninical help.     

An exception to the matched filter hypothesis: A mismatch of male call frequency and female best hearing frequency in a torrent frog

The matched filter hypothesis proposes that the tuning of auditory sensitivity and the spectral character of calls will match in order to maximize auditory processing efficiency during courtship. In this study, we analyzed the acoustic structure of male calls and both male and female hearing sensitivities in the little torrent frog (Amolops torrentis), an anuran species who transmits acoustic signals across streams. The results were in striking contradiction to the matched filter hypothesis. Auditory brainstem response results showed that the best hearing range was 1.6–2 kHz consistent with the best sensitive frequency of most terrestrial lentic taxa, yet completely mismatched with the dominant frequency of conspecific calls (4.3 kHz). Moreover, phonotaxis tests show that females strongly prefer high‐frequency (4.3 kHz) over low‐frequency calls (1.6 kHz) regardless of ambient noise levels, although peripheral auditory sensitivity is highest in the 1.6–2 kHz range. These results are consistent with the idea that A. torrentis evolved from nonstreamside species and that high‐frequency calls evolved under the pressure of stream noise. Our results also suggest that female preferences based on central auditory system characteristics may evolve independently of peripheral auditory system sensitivity in order to maximize communication effectiveness in noisy environments.     

Resonance phenomena in the cochlea of the mustache bat and their contribution to neuronal response characteristics in the cochlear nucleus

Summary The cochlea of the mustache bat, Pteronotus parnellii, is very sensitive and sharply tuned to the frequency range of the dominant second harmonic of the echolocation call around 61 kHz. About 900 Hz above this frequency the cochlear microphonic potential (CM) reaches its maximum amplitude and lowest threshold. At exactly the same frequency, pronounced evoked otoacoustic emissions (OAE) can be measured in the outer ear canal, indicating mechanical resonance. The CM amplitude maximum and the OAE are most severely masked by simultaneous exposure to tones within the range from about 61–62 kHz up to about 70 kHz. The data suggest that the mechanism of mechanical resonance involves cochlear loci basal to the 61 kHz position.The resonance contributes to auditory sensitivity and sharp tuning: At the frequency of the OAE, single unit responses in the cochlear nucleus have the lowest thresholds. Maximum tuning sharpness occurs at frequencies about 300 Hz below the OAE-frequency, where the threshold is about 10 dB less sensitive than at the OAE-frequency. In addition, in the frequency range around the OAE-frequency several specialized neuronal response features can be related to mechanical resonance: Long lasting excitation after the end of the stimulus, asymmetrical tuning curves with a shallow high frequency slope and phasic on-off neuronal response patterns. In particular the latter phenomenon indicates the occurrence of local mechanical cancellations in the cochlea.Abbreviations CF constant frequency component of echolocation calls - CM cochlear microphonic potential - FM frequency modulated component of echolocation calls - N1 compound action potential of the auditory nerve - OAE octoacoustic emission - SEOAE synchronous evoked OAE     

Optimal degree of protonation for 1H detection of aliphatic sites in randomly deuterated proteins as a function of the MAS frequency

The ¹H dipolar network, which is the major obstacle for applying proton detection in the solid-state, can be reduced by deuteration, employing the RAP (Reduced Adjoining Protonation) labeling scheme, which yields random protonation at non-exchangeable sites. We present here a systematic study on the optimal degree of random sidechain protonation in RAP samples as a function of the MAS (magic angle spinning) frequency. In particular, we compare ¹H sensitivity and linewidth of a microcrystalline protein, the SH3 domain of chicken ??-spectrin, for samples, prepared with 5?C25?% H₂O in the E. coli growth medium, in the MAS frequency range of 20?C60?kHz. At an external field of 19.96?T (850?MHz), we find that using a proton concentration between 15 and 25?% in the M9 medium yields the best compromise in terms of sensitivity and resolution, with an achievable average ¹H linewidth on the order of 40?C50?Hz. Comparing sensitivities at a MAS frequency of 60 versus 20?kHz, a gain in sensitivity by a factor of 4?C4.5 is observed in INEPT-based ¹H detected 1D ¹H,¹³C correlation experiments. In total, we find that spectra recorded with a 1.3?mm rotor at 60?kHz have almost the same sensitivity as spectra recorded with a fully packed 3.2?mm rotor at 20?kHz, even though ~20×?less material is employed. The improved sensitivity is attributed to ¹H line narrowing due to fast MAS and to the increased efficiency of the 1.3?mm coil.     

Ontogenesis of auditory fovea representation in the inferior colliculus of the Sri Lankan rufous horseshoe bat,Rhinolophus rouxi

Summary This report describes the ontogenesis of tonotopy in the inferior colliculus (IC) of the rufous horseshoe bat (Rhinolophus rouxi). Horseshoe bats are deaf at birth, but consistent tonotopy with a low-to-high frequency gradient from dorsolateral to ventromedial develops from the 2nd up to the 5th week. The representation of the auditory fovea is established in ventro-mediocaudal parts of the IC during the 3rd postnatal week (Fig. 3). Then, a narrow frequency band 5 kHz in width, comprising 16% of the bat's auditory range, captures 50–60 vol% of the IC (Fig. 3c). However, foveal tuning is 10–12 kHz (1/3 octave) lower than in adults; foveal tuning in females (65–68 kHz) is 2–3 kHz higher than in males (62–65 kHz). Thereafter, foveal tuning increases by 1–1.5 kHz per day up to the 5th postnatal week, when the adult hearing range is established (Figs. 4, 5). The increase of sensitivity and of tuning sharpness of single units also follows a low-to-high frequency gradient (Fig. 6).Throughout this development the foveal tuning matches the second harmonic of the echolocation pulses vocalised by these young bats. The results confirm the hypothesis of developmental shifts in the frequency-place code for the foveal high frequency representation in the IC.Abbreviations BF best frequency - CF constant frequency - FM frequency modulation - IC inferior colliculus - IHC inner hair cell; - OHC outer hair cell - RR Rhinolophus rouxi     

Hearing in coruros (<Emphasis Type="Italic">Spalacopus cyanus</Emphasis>): special audiogram features of a subterranean rodent

Learning curves and behavioural audiograms of subterranean, socially living coruros (Spalacopus cyanus) were obtained using a positive reinforcement conditioning procedure. The individually varying audiograms revealed best hearing at frequencies between 1.25 and 1.6 kHz, which corresponds with the common pattern established in subterranean rodents studied so far. However, the broad hearing range covering frequencies at least between 0.25 and 20 kHz coupled with the high sensitivity (average minimum 7 dB) that is found in coruros are atypical features for audiograms of subterranean rodents, which usually show restricted high-frequency hearing ranges and very poor sensitivity. Hearing at low frequencies (peaks at frequencies <1 kHz), which may be related to sound transmission in underground burrows, and high sensitivity at 1.25/1.6 kHz are discussed in relation to vocalization. In addition to these peaks, a third peak at 8 kHz—probably a plesiomorphic feature of mammals—may be of significance in aboveground communication.     

The directionality of the ear of the pigeon (Columba livia)

Summary The directionality of cochlear microphonic potentials in the azimuthal plane was investigated in the pigeon (Columba livia), using acoustic free-field stimulation (pure tones of 0.25–6 kHz).At high frequencies in the pigeon's hearing range (4–6 kHz), changing azimuth resulted in a maximum change of the cochlear microphonic amplitude by about 20 dB (SPL). The directionality decreased clearly with decreasing frequency.Acoustic blocking of the contralateral ear canal could reduce the directional sensitivity of the ipsilateral ear by maximally 8 dB. This indicates a significant sound transmission through the bird's interaural pathways. However, the magnitude of these effects compared to those obtained by sound diffraction (maximum > 15 dB) suggests that pressure gradients at the tympanic membrane are only of subordinate importance for the generation of directional cues.The comparison of interaural intensity differences with previous behavioral results confirms the hypothesis that interaural intensity difference is the primary directional cue of azimuthal sound localization in the high-frequency range (2–6 kHz).Abbreviations CM cochlear microphonic potential - IID interaural intensity difference - IID-MRA minimum resolvable angle calculated from interaural intensity difference - MRA minimum resolvable angle - OTD interaural ongoing time difference - RMS root mean square - SPL sound pressure level     

Viscoelastic properties of protein crystals: Triclinic crystals of hen egg white lysozyme in different conditions

A technique for the measurement of the dynamic Young's modulus E and logarithmic decrement ?? of protein crystals and other microscopic samples by the resonance method modified to a microscale is described. Monoclinic crystals of horse hemoglobin and sperm whale myoglobin; triclinic hen egg white lysozyme crystals, crosslinked by glutaraldehyde; and native and crosslinked needlelike lysozyme crystals were studied, as were amorphous protein films. The E of wet protein crystals is shown to be in the range (3–15) × 10³ kg/cm², ?? = 0.3–0.7. The crosslinking does not significantly affect the values. General elastic properties were analyzed for triclinic lysozyme crystals. No frequency dependence of E and ?? was found over the frequency range of 2.5–65 kHz. The temperature dependence was found to be characteristic for glassy polymers; the decrement of Young's modulus was ?2.4 ± 0.1%/°C. The guanidine HCl denaturation caused a 1000-fold decrease of E, its temperature dependence becoming similar to that of rubberlike materials. Studies of pH and salt effects showed E to be influenced by ionization of the acidic groups at pH 3–4.5. A humidity decrease from 100 to 30% caused a three- to fourfold increase of E and a decrease of ??. The final values of E = (40–60) × 10³ kg/cm² and ?? ? 0.1 were the same for dry crystals and amorphous films, whether crosslinked or not. These values may be attributed to the protein globular material.     

Comparative collicular tonotopy in two bat species adapted to movement detection,Hipposideros speoris andMegaderma lyra

Summary The tonotopic organization of the inferior colliculus (IC) in two echolocating bats,Hipposideros speoris andMegaderma lyra, was studied by multiunit recordings.InHipposideros speoris frequencies below the range of the echolocation signals (i.e. below 120 kHz) are compressed into a dorsolateral cap about 400–600 m thick. Within this region, neuronal sheets of about 4–5 m thickness represent a 1 kHz-band.In contrast, the frequencies of the echolocation signals (120–140 kHz) are overrepresented and occupy the central and ventral parts of the IC (Fig. 3). In this region, neuronal sheets of about 80 m thickness represent a 1 kHz-band. The largest 1 kHz-slabs (400–600 m) represent frequencies of the pure tone components of the echolocation signals (130–140 kHz).The frequency of the pure tone echolocation component is specific for any given individual and always part of the overrepresented frequency range but did not necessarily coincide with the BF of the thickest isofrequency slab. Thus hipposiderid bats have an auditory fovea (Fig. 10).In the IC ofMegaderma lyra the complete range of audible frequencies, from a few kHz to 110 kHz, is represented in fairly equal proportions (Fig. 7). On the average, a neuronal sheet of 30 m thickness is dedicated to a 1 kHz-band, however, frequencies below 20 kHz, i.e. below the range of the echolocation signals, are overrepresented.Audiograms based on thresholds determined from multiunit recordings demonstrate the specific sensitivities of the two bat species. InHipposideros speoris the audiogram shows two sensitivity peaks, one in the nonecholocating frequency range (10–60 kHz) and one within the auditory fovea for echolocation (130–140 kHz).Megaderma lyra has extreme sensitivity between 15–20 kHz, with thresholds as low as –24 dB SPL, and a second sensitivity peak at 50 kHz (Fig. 8).InMegaderma lyra, as in common laboratory mammals, Q_10dB-values of single units do not exceed 30, whereas inHipposideros speoris units with BFs within the auditory fovea reach Q_10dB-values of up to 130.InMegaderma lyra, many single units and multiunit clusters with BFs below 30 kHz show upper thresholds of 40–50 dB SPL and respond most vigorously to sound intensities below 30 dB SPL (Fig. 9). Many of these units respond preferentially or exclusively to noise. These features are interpreted as adaptations to detection of prey-generated noises.The two different tonotopic arrangements (compare Figs. 3 and 7) in the ICs of the two species are correlated with their different foraging behaviours. It is suggested that pure tone echolocation and auditory foveae are primarily adaptations to echo clutter rejection for species foraging on the wing close to vegetation.Abbreviations BF Best frequency - CF constant frequency - FM frequency modulated - IC inferior colliculus - HS Hipposideros speoris     

Assessing the effect of different ultrasonic frequencies on bacterial viability using flow cytometry

Aims: This research investigated the effect of sonication at frequencies of 20, 40 and 580 kHz and approximately the same acoustic intensity on the viability and declumping of two micro‐organisms (Escherichia coli and Klebsiella pneumonia). Methods and Results: Two analytical methods were employed; viable plate counts (CFU ml^?1) and flow cytometry to identify and quantify both live/viable and dead bacteria in the bulk liquid. Flow cytometry results for E. coli and Kl. pneumonia indicated a high sensitivity to 20 and 40 kHz frequency with a continuous decrease in the viable cells and an increase in dead cells during experiments. In contrast, results using the higher frequency of 580 kHz indicate predominantly deagglomeration of bacterial clumps rather than cell membrane disruption (Joyce et al. 2003). Results indicate a good correlation between flow cytometry and viable plate count methodology. Conclusions: Sonication has two different effects on bacteria (i) inactivation and (ii) declumping; however, the scale of these effects is dependent on intensity and frequency. Flow cytometry provides a method to distinguish between and quantify the effects through the observation of two subpopulations: (i) live/viable and (ii) dead bacterial cells. Significance and Impact of the study: Treatment using power ultrasound has been shown to have a significant impact on microbial activity. This is the first time a study has compared the influence of a range of different frequencies, but at similar power settings on the survival of bacteria in phosphate buffer saline (PBS). This work is of importance for applications where ultrasound has been considered for use in industry as a means of disinfection including the treatment and pretreatment of water and also for the sterilization of liquid foods.     

Hearing of old world monkeys (Cercopithecinae)

The characteristics of normal hearing were examined in the laboratory for seven species of Old World monkeys. Operant conditioning procedures, coupled with standard audiometric testing methods, were used to assess thresholds of hearing, frequency range of hearing, and differential sensitivity to auditory intensity and frequency. To produce tonal stimulation, an animal was trained to touch and maintain manual contact with a contact-sensitive key and to report hearing the tone by lifting his hand from the key; this response was followed by food reinforcement. When the reporting response occurred without the auditory signal, the animal was punished by a short suspension from the experiment. Additional contingencies were added to ensure stable and reliable responding, and threshold and differential acuity determinations were then made. Threshold was defined as the stimulus value responded to correctly 50% of the time. The frequency range of hearing of all the cercopithecoids tested extended from 60 to 40,000 Hz, an octave above the upper bound of 20,000 Hz for man but well below the 60–70,000 Hz limit for some prosimians. Absolute sensitivity for tonal stimulation in the most sensitive frequency range (1–8 kHz) was about 2 × 10^?4 microbars, comparable to that of other primates tested, including man. Thus, the Old World monkey appears only slightly less sensitive than man to small changes in intensity and frequency of acoustic stimulation. At 1000 Hz at 60 dB above the threshold of audibility, his limit of resolution is about 5 Hz for frequency and 2 dB for intensity.     

Functional organization of the tympanal organ of the flour moth, Ephestia kuehniella

Data about electrical recordings from the tympanic organ of the flour moth, Ephestia kuehniella, to acoustic stimuli is presented. The stimuli had a frequency that ranged from 5 to 100 kHz, with minimal intensities of 40 to 50 db (Odb = 0.0002 dynes/cm²) and maximal up to 110 db. The tympanic organ of E. kuehniella responded in the whole range of frequencies used and showed two sensitivity maxima, one at 20 kHz and the other at 60 kHz. It responded from 45 to 110 db. The electrical activity of the tympanic nerve consisted of a spontaneous discharge of a type B cell and a tonic discharge in response to acoustic stimulation, produced by four acoustic sense cells, called A₁, A₂, A₃, and A₄. All these acoustic sense cells respond in the whole frequency range used and they differ in the heights of their action potentials and in their sensitivity to acoustic stimuli. The possible biological significance of hearing in pyralid moths is discussed.     

Size, peripheral auditory tuning and target strength in noctuid moths

We investigated relationships among body size, the frequency of peak auditory sensitivity (best frequency) and acoustic conspicuousness (measured as target strength) to simulated bat echolocation calls in a range of tympanate moths (Lepidoptera: Noctuidae). Audiograms of Amphipyra pyramidea Linnaeus, Agrotis exclamationis Linnaeus, Omphaloscelis lunosa Haworth and Xestia xanthographa Denis and Schiffermüller are described for the first time. Best frequency was inversely related to forewing length, an index of body size. Models predict that target strength falls off rapidly once wavelength (1/frequency) exceeds some defined feature of target size (e.g. circumference for spheres). We investigated how target strength varies in relation to target size and emitted frequency for simple targets (paper discs) and for moths. Target strength fell rapidly when target radius/wavelength < 2 for paper discs of similar size to many noctuid moths. Target strength fell rapidly below wing‐length/wavelength ratios of 2 in relatively small (O. lunosa, wing‐length = 15.2 ± 0.4 mm, best frequency = 45 kHz) and large (N. pronuba, wing‐length = 24.6 ± 0.8 mm, best frequency = 15 kHz) noctuid species, and decreased rapidly at frequencies below 25 kHz in both species. These target strengths were used to predict the detection distance of the moths by bat sonar between 10 and 55 kHz. Predicted detection distances of both species were maximal for fictive call frequencies of 20 kHz, and were reduced at lower frequencies due to decreased target strength and at higher frequencies by excess atmospheric attenuation. Both relatively large and small noctuid moths are therefore strong acoustic targets to bats that echolocate at relatively low frequencies. Bats may emit allotonic calls at low frequency because the costs of reduced detection range are smaller than the benefits of reduced audibility to moths. Because best frequency scales with body size and maximum detection distance is not very sensitive to body size, noctuid moths in the size range examined do not necessarily have best frequencies that would match the call frequencies of bats that may detect the moths at greatest distance precisely. Hence, best frequency may be constrained in part by body size.     

Evoked potentials of the auditory cortex of the porpoise,Phocoena phocoena

Summary Evoked potential (EP) recordings in the auditory cortex of the porpoise,Phocoena phocoena, were used to obtain data characterizing the auditory perception of this dolphin. The frequency threshold curves showed that the lowest EP thresholds were within 120–130 kHz. An additional sensitivity peak was observed between 20 and 30 kHz. The minimal EP threshold to noise burst was 3·10^–4–10^/s-3 Pa. The threshold for response to modulations in sound intensity was below 0.5 dB and about 0.1% for frequency modulations. Special attention was paid to the dependence of the auditory cortex EP on the temporal parameters of the acoustic stimuli: sound burst duration, rise time, and repetition rate. The data indicate that the porpoise auditory cortex is adapted to detect ultrasonic, brief, fast rising, and closely spaced sounds like echolocating clicks.Abbreviation EP evoked potential     

HEARING IN THE RED-BILLED FIREFINCH LAGON O S TIC TA SENEGALA AND THE SPANISH TIMBRADO CANARY SERINUS CANARIA: THE INFLUENCE OF NATURAL AND ARTIFICIAL SELECTION ON AUDITORY ABILITIES AND VOCAL STRUCTURE

ABSTRACT

We tested the auditory sensitivity of red-billed firefinches Lagonosticta senegal0061 and Spanish timbrado canaries Serinus canaria. Both these species produce songs and calls that are narrowband and relatively high in frequency, with spectral energy falling predominantly in the region of 3–6 kHz. Hearing thresholds were measured in these two species and compared to the auditory sensitivity of closely related species: the well studied zebra finch Taeniopygia guttata, and other strains of canary bred for song. Auditory thresholds were similar in both groups of birds, with firefinches having an audiogram typical for that of small birds. Timbrado canaries exhibited an audiogram with its greatest sensitivity in the relatively high region of 4–6 kHz, corresponding to the peak frequency of its calls. Critical ratios measured over a range of several octaves increased in a monotonie fashion at a rate of 2–3 dB per octave for both firefinches and timbrado canaries. Critical ratios in these two species are similar to what has been found in most other small passerine species, suggesting spectral resolving abilities similar to most small birds tested to date.     

Hearing and frequency dependence of auditory interneurons in the parasitoid fly <Emphasis Type="Italic">Homotrixa alleni</Emphasis> (Tachinidae: Ormiini)

The parasitoid tachinid fly Homotrixa alleni detects its hosts by their acoustic signals. The tympanal organ of the fly is located at the prothorax and contains scolopidial sensory units of different size and orientation. The tympanal membrane vibrates in the frequency range of approximately 4–35 kHz, which is also reflected in the hearing threshold measured at the neck connective. The auditory organ is not tuned to the peak frequency (5 kHz) of the main host, the bush cricket Sciarasaga quadrata. Auditory afferents project in the three thoracic neuromeres. Most of the ascending interneurons branch in all thoracic neuromeres and terminate in the deutocerebrum of the brain. The interneurons do not differ considerably in frequency tuning, but in their sensitivity with lowest thresholds around 30 dB SPL. Suprathreshold responses of most neurons depend on frequency and intensity, indicating inhibitory influence at higher intensities. Some neurons respond particularly well at low frequency sounds (around 5 kHz) and high intensities (80–90 dB SPL), and thus may be involved in detection of the primary host, S. quadrata. The auditory system of H. alleni contains auditory interneurons reacting in a wide range of temporal patterns from strictly phasic to tonic and with clear differences in frequency responses.