Otoacoustic emissions from insect ears having just one auditory neuron

Hearing diversity in moths confronting a neotropical bat assemblage

The tympanal ear is an evolutionary acquisition which helps moths survive predation from bats. The greater diversity of bats and echolocation strategies in the Neotropics compared with temperate zones would be expected to impose different sensory requirements on the neotropical moths. However, even given some variability among moth assemblages, the frequencies of best hearing of moths from different climate zones studied to date have been roughly the same: between 20 and 60 kHz. We have analyzed the auditory characteristics of tympanate moths from Cuba, a neotropical island with high levels of bat diversity and a high incidence of echolocation frequencies above those commonly at the upper limit of moths' hearing sensitivity. Moths of the superfamilies Noctuoidea, Geometroidea and Pyraloidea were examined. Audiograms were determined by non-invasively measuring distortion-product otoacoustic emissions. We also quantified the frequency spectrum of the echolocation sounds to which this moth community is exposed. The hearing ranges of moths in our study showed best frequencies between 36 and 94 kHz. High sensitivity to frequencies above 50 kHz suggests that the auditory sensitivity of moths is suited to the sounds used by sympatric echolocating bat fauna. Biodiversity characterizes predators and prey in the Neotropics, but the bat-moth acoustic interaction keeps spectrally matched.

Unexpected dynamic up-tuning of auditory organs in day-flying moths

In certain nocturnal moth species the frequency range of best hearing shifts to higher frequencies during repeated sound stimulation. This could provide the moths with a mechanism to better detect approaching echolocating bats. However, such a dynamic up-tuning would be of little value for day-flying moths that use intra-specific acoustic communication. Here we examined if the ears of day-flying moths provide stable tuning during longer sound stimulation. Contrary to our expectations, dynamic up-tuning was found in the ear of the day-flying species Urania boisduvalii and Empyreuma pugione. Audiograms were measured with distortion-product otoacoustic emissions (DPOAEs). The level of the dominant distortion product (i.e. 2f1-f2) varied as a function of time by as much as 45 dB during ongoing acoustic stimulation, showing a systematic decrease at low frequencies and an increase at high frequencies. As a consequence, within about 2 s of acoustic stimulation, the DPOAEs audiogram shifted from low to high frequencies. Despite the up-tuning, the range of best audition still fell within the frequency band of the species-specific communication signals, suggesting that intra-specific communication should not be affected adversely. Up-tuning could be an ancestral condition in moth ears that in day-flying moths does not underlie larger selection pressure.

Mechanical basis of otoacoustic emissions in tympanal hearing organs

Tympanal hearing organs of insects emit distortion-product otoacoustic emissions (DPOAEs), which in mammals are used as indicator for nonlinear cochlear amplification, and which are highly vulnerable to manipulations interfering with the animal's physiological state. Although in previous studies, evidence was provided for the involvement of auditory mechanoreceptors, the source of DPOAE generation and possible active mechanisms in tympanal organs remained unknown. Using laser Doppler vibrometry in the locust ear, we show that DPOAEs mechanically emerge at the tympanum region where the auditory mechanoreceptors are attached. Those emission-coupled vibrations differed remarkably from tympanum waves evoked by external pure tones of the same frequency, in terms of wave propagation, energy distribution, and location of amplitude maxima. Selective inactivation of the auditory receptor cells by mechanical lesions did not affect the tympanum's response to external pure tones, but abolished the emission's displacement amplitude peak. These findings provide evidence that tympanal auditory receptors, comparable to the situation in mammals, comprise the required nonlinear response characteristics, which during two-tone stimulation lead to additional, highly localized deflections of the tympanum.

Mechanical tuning of the moth ear: distortion-product otoacoustic emissions and tympanal vibrations

The mechanical tuning of the ear in the moth Empyreuma pugione was investigated by distortion-product otoacoustic emissions (DPOAE) and laser Doppler vibrometry (LDV). DPOAE audiograms were assessed using a novel protocol that may be advantageous for non-invasive auditory studies in insects. To evoke DPOAE, two-tone stimuli within frequency and level ranges that generated a large matrix of values (960 frequency-level combinations) were used to examine the acoustic space in which the moth tympanum shows its best mechanical and acoustical responses. The DPOAE tuning curve derived from the response matrix resembles that obtained previously by electrophysiology, and is V-shaped and tuned to frequencies between 25 and 45 kHz with low Q10dB values of 1.21±0.26. In addition, while using a comparable stimulation regime, mechanical distortion in the displacement of the moth's tympanal membrane at the stigma was recorded with a laser Doppler vibrometer. The corresponding mechanical vibration audiograms were compared with DPOAE audiograms. Both types of audiograms have comparable shape, but most of the mechanical response fields are shifted towards lower frequencies. We showed for the first time in moths that DPOAE have a pronounced analogy in the vibration of the tympanic membrane where they may originate. Our work supports previous studies that point to the stigma (and the internally associated transduction machinery) as an important place of sound amplification in the moth ear, but also suggests a complex mechanical role for the rest of the transparent zone.

Temperature dependence of distortion-product otoacoustic emissions in tympanal organs of locusts

Distortion-product otoacoustic emissions (DPOAEs) in tympanal organs of insects are vulnerable to manipulations that interfere with the animal's physiological state. Starting at a medium temperature, we raised and lowered the locust's body temperature within the range of 12 to 35°C by changing the temperature of the surrounding air, while recording DPOAEs. These experimental manipulations resulted in reversible amplitude changes of the 2f1–f2 emission, which were dependent on stimulus frequency and level. Using low f2 frequencies of up to 10 kHz, a temperature increase (median +8–9°C) led to an upward shift of DPOAE amplitudes of approximately +10 dB, whereas a temperature decrease (median −7°C) was followed by a reduction of DPOAE amplitudes by 3 to 5 dB. Both effects were only present in the range of the low-level component of DPOAE growth functions below L2 levels (levels of the f2 stimulus) of approximately 30 dB SPL. DPOAEs evoked by higher stimulus levels as well as measurements using higher stimulation frequencies above 10 kHz remained unaffected by any temperature shifts. The Arrhenius activation energy was calculated from the −10 dB SPL thresholds (representing the low-level component) of growth functions, which had been measured with 8 and 10 kHz as f2 frequencies and amounted to up to ~34 and 41 kJ mol−1, respectively. Such activation energy values provide a hint that the dynein-tubulin system within the scolopidial receptors could play an essential part in the DPOAE generation in tympanal organs.

Otoacoustic emissions in bushcricket ears: general characteristics and the influence of the neuroactive insecticide pymetrozine

The tympanal organ of the bushcricket Mecopoda elongata emits pronounced distortion-product otoacoustic emissions (DPOAEs). Their characteristics are comparable to those measured in other insects, such as locusts and moths, with the 2f1-f2 emission being the most prominent one. Yet the site of their generation is still unclear. The spatial separation between the sound receiving spiracle and the hearing organ in this species allows manipulations of the sensory cells without interfering with the acoustical measurements. We tried to interfere with the DPOAE generation by pharmacologically influencing the tympanal organ using the insecticide pymetrozine. The compound appears to act selectively on scolopidia, i.e., the mechanosensor type characteristically constituting tympanal organs. Pymetrozine solutions were applied as closely as possible to the scolopidia via a cuticle opening in the tibia, distally to the organ. Applications of pymetrozine at concentrations between 10(-3) and 10(-7) M to the tympanal organ led to a pronounced and irreversible decrease of the DPOAE amplitudes.

Otoacoustic emissions from insect ears: evidence of active hearing?

Sensitive hearing organs often employ nonlinear mechanical sound processing which generates distortion-product otoacoustic emissions (DPOAE). Such emissions are also recordable from tympanal organs of insects. In vertebrates (including humans), otoacoustic emissions are considered by-products of active sound amplification through specialized sensory receptor cells in the inner ear. Force generated by these cells primarily augments the displacement amplitude of the basilar membrane and thus increases auditory sensitivity. As in vertebrates, the emissions from insect ears are based on nonlinear mechanical properties of the sense organ. Apparently, to achieve maximum sensitivity, convergent evolutionary principles have been realized in the micromechanics of these hearing organs-although vertebrates and insects possess quite different types of receptor cells in their ears. Just as in vertebrates, otoacoustic emissions from insects ears are vulnerable and depend on an intact metabolism, but so far in tympanal organs, it is not clear if auditory nonlinearity is achieved by active motility of the sensory neurons or if passive cellular characteristics cause the nonlinear behavior. In the antennal ears of flies and mosquitoes, however, active vibrations of the flagellum have been demonstrated. Our review concentrates on experiments studying the tympanal organs of grasshoppers and moths; we show that their otoacoustic emissions are produced in a frequency-specific way and can be modified by electrical stimulation of the sensory cells. Even the simple ears of notodontid moths produce distinct emissions, although they have just one auditory neuron. At present it is still uncertain, both in vertebrates and in insects, if the nonlinear amplification so essential for sensitive sound processing is primarily due to motility of the somata of specialized sensory cells or to active movement of their (stereo-)cilia. We anticipate that further experiments with the relatively simple ears of insects will help answer these questions.