Mechanical signatures of transducer gating in the Drosophila ear

Distinct roles of TRP channels in auditory transduction and amplification in Drosophila

Auditory receptor cells rely on mechanically gated channels to transform sound stimuli into neural activity. Several TRP channels have been implicated in Drosophila auditory transduction, but mechanistic studies have been hampered by the inability to record subthreshold signals from receptor neurons. Here, we develop a non-invasive method for measuring these signals by recording from a central neuron that is electrically coupled to a genetically defined population of auditory receptor cells. We find that the TRPN family member NompC, which is necessary for the active amplification of sound-evoked motion by the auditory organ, is not required for transduction in auditory receptor cells. Instead, NompC sensitizes the transduction complex to movement and precisely regulates the static forces on the complex. In contrast, the TRPV channels Nanchung and Inactive are required for responses to sound, suggesting they are components of the transduction complex. Thus, transduction and active amplification are genetically separable processes in Drosophila hearing.

Cell-type-specific roles of Na+/K+ ATPase subunits in Drosophila auditory mechanosensation

Ion homeostasis is a fundamental cellular process particularly important in excitable cell activities such as hearing. It relies on the Na(+)/K(+) ATPase (also referred to as the Na pump), which is composed of a catalytic α subunit and a β subunit required for its transport to the plasma membrane and for regulating its activity. We show that α and β subunits are expressed in Johnston's organ (JO), the Drosophila auditory organ. We knocked down expression of α subunits (ATPα and α-like) and β subunits (nrv1, nrv2, and nrv3) individually in JO with UAS/Gal4-mediated RNAi. ATPα shows elevated expression in the ablumenal membrane of scolopale cells, which enwrap JO neuronal dendrites in endolymph-like compartments. Knocking down ATPα, but not α-like, in the entire JO or only in scolopale cells using specific drivers, resulted in complete deafness. Among β subunits, nrv2 is expressed in scolopale cells and nrv3 in JO neurons. Knocking down nrv2 in scolopale cells blocked Nrv2 expression, reduced ATPα expression in the scolopale cells, and caused almost complete deafness. Furthermore, knockdown of either nrv2 or ATPα specifically in scolopale cells causes abnormal, electron-dense material accumulation in the scolopale space. Similarly, nrv3 functions in JO but not in scolopale cells, suggesting neuron specificity that parallels nrv2 scolopale cell-specific support of the catalytic ATPα. Our studies provide an amenable model to investigate generation of endolymph-like extracellular compartments.

Dissection of gain control mechanisms in Drosophila mechanotransduction

Mechanoreceptor cells respond to a vast span of stimulus intensities, which they transduce into a limited response-range using a dynamic regulation of transduction gain. Weak stimuli are detected by enhancing the gain of responses through the process of active mechanical amplification. To preserve responsiveness, the gain of responses to prolonged activation is rapidly reduced through the process of adaptation. We investigated long-term processes of mechanotransduction gain control by studying responses from single mechanoreceptor neurons in Drosophila. We found that mechanical stimuli elicited a sustained reduction of gain that we termed long-term adaptation. Long-term adaptation and the adaptive decay of responses during stimuli had distinct kinetics and they were independently affected by manipulations of mechanotransduction. Therefore, long-term adaptation is not associated with the reduction of response gain during stimulation. Instead, the long-term adaptation suppressed canonical features of active amplification which were the high gain of weak stimuli and the spontaneous emission of noise. In addition, depressing amplification using energy deprivation recapitulated the effects of long-term adaptation. These data suggest that long-term adaptation is mediated by suppression of active amplification. Finally, the extent of long-term adaptation matched with cytoplasmic Ca(2+) levels and dTrpA1-induced Ca(2+) elevation elicited the effects of long-term adaptation. Our data suggest that mechanotransduction employs parallel adaptive mechanisms: while a rapid process exerts immediate gain reduction, long-term adjustments are achieved by attenuating active amplification. The slow adjustment of gain, manifest as diminished sensitivity, is associated with the accumulation of Ca(2+).

Drosophila auditory organ genes and genetic hearing defects

The Drosophila auditory organ shares equivalent transduction mechanisms with vertebrate hair cells, and both are specified by atonal family genes. Using a whole-organ knockout strategy based on atonal, we have identified 274 Drosophila auditory organ genes. Only four of these genes had previously been associated with fly hearing, yet one in five of the genes that we identified has a human cognate that is implicated in hearing disorders. Mutant analysis of 42 genes shows that more than half of them contribute to auditory organ function, with phenotypes including hearing loss, auditory hypersusceptibility, and ringing ears. We not only discover ion channels and motors important for hearing, but also show that auditory stimulus processing involves chemoreceptor proteins as well as phototransducer components. Our findings demonstrate mechanosensory roles for ionotropic receptors and visual rhodopsins and indicate that different sensory modalities utilize common signaling cascades.

Direct gating and mechanical integrity of Drosophila auditory transducers require TRPN1

The elusive transduction channels for hearing are directly gated mechanically by the pull of gating springs. We found that the transient receptor potential (TRP) channel TRPN1 (NOMPC) is essential for this direct gating of Drosophila auditory transduction channels and that the channel-spring complex was disrupted if TRPN1 was lost. Our results identify TRPN1 as a mechanical constituent of the fly's auditory transduction complex that may act as the channel and/or gating spring.

Neural representations of courtship song in the Drosophila brain

Acoustic communication in drosophilid flies is based on the production and perception of courtship songs, which facilitate mating. Despite decades of research on courtship songs and behavior in Drosophila, central auditory responses have remained uncharacterized. In this study, we report on intracellular recordings from central neurons that innervate the Drosophila antennal mechanosensory and motor center (AMMC), the first relay for auditory information in the fly brain. These neurons produce graded-potential (nonspiking) responses to sound; we compare recordings from AMMC neurons to extracellular recordings of the receptor neuron population [Johnston's organ neurons (JONs)]. We discover that, while steady-state response profiles for tonal and broadband stimuli are significantly transformed between the JON population in the antenna and AMMC neurons in the brain, transient responses to pulses present in natural stimuli (courtship song) are not. For pulse stimuli in particular, AMMC neurons simply low-pass filter the receptor population response, thus preserving low-frequency temporal features (such as the spacing of song pulses) for analysis by postsynaptic neurons. We also compare responses in two closely related Drosophila species, Drosophila melanogaster and Drosophila simulans, and find that pulse song responses are largely similar, despite differences in the spectral content of their songs. Our recordings inform how downstream circuits may read out behaviorally relevant information from central neurons in the AMMC.

Drosophila nociceptors mediate larval aversion to dry surface environments utilizing both the painless TRP channel and the DEG/ENaC subunit, PPK1

A subset of sensory neurons embedded within the Drosophila larval body wall have been characterized as high-threshold polymodal nociceptors capable of responding to noxious heat and noxious mechanical stimulation. They are also sensitized by UV-induced tissue damage leading to both thermal hyperalgesia and allodynia very similar to that observed in vertebrate nociceptors. We show that the class IV multiple-dendritic(mdIV) nociceptors are also required for a normal larval aversion to locomotion on to a dry surface environment. Drosophila melanogaster larvae are acutely susceptible to desiccation displaying a strong aversion to locomotion on dry surfaces severely limiting the distance of movement away from a moist food source. Transgenic inactivation of mdIV nociceptor neurons resulted in larvae moving inappropriately into regions of low humidity at the top of the vial reflected as an increased overall pupation height and larval desiccation. This larval lethal desiccation phenotype was not observed in wild-type controls and was completely suppressed by growth in conditions of high humidity. Transgenic hyperactivation of mdIV nociceptors caused a reciprocal hypersensitivity to dry surfaces resulting in drastically decreased pupation height but did not induce the writhing nocifensive response previously associated with mdIV nociceptor activation by noxious heat or harsh mechanical stimuli. Larvae carrying mutations in either the Drosophila TRP channel, Painless, or the degenerin/epithelial sodium channel subunit Pickpocket1(PPK1), both expressed in mdIV nociceptors, showed the same inappropriate increased pupation height and lethal desiccation observed with mdIV nociceptor inactivation. Larval aversion to dry surfaces appears to utilize the same or overlapping sensory transduction pathways activated by noxious heat and harsh mechanical stimulation but with strikingly different sensitivities and disparate physiological responses.

No evidence for DPOAEs in the mechanical motion of the locust tympanum

Distortion-product otoacoustic emissions (DPOAEs) are present in non-linear hearing organs, and for low-intensity sounds are a by-product of active processes. In vertebrate ears they are considered to be due to hair cell amplification of sound in the cochlea; however, certain animals lacking a cochlea and hair cells are also reported to be capable of DPOAEs. In the Insecta, DPOAEs have been recorded from the locust auditory organ. However, the site of generation of these DPOAEs and the physiological mechanisms causing their presence in the locust ear are not yet understood, despite there being a number of potential places in the tympanal organ that could be capable of generating DPOAEs. This study aimed to record locust tympanal membrane vibration using a laser Doppler vibrometer in order to identify a distinct place of DPOAE generation on the membrane. Two species of locust were investigated over a range of frequencies and levels of acoustic stimulus, mirroring earlier acoustic recording studies; however, the current experiments were carried out in an open acoustic system. The laser measurements did not find any evidence of mechanical motion on the tympanal membrane related to the expected DPOAE frequencies. The results of the current study therefore could not confirm the presence of DPOAEs in the locust ear through the mechanics of the tympanal membrane. Experiments were also carried out to test how membrane behaviour altered when the animals were in a state of hypoxia, as this was previously found to decrease DPOAE magnitude, suggesting a metabolic sensitivity. However, hypoxia did not have any significant effect on the membrane mechanics. The location of the mechanical generation of DPOAEs in the locust's ear, and therefore the basis for the related physiological mechanisms, thus remains unknown.

Level-dependent auditory tuning: Transducer-based active processes in hearing and best-frequency shifts

Ears boost their sensitivity by means of active, force-generating processes that augment the minute vibrations induced by soft sounds. These processes can alter auditory frequency-tuning in a level-dependent way. In the antennal hearing organ of Drosophila, for example, the active process shifts the best frequency (BF) of the antennal sound receiver when the sound intensity is varied, tuning the receiver to conspecific songs. Here we show that this level-dependent tuning can be reproduced by an active transduction model as proposed for vertebrate hair cells and the Drosophila ear. We further show that the direction of the frequency shift depends on the system to which the molecular modules for auditory transduction connect: If this system is mass-less such as the sensory hair bundles of bullfrog saccular hair cells, the BF of the displacement response will increase as the sound intensity declines. Conversely, BF will decrease with declining intensity if the transduction modules couple to inertial systems such as the fly's antennal sound receiver or cupulae in the fish lateral line.

NompC TRP channel is essential for Drosophila sound receptor function

The idea that the NompC TRPN1 channel is the Drosophila transducer for hearing has been challenged by remnant sound-evoked nerve potentials in nompC nulls. We now report that NompC is essential for the function of Drosophila sound receptors and that the remnant nerve potentials of nompC mutants are contributed by gravity/wind receptor cells. Ablating the sound receptors reduces the amplitude and sensitivity of sound-evoked nerve responses, and the same effects ensued from mutations in nompC. Ablating the sound receptors also suffices to abolish mechanical amplification, which arises from active receptor motility, is linked to transduction, and also requires NompC. Calcium imaging shows that the remnant nerve potentials in nompC mutants are associated with the activity of gravity/wind receptors and that the sound receptors of the mutants fail to respond to sound. Hence, Drosophila sound receptors require NompC for mechanical signal detection and amplification, demonstrating the importance of this transient receptor potential channel for hearing and reviving the idea that the fly's auditory transducer might be NompC.

Active process mediates species-specific tuning of Drosophila ears

The courtship behavior of Drosophilid flies has served as a long-standing model for studying the bases of animal communication. During courtship, male flies flap their wings to send a complex pattern of airborne vibrations to the antennal ears of the females. These "courtship songs" differ in their spectrotemporal composition across species and are considered a crucial component of the flies' premating barrier. However, whether the species-specific differences in song structure are also reflected in the receivers of this communication system, i.e., the flies' antennal ears, has remained unexplored. Here we show for seven members of the melanogaster species group that (1) their ears are mechanically tuned to different best frequencies, (2) the ears' best frequencies correlate with high-frequency pulses of the conspecific courtship songs, and (3) the species-specific tuning relies on amplificatory mechanical feedback from the flies' auditory neurons. As a result of its level-dependent nature, the active mechanical feedback amplification is particularly useful for the detection of small stimuli, such as conspecific song pulses, and becomes negligible for sensing larger stimuli, such as the flies' own wingbeat during flight.

The role of the TRP channel NompC in Drosophila larval and adult locomotion

The generation of coordinated body movements relies on sensory feedback from mechanosensitive proprioceptors. We have found that the proper function of NompC, a putative mechanosensitive TRP channel, is not only required for fly locomotion, but also crucial for larval crawling. Calcium imaging revealed that NompC is required for the activation of two subtypes of sensory neurons during peristaltic muscle contractions. Having isolated a full-length nompC cDNA with a protein coding sequence larger than previously predicted, we demonstrate its function by rescuing locomotion defects in nompC mutants, and further show that antibodies against the extended C terminus recognize NompC in chordotonal ciliary tips. Moreover, we show that the ankyrin repeats in NompC are required for proper localization and function of NompC in vivo and are required for association of NompC with microtubules. Taken together, our findings suggest that NompC mediates proprioception in locomotion and support its role as a mechanosensitive channel.

Transcuticular optical imaging of stimulus-evoked neural activities in the Drosophila peripheral nervous system

The nervous system of Drosophila is widely used to study neuronal signal processing because the activities of neurons can be controlled and monitored by cell type-specific expression of genetically encoded actuator and sensor proteins. Measuring neural activities in adult flies, however, usually requires surgical approaches to penetrate the firm and pigmented cuticular exoskeleton. Interfering with this exoskeleton is critical in the case of the peripheral nervous system (PNS), as sensory neurons are often located directly beneath the cuticle and are associated with specialized stimulus-receiving and -conducting cuticular structures. In this article, we describe how the activities of these neurons can be probed nondestructively through the cuticle if a genetically encoded fluorescent protein sensor with strong baseline fluorescence is used. The method is exemplified for mechanosensory neurons in the adult antenna but can also be applied to many other PNS neurons, as is shown for the femoral chordotonal organ located in the fly's leg.

Antennal hearing in insects--new findings, new questions

Mosquitoes, certain Drosophila species, and honey bees use Johnston's organ in their antennae to detect the wing-beat sounds of conspecifics. Recent studies on these insects have provided novel insights into the intricacies of insect hearing and sound communication, with main discoveries including transduction and amplification mechanisms as known from vertebrate hearing, functional and molecular diversifications of mechanosensory cells, and complex mating duets that challenge the frequency-limits of insect antennal ears. This review discusses these recent advances and outlines potential avenues for future research.

A doublecortin containing microtubule-associated protein is implicated in mechanotransduction in Drosophila sensory cilia

Mechanoreceptors are sensory cells that transduce mechanical stimuli into electrical signals and mediate the perception of sound, touch and acceleration. Ciliated mechanoreceptors possess an elaborate microtubule cytoskeleton that facilitates the coupling of external forces to the transduction apparatus. In a screen for genes preferentially expressed in Drosophila campaniform mechanoreceptors, we identified DCX-EMAP, a unique member of the EMAP family (echinoderm-microtubule-associated proteins) that contains two doublecortin domains. DCX-EMAP localizes to the tubular body in campaniform receptors and to the ciliary dilation in chordotonal mechanoreceptors in Johnston's organ, the fly's auditory organ. Adult flies carrying a piggyBac insertion in the DCX-EMAP gene are uncoordinated and deaf and display loss of mechanosensory transduction and amplification. Electron microscopy of mutant sensilla reveals loss of electron-dense materials within the microtubule cytoskeleton in the tubular body and ciliary dilation. Our results establish a catalogue of candidate genes for Drosophila mechanosensation and show that one candidate, DCX-EMAP, is likely to be required for mechanosensory transduction and amplification.

Unraveling the auditory system of Drosophila

Acoustic communication in flies is based on the production and perception of courtship song. Drosophila males sing to females during the courtship ritual, while females listen for the correct species-specific song parameters before deciding to mate. While we know that song is important for mating, the neural mechanisms involved in song recognition remain mysterious. However, the last few years have seen major advances in our understanding of the auditory system of Drosophila, including delineation of the neurons involved in song production, detailed characterization of the auditory receptor organ, and mapping of auditory projections into the brain. The stage is being set to tackle the auditory system of Drosophila in much the same way as has been done for its olfactory system. This review covers recent work and discusses prospects for future research on Drosophila audition.

Hearing in Drosophila requires TilB, a conserved protein associated with ciliary motility

Cilia were present in the earliest eukaryotic ancestor and underlie many biological processes ranging from cell motility and propulsion of extracellular fluids to sensory physiology. We investigated the contribution of the touch insensitive larva B (tilB) gene to cilia function in Drosophila melanogaster. Mutants of tilB exhibit dysfunction in sperm flagella and ciliated dendrites of chordotonal organs that mediate hearing and larval touch sensitivity. Mutant sperm axonemes as well as sensory neuron dendrites of Johnston's organ, the fly's auditory organ, lack dynein arms. Through deficiency mapping and sequencing candidate genes, we identified tilB mutations in the annotated gene CG14620. A genomic CG14620 transgene rescued deafness and male sterility of tilB mutants. TilB is a 395-amino-acid protein with a conserved N-terminal leucine-rich repeat region at residues 16-164 and a coiled-coil domain at residues 171-191. A tilB-Gal4 transgene driving fluorescently tagged TilB proteins elicits cytoplasmic expression in embryonic chordotonal organs, in Johnston's organ, and in sperm flagella. TilB does not appear to affect tubulin polyglutamylation or polyglycylation. The phenotypes and expression of tilB indicate function in cilia construction or maintenance, but not in intraflagellar transport. This is also consistent with phylogenetic association of tilB homologs with presence of genes encoding axonemal dynein arm components. Further elucidation of tilB functional mechanisms will provide greater understanding of cilia function and will facilitate understanding ciliary diseases.

Pickpocket is a DEG/ENaC protein required for mechanical nociception in Drosophila larvae

Highly branched class IV multidendritic sensory neurons of the Drosophila larva function as polymodal nociceptors that are necessary for behavioral responses to noxious heat (>39 degrees C) or noxious mechanical (>30 mN) stimuli. However, the molecular mechanisms that allow these cells to detect both heat and force are unknown. Here, we report that the pickpocket (ppk) gene, which encodes a Degenerin/Epithelial Sodium Channel (DEG/ENaC) subunit, is required for mechanical nociception but not thermal nociception in these sensory cells. Larvae mutant for pickpocket show greatly reduced nociception behaviors in response to harsh mechanical stimuli. However, pickpocket mutants display normal behavioral responses to gentle touch. Tissue-specific knockdown of pickpocket in nociceptors phenocopies the mechanical nociception impairment without causing defects in thermal nociception behavior. Finally, optogenetically triggered nociception behavior is unaffected by pickpocket RNAi, which indicates that ppk is not generally required for the excitability of the nociceptors. Interestingly, DEG/ENaCs are known to play a critical role in detecting gentle touch stimuli in Caenorhabditis elegans and have also been implicated in some aspects of harsh touch sensation in mammals. Our results suggest that neurons that detect harsh touch in Drosophila utilize similar mechanosensory molecules.

The dynein-tubulin motor powers active oscillations and amplification in the hearing organ of the mosquito

The design principles and specific proteins of the dynein-tubulin motor, which powers the flagella and cilia of eukaryotes, have been conserved throughout the evolution of life from algae to humans. Cilia and flagella can support both motile and sensory functions independently, or sometimes in parallel to each other. In this paper we show that this dual sensory-motile role of eukaryotic cilia is preserved in the most sensitive of all invertebrate hearing organs, the Johnston's organ of the mosquito. The Johnston's organ displays spontaneous oscillations, which have been identified as being a characteristic of amplification in the ears of mosquitoes and Drosophila. In the auditory organs of Drosophila and vertebrates, the molecular basis of amplification has been attributed to the gating and adaptation of the mechanoelectrical transducer channels themselves. On the basis of their temperature-dependence and sensitivity to colchicine, we attribute the molecular basis of spontaneous oscillations by the Johnston's organ of the mosquito Culex quinquefasciatus, to the dynein-tubulin motor of the ciliated sensillae. If, as has been claimed for insect and vertebrate hearing organs, spontaneous oscillations epitomize amplification, then in the mosquito ear, this process is independent of mechanotransduction.

The origin of adaptation in the auditory pathway of locusts is specific to cell type and function

We investigated the origin of spike frequency adaptation within a layered sensory network: the auditory pathway of locusts. Spike frequency adaptation as observed in an individual neuron may arise because of intrinsic or presynaptic adaptation mechanisms. To separate the contribution of different mechanisms, we recorded from the same cell during acoustic and intracellular current stimulation. We studied three identified neuron types that are representative for each network layer and participate in processing auditory patterns and localizing sound sources. By comparing current and acoustic stimulation, three distinct patterns of the distribution of adaptation mechanisms within the sensory network emerged: (1) balanced influence of both intrinsic and presynaptic adaptation mechanisms in an interneuron that summates over several receptor afferents (TN1), (2) predominantly inhibiting input as the source for spike frequency adaptation in a cell that transmits both pattern representation and directional information (BSN1), (3) primarily intrinsic, spike-triggered adaptation currents within an interneuron coding exclusively for direction (AN2). The time courses of spike frequency adaptation differed significantly between the cells types. Using the adaptation time constants, we were able to predict signal transmission properties for the different cells. We conclude that the adaptation mechanisms differ greatly among interneurons within this sensory pathway and are a function of their role in information processing.

Neurosensory mechanotransduction

Neurons that sense touch, sound and acceleration respond rapidly to specific mechanical signals. The proteins that transduce these signals and underlie these senses, however, are mostly unknown. Research over the past decade has suggested that members of three families of channel proteins are candidate transduction molecules. Current studies are directed towards characterizing these candidates, determining how they are mechanically gated and discovering new molecules that are involved in mechanical sensing.

Wired for sex: the neurobiology of Drosophila mating decisions

Decisions about whom to mate with can sometimes be difficult, but making the right choice is critical for an animal's reproductive success. The ubiquitous fruit fly, Drosophila, is clearly very good at making these decisions. Upon encountering another fly, a male may or may not choose to court. He estimates his chances of success primarily on the basis of pheromone signals and previous courtship experience. The female decides whether to accept or reject the male, depending on her perception of his pheromone and acoustic signals, as well as her own readiness to mate. This simple and genetically tractable system provides an excellent model to explore the neurobiology of decision making.

Making an effort to listen: mechanical amplification in the ear

The inner ear's performance is greatly enhanced by an active process defined by four features: amplification, frequency selectivity, compressive nonlinearity, and spontaneous otoacoustic emission. These characteristics emerge naturally if the mechanoelectrical transduction process operates near a dynamical instability, the Hopf bifurcation, whose mathematical properties account for specific aspects of our hearing. The active process of nonmammalian tetrapods depends upon active hair-bundle motility, which emerges from the interaction of negative hair-bundle stiffness and myosin-based adaptation motors. Taken together, these phenomena explain the four characteristics of the ear's active process. In the high-frequency region of the mammalian cochlea, the active process is dominated instead by the phenomenon of electromotility, in which the cell bodies of outer hair cells extend and contract as the protein prestin alters its membrane surface area in response to changes in membrane potential.

Transducer-based force generation explains active process in Drosophila hearing

BACKGROUND

Like vertebrate hair cells, Drosophila auditory neurons are endowed with an active, force-generating process that boosts the macroscopic performance of the ear. The underlying force generator may be the molecular apparatus for auditory transduction, which, in the fly as in vertebrates, seems to consist of force-gated channels that occur in series with adaptation motors and gating springs. This molecular arrangement explains the active properties of the sensory hair bundles of inner-ear hair cells, but whether it suffices to explain the active macroscopic performance of auditory systems is unclear.

RESULTS

To relate transducer dynamics and auditory-system behavior, we have devised a simple model of the Drosophila hearing organ that consists only of transduction modules and a harmonic oscillator that represents the sound receiver. In vivo measurements show that this model explains the ear's active performance, quantitatively capturing displacement responses of the fly's antennal sound receiver to force steps, this receiver's free fluctuations, its response to sinusoidal stimuli, nonlinearity, and activity and cycle-by-cycle amplification, and properties of electrical compound responses in the afferent nerve.

CONCLUSIONS

Our findings show that the interplay between transduction channels and adaptation motors accounts for the entire macroscopic phenomenology of the active process in the Drosophila auditory system, extending transducer-based amplification from hair cells to fly ears and demonstrating that forces generated by transduction modules can suffice to explain active processes in ears.

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.