Evolutionary transition from stretch to hearing organs in ancient grasshoppers

What can insects teach us about hearing loss?

Over the last three decades, insects have been utilized to provide a deep and fundamental understanding of many human diseases and disorders. Here, we present arguments for insects as models to understand general principles underlying hearing loss. Despite ∼600 million years since the last common ancestor of vertebrates and invertebrates, we share an overwhelming degree of genetic homology particularly with respect to auditory organ development and maintenance. Despite the anatomical differences between human and insect auditory organs, both share physiological principles of operation. We explain why these observations are expected and highlight areas in hearing loss research in which insects can provide insight. We start by briefly introducing the evolutionary journey of auditory organs, the reasons for using insect auditory organs for hearing loss research, and the tools and approaches available in insects. Then, the first half of the review focuses on auditory development and auditory disorders with a genetic cause. The second half analyses the physiological and genetic consequences of ageing and short- and long-term changes as a result of noise exposure. We finish with complex age and noise interactions in auditory systems. In this review, we present some of the evidence and arguments to support the use of insects to study mechanisms and potential treatments for hearing loss in humans. Obviously, insects cannot fully substitute for all aspects of human auditory function and loss of function, although there are many important questions that can be addressed in an animal model for which there are important ethical, practical and experimental advantages.

Neurophysiology goes wild: from exploring sensory coding in sound proof rooms to natural environments

To perform adaptive behaviours, animals have to establish a representation of the physical "outside" world. How these representations are created by sensory systems is a central issue in sensory physiology. This review addresses the history of experimental approaches toward ideas about sensory coding, using the relatively simple auditory system of acoustic insects. I will discuss the empirical evidence in support of Barlow's "efficient coding hypothesis", which argues that the coding properties of neurons undergo specific adaptations that allow insects to detect biologically important acoustic stimuli. This hypothesis opposes the view that the sensory systems of receivers are biased as a result of their phylogeny, which finally determine whether a sound stimulus elicits a behavioural response. Acoustic signals are often transmitted over considerable distances in complex physical environments with high noise levels, resulting in degradation of the temporal pattern of stimuli, unpredictable attenuation, reduced signal-to-noise levels, and degradation of cues used for sound localisation. Thus, a more naturalistic view of sensory coding must be taken, since the signals as broadcast by signallers are rarely equivalent to the effective stimuli encoded by the sensory system of receivers. The consequences of the environmental conditions for sensory coding are discussed.

The Subgenual Organ Complex in Stick Insects: Functional Morphology and Mechanical Coupling of a Complex Mechanosensory Organ

Leg chordotonal organs in insects show different adaptations to detect body movements, substrate vibrations, or airborne sound. In the proximal tibia of stick insects occur two chordotonal organs: the subgenual organ, a highly sensitive vibration receptor organ, and the distal organ, of which the function is yet unknown. The distal organ consists of a linear set of scolopidial sensilla extending in the tibia in distal direction toward the tarsus. Similar organs occur in the elaborate hearing organs in crickets and bushcrickets, where the auditory sensilla are closely associated with thin tympanal membranes and auditory trachea in the leg. Here, we document the position and attachment points for the distal organ in three species of stick insects without auditory adaptations (Ramulus artemis,Sipyloidea sipylus, andCarausius morosus). The distal organ is located in the dorsal hemolymph channel and attaches at the proximal end to the dorsal and posterior leg cuticle by tissue strands. The central part of the distal organ is placed closer to the dorsal cuticle and is suspended by fine tissue strands. The anterior part is clearly separated from the tracheae, while the distal part of the organ is placed over the anterior trachea. The distal organ is not connected to a tendon or muscle, which would indicate a proprioceptive function. The sensilla in the distal organ have dendrites oriented in distal direction in the leg. This morphology does not reveal obvious auditory adaptations as in tympanal organs, while the position in the hemolymph channel and the direction of dendrites indicate responses to forces in longitudinal direction of the leg, likely vibrational stimuli transmitted in the leg’s hemolymph. The evolutionary convergence of complex chordotonal organs with linear sensilla sets between tympanal hearing organs and atympanate organs in stick insects is emphasized by the different functional morphologies and sensory specializations.

Phylogenomic analysis sheds light on the evolutionary pathways towards acoustic communication in Orthoptera

Acoustic communication is enabled by the evolution of specialised hearing and sound producing organs. In this study, we performed a large-scale macroevolutionary study to understand how both hearing and sound production evolved and affected diversification in the insect order Orthoptera, which includes many familiar singing insects, such as crickets, katydids, and grasshoppers. Using phylogenomic data, we firmly establish phylogenetic relationships among the major lineages and divergence time estimates within Orthoptera, as well as the lineage-specific and dynamic patterns of evolution for hearing and sound producing organs. In the suborder Ensifera, we infer that forewing-based stridulation and tibial tympanal ears co-evolved, but in the suborder Caelifera, abdominal tympanal ears first evolved in a non-sexual context, and later co-opted for sexual signalling when sound producing organs evolved. However, we find little evidence that the evolution of hearing and sound producing organs increased diversification rates in those lineages with known acoustic communication.

Innate releasing mechanisms and fixed action patterns: basic ethological concepts as drivers for neuroethological studies on acoustic communication in Orthoptera

This review addresses the history of neuroethological studies on acoustic communication in insects. One objective is to reveal how basic ethological concepts developed in the 1930s, such as innate releasing mechanisms and fixed action patterns, have influenced the experimental and theoretical approaches to studying acoustic communication systems in Orthopteran insects. The idea of innateness of behaviors has directly fostered the search for central pattern generators that govern the stridulation patterns of crickets, katydids or grasshoppers. A central question pervading 50 years of research is how the essential match between signal features and receiver characteristics has evolved and is maintained during evolution. As in other disciplines, the tight interplay between technological developments and experimental and theoretical advances becomes evident throughout this review. While early neuroethological studies focused primarily on proximate questions such as the implementation of feature detectors or central pattern generators, later the interest shifted more towards ultimate questions. Orthoptera offer the advantage that both proximate and ultimate questions can be tackled in the same system. An important advance was the transition from laboratory studies under well-defined acoustic conditions to field studies that allowed to measure costs and benefits of acoustic signaling as well as constraints on song evolution.

Listening in the bog: II. Neural correlates for acoustic interactions and spacing between Sphagniana sphagnorum males

Males of the katydid Sphagniana sphagnorum maintain inter-male distances from one another using agonistic song interactions with a frequency-modulated song that consists of alternating audio and ultrasonic parts. We studied the neuronal representation of this song in auditory receptors and interneurons of receivers, using playbacks of songs that mimicked the absolute and relative sound pressure levels of the two song modes varying with distance. The tuning and sensitivity of both receptors and interneurons strongly determine their responses to the two song modes at different distances. Low-frequency interneurons respond preferentially to the audio mode of the song at larger distances. High-frequency (HF) interneurons respond preferentially to the HF component of the song at close range. ‘Switch interneurons’ are sensitive to both spectral song components, but exhibit a typical activity switch towards the high-frequency mode at distances nearer than 3–6 m. The activity of the latter two groups of interneurons correlates with the distance in the field at which males begin to interact acoustically with their neighbours. Important information about the rate of changes in the song mode is represented by the afferent activity despite the influence of the masking song produced by a sympatric katydid species.

The scolopidial accessory organs and Nebenorgans in orthopteroid insects: Comparative neuroanatomy, mechanosensory function, and evolutionary origin

Scolopidial sensilla in insects often form large sensory organs involved in proprioception or exteroception. Here the knowledge on Nebenorgans and accessory organs, two organs consisting of scolopidial sensory cells, is summarised. These organs are present in some insects which are model organisms for the physiology of mechanosensory systems (cockroaches and tettigoniids). Recent comparative studies documented the accessory organ in several taxa of Orthoptera (including tettigoniids, cave crickets, Jerusalem crickets) and the Nebenorgan in related insects (Mantophasmatodea). The accessory organ or Nebenorgan is usually a small organ of 8-15 sensilla located in the posterior leg tibia of all leg pairs. The physiological properties of the accessory organs and Nebenorgans are so far largely unknown. Taking together neuroanatomical and electrophysiological data from disparate taxa, there is considerable evidence that the accessory organ and Nebenorgan are vibrosensitive. They thus complement the larger vibrosensitive subgenual organ in the tibia. This review summarises the comparative studies of these sensory organs, in particular the arguments and criteria for the homology of the accessory organ and Nebenorgan among orthopteroid insects. Different scenarios of repeated evolutionary origins or losses of these sensory organs are discussed. Neuroanatomy allows to distinguish individual sensory organs for analysis of sensory physiology, and to infer scenarios of sensory evolution.

The complex tibial organ of the New Zealand ground weta: sensory adaptations for vibrational signal detection

In orthopteran insects, a complex tibial organ has evolved to detect substrate vibrations and/or airborne sound. Species of New Zealand weta (Anostostomatidae) with tympanal ears on the foreleg tibia use this organ to communicate by sound, while in atympanate species (which communicate by substrate drumming) the organ is unstudied. We investigated the complex tibial organ of the atympanate ground weta, Hemiandrus pallitarsis, for vibration detection adaptations. This system contains four sensory components (subgenual organ, intermediate organ, crista acustica homolog, accessory organ) in all legs, together with up to 90 scolopidial sensilla. Microcomputed tomography shows that the subgenual organ spans the hemolymph channel, with attachments suggesting that hemolymph oscillations displace the organ in a hinged-plate fashion. Subgenual sensilla are likely excited by substrate oscillations transmitted within the leg. Instead of the usual suspension within the middle of the tibial cavity, we show that the intermediate organ and crista acustica homolog comprise a cellular mass broadly attached to the anterior tibial wall. They likely detect cuticular vibrations, and not airborne sound. This atympanate complex tibial organ shows elaborate structural changes suggesting detection of vibrational stimuli by parallel input pathways, thus correlating well with the burrowing lifestyle and communication by substrate-transmitted vibration.

Directional hearing in insects with internally coupled ears

Compared to all other hearing animals, insects are the smallest ones, both in absolute terms and in relation to the wavelength of most biologically relevant sounds. The ears of insects can be located at almost any possible body part, such as wings, legs, mouthparts, thorax or abdomen. The interaural distances are generally so small that cues for directional hearing such as interaural time and intensity differences (IITs and IIDs) are also incredibly small, so that the small body size should be a strong constraint for directional hearing. Yet, when tested in behavioral essays for the precision of sound source localization, some species demonstrate hyperacuity in directional hearing and can track a sound source deviating from the midline by only [Formula: see text]-[Formula: see text]. They can do so by using internally coupled ears, where sound pressure can act on both sides of a tympanic membrane. Here we describe their varying anatomy and mode of operation for some insect groups, with a special focus on crickets, exhibiting probably one of the most sophisticated of all internally coupled ears in the animal kingdom.

Chordotonal organs in hemipteran insects: unique peripheral structures but conserved central organization revealed by comparative neuroanatomy

Hemipteran insects use sophisticated vibrational communications by striking body appendages on the substrate or by oscillating the abdominal tymbal. There has been, however, little investigation of sensory channels for processing vibrational signals. Using sensory nerve stainings and low invasive confocal analyses, we demonstrate the comprehensive neuronal mapping of putative vibration-responsive chordotonal organs (COs) in stink bugs (Pentatomidae and Cydinidae) and cicadas (Cicadidae). The femoral CO (FCO) in stink bugs consists of ventral and dorsal scoloparia, homologous to distal and proximal scoloparia in locusts, which are implicated in joint movement detection and vibration detection, respectively. The ligament of the dorsal scoloparium is distally attached to the accessory extensor muscle, whereas that of the ventral scoloparium is attached to a specialized tendon. Their afferents project to the dorso-lateral neuropil and the central region of the medial ventral association center (mVAC) in the ipsilateral neuromere, where presumed dorsal scoloparium afferents and subgenual organ afferents are largely intermingled. In contrast, FCOs in cicadas have decreased dorsal scoloparium neurons and lack projections to the mVAC. The tymbal CO of stink bugs contains four sensory neurons that are distally attached to fat body cells via a ligament. Their axons project intersegmentally to the dorsal region of mVACs in all neuromeres. Together with comparisons of COs in different insect groups, the results suggest that hemipteran COs have undergone structural modification for achieving faster signaling of resonating peripheral tissues. The conserved projection patterns of COs suggest functional importance of the FCO and subgenual organ for vibrational communications.

Hearing in Insects

Insect hearing has independently evolved multiple times in the context of intraspecific communication and predator detection by transforming proprioceptive organs into ears. Research over the past decade, ranging from the biophysics of sound reception to molecular aspects of auditory transduction to the neuronal mechanisms of auditory signal processing, has greatly advanced our understanding of how insects hear. Apart from evolutionary innovations that seem unique to insect hearing, parallels between insect and vertebrate auditory systems have been uncovered, and the auditory sensory cells of insects and vertebrates turned out to be evolutionarily related. This review summarizes our current understanding of insect hearing. It also discusses recent advances in insect auditory research, which have put forward insect auditory systems for studying biological aspects that extend beyond hearing, such as cilium function, neuronal signal computation, and sensory system evolution.

Diel variation in signalling and signal transmission in the bladder grasshopper, Bullacris unicolor (Orthoptera; Pneumoridae)

Here we investigate intraspecific variation in diel patterns of acoustic signalling in the bladder grasshopper, Bullacris unicolor. We observed that B. unicolor calls at different times during the night in different parts of its distribution. Males further north typically call just before dawn, while those further south signal throughout the night. Sound transmission experiments were conducted in order to determine whether the observed discrepancy in signal timing can be explained by differences in signal propagation at different times in the night, which might vary geographically. We found significant differences in signal attenuation and fidelity at different broadcast times as well as between locations. However, there was only partial support for the hypothesis that males time their calls to coincide with the most ideal transmission conditions. We suggest that other factors, such as predation pressure, might also contribute to the observed discrepancy in signal timing between populations.

Sensory neuroanatomy of stick insects highlights the evolutionary diversity of the orthopteroid subgenual organ complex

The subgenual organ is a scolopidial sense organ located in the tibia of many insects. In this study the neuroanatomy of the subgenual organ complex of stick insects is clarified for two species, Carausius morosus and Siyploidea sipylus. Neuronal tracing shows a subgenual organ complex that consists of a subgenual organ and a distal organ. There are no differences in neuroanatomy between the three thoracic leg pairs, and the sensory structures are highly similar in both species. A comparison of the neuroanatomy with other orthopteroid insects highlights two features unique in Phasmatodea. The subgenual organ contains a set of densely arranged sensory neurons in the anterior-ventral part of the organ, and a distal organ with 16-17 scolopidial sensilla in C. morosus and 20-22 scolopidial sensilla in S. sipylus. The somata of sensory neurons in the distal organ are organized in a linear array extending distally into the tibia, with only a few exceptions of closely associated neurons. The stick insect sense organs show a case of an elaborate scolopidial sense organ that evolved in addition to the subgenual organ. The neuroanatomy of stick insects is compared to that studied in other orthopteroid taxa (cockroaches, locusts, crickets, tettigoniids). The comparison of sensory structures indicates that elaborate scolopidial organs have evolved repeatedly among orthopteroids. The distal organ in stick insects has the highest number of sensory neurons known for distal organs so far.

From microseconds to seconds and minutes-time computation in insect hearing

The computation of time in the auditory system of insects is of relevance at rather different time scales, covering a large range from microseconds to several minutes. At the one end of this range, only a few microseconds of interaural time differences are available for directional hearing, due to the small distance between the ears, usually considered too small to be processed reliably by simple nervous systems. Synapses of interneurons in the afferent auditory pathway are, however, very sensitive to a time difference of only 1–2 ms provided by the latency shift of afferent activity with changing sound direction. At a much larger time scale of several tens of milliseconds to seconds, time processing is important in the context species recognition, but also for those insects where males produce acoustic signals within choruses, and the temporal relationship between song elements strongly deviates from a random distribution. In these situations, some species exhibit a more or less strict phase relationship of song elements, based on phase response properties of their song oscillator. Here we review evidence on how this may influence mate choice decisions. In the same dimension of some tens of milliseconds we find species of katydids with a duetting communication scheme, where one sex only performs phonotaxis to the other sex if the acoustic response falls within a very short time window after its own call. Such time windows show some features unique to insects, and although its neuronal implementation is unknown so far, the similarity with time processing for target range detection in bat echolocation will be discussed. Finally, the time scale being processed must be extended into the range of many minutes, since some acoustic insects produce singing bouts lasting quite long, and female preferences may be based on total signaling time.

Lateral line diversity among ecologically divergent threespine stickleback populations

The lateral line is a mechanoreceptive sensory system that allows fish to sense objects and motion in their local environment. Variation in lateral line morphology may allow fish in different habitats to differentially sense and respond to salient cues. Threespine sticklebacks (Gasterosteus aculeatus) occupy a diverse range of aquatic habitats; we therefore hypothesized that populations within the G. aculeatus species complex might show variation in the morphology of the lateral line sensory system. We sampled 16 threespine stickleback populations from marine, stream and lake (including benthic and limnetic) habitats and examined the distribution, type and number of neuromasts on different regions of the body. We found that the threespine stickleback has a reduced lateral line canal system, completely lacking canal neuromasts. Although the arrangement of lines of superficial neuromasts on the body was largely the same in all populations, the number of neuromasts within these lines varied across individuals, populations and habitats. In pairwise comparisons between threespine sticklebacks adapted to divergent habitats, we found significant differences in neuromast number. Stream residents had more neuromasts than marine sticklebacks living downstream in the same watershed. In two independent lakes, benthic sticklebacks had more trunk neuromasts than sympatric limnetic sticklebacks, providing evidence for parallel evolution of the lateral line system. Our data provide the first demonstration that the lateral line sensory system can vary significantly between individuals and among populations within a single species, and suggest that this sensory system may experience different selection regimes in alternative habitats.

Postembryonic development of the auditory system of the cicada Okanagana rimosa (Say) (Homoptera: Auchenorrhyncha: Cicadidae)

Cicadas (Homoptera: Auchenorrhyncha: Cicadidae) use acoustic signalling for mate attraction and perceive auditory signals by a tympanal organ in the second abdominal segment. The main structural features of the ear are the tympanum, the sensory organ consisting of numerous scolopidial cells, and the cuticular link between sensory neurones and tympanum (tympanal ridge and apodeme). Here, a first investigation of the postembryonic development of the auditory system is presented. In insects, sensory neurones usually differentiate during embryogenesis, and sound-perceiving structures form during postembryogenesis. Cicadas have an elongated and subterranian postembryogenesis which can take several years until the final moult. The neuroanatomy and functional morphology of the auditory system of the cicada Okanagana rimosa (Say) are documented for the adult and the three last larval stages. The sensory organ and the projection of sensory afferents to the CNS are present in the earliest stages investigated. The cuticular structures of the tympanum, the tympanal frame holding the tympanum, and the tympanal ridge differentiate in the later stages of postembryogenesis. Thus, despite the different life styles of larvae and adults, the neuronal components of the cicada auditory system develop already during embryogenesis or early postembryogenesis, and sound-perceiving structures like tympana are elaborated later in postembryogenesis. The life cycle allows comparison of cicada development to other hemimetabolous insects with respect to the influence of specially adapted life cycle stages on auditory maturation. The neuronal development of the auditory system conforms to the timing in other hemimetabolous insects.

Time-resolved tympanal mechanics of the locust

A salient characteristic of most auditory systems is their capacity to analyse the frequency of sound. Little is known about how such analysis is performed across the diversity of auditory systems found in animals, and especially in insects. In locusts, frequency analysis is primarily mechanical, based on vibrational waves travelling across the tympanal membrane. Different acoustic frequencies generate travelling waves that direct vibrations to distinct tympanal locations, where distinct groups of correspondingly tuned mechanosensory neurons attach. Measuring the mechanical tympanal response, for the first time, to acoustic impulses in the time domain, nanometre-range vibrational waves are characterized with high spatial and temporal resolutions. Conventional Fourier analysis is also used to characterize the response in the frequency domain. Altogether these results show that travelling waves originate from a particular tympanal location and travel across the membrane to generate oscillations in the exact region where mechanosensory neurons attach. Notably, travelling waves are unidirectional; no strong back reflection or wave resonance could be observed across the membrane. These results constitute a key step in understanding tympanal mechanics in general, and in insects in particular, but also in our knowledge of the vibrational behaviour of anisotropic media.

Evolutionarily conserved coding properties of auditory neurons across grasshopper species

We investigated encoding properties of identified auditory interneurons in two not closely related grasshopper species (Acrididae). The neurons can be homologized on the basis of their similar morphologies and physiologies. As test stimuli, we used the species-specific stridulation signals of Chorthippus biguttulus, which evidently are not relevant for the other species, Locusta migratoria. We recorded spike trains produced in response to these signals from several neuron types at the first levels of the auditory pathway in both species. Using a spike train metric to quantify differences between neuronal responses, we found a high similarity in the responses of homologous neurons: interspecific differences between the responses of homologous neurons in the two species were not significantly larger than intraspecific differences (between several specimens of a neuron in one species). These results suggest that the elements of the thoracic auditory pathway have been strongly conserved during the evolutionary divergence of these species. According to the 'efficient coding' hypothesis, an adaptation of the thoracic auditory pathway to the specific needs of acoustic communication could be expected. We conclude that there must have been stabilizing selective forces at work that conserved coding characteristics and prevented such an adaptation.

Morphology and physiology of the prosternal chordotonal organ of the sarcophagid fly Sarcophaga bullata (Parker)

The anatomy and the physiology of the prosternal chordotonal organ (pCO) within the prothorax of Sarcophaga bullata is analysed. Neuroanatomical studies illustrate that the approximately 35 sensory axons terminate within the median ventral association centre of the different neuromeres of the thoracico-abdominal ganglion. At the single-cell level two classes of receptor cells can be discriminated physiologically and morphologically: receptor cells with dorso-lateral branches in the mesothoracic neuromere are insensitive to frequencies below approximately 1 kHz. Receptor cells without such branches respond most sensitive at lower frequencies. Absolute thresholds vary between 0.2 and 8m/s(2) for different frequencies. The sensory information is transmitted to the brain via ascending interneurons. Functional analyses reveal a mechanical transmission of forced head rotations and of foreleg vibrations to the attachment site of the pCO. In summed action potential recordings a physiological correlate was found to stimuli with parameters of leg vibrations, rather than to those of head rotation. The data represent a first physiological study of a putative predecessor organ of an insect ear.

Diversity of intersegmental auditory neurons in a bush cricket

Various auditory interneurons of the duetting bush cricket Ancistrura nigrovittata with axons ascending to the brain are presented. In this species, more intersegmental sound-activated neurons have been identified than in any other bush cricket so far, among them a new type of ascending neuron with posterior soma in the prothoracic ganglion (AN4). These interneurons show not only morphological differences in the prothoracic ganglion and the brain, but also respond differently to carrier frequencies, intensity and direction. As a set of neurons, they show graded differences for all of these parameters. A response type not described among intersegmental neurons of crickets and other bush crickets so far is found in the AN3 neuron with a tonic response, broad frequency tuning and little directional dependence. All neurons, with the exception of AN3, respond in a relatively similar manner to the temporal patterns of the male song: phasically to high syllable repetitions and rhythmically to low syllable repetitions. The strongest coupling to the temporal pattern is found in TN1. In contrast to behavior the neuronal responses depend little on syllable duration. AN4, AN5 and TN1 respond well to the female song. AN4 (at higher intensities) and TN1 respond well to a complete duet.

Synaptic ultrastructure of Drosophila Johnston's organ axon terminals as revealed by an enhancer trap

The role of auditory circuitry is to decipher relevant information from acoustic signals. Acoustic parameters used by different insect species vary widely. All these auditory systems, however, share a common transducer: tympanal organs as well as the Drosophila flagellar ears use chordotonal organs as the auditory mechanoreceptors. We here describe the central neural projections of the Drosophila Johnston's organ (JO). These neurons, which represent the antennal auditory organ, terminate in the antennomechanosensory center. To ensure correct identification of these terminals we made use of a beta-galactosidase-expressing transgene that labels JO neurons specifically. Analysis of these projection pathways shows that parallel JO fibers display extensive contacts, including putative gap junctions. We find that the synaptic boutons show both chemical synaptic structures as well as putative gap junctions, indicating mixed synapses, and belong largely to the divergent type, with multiple small postsynaptic processes. The ultrastructure of JO fibers and synapses may indicate an ability to process temporally discretized acoustic information.

Cockroach homologs of praying mantis peripheral auditory system components

This study identifies the cuticular metathoracic structures in earless cockroaches that are the homologs to the peripheral auditory components in their sister taxon, praying mantids, and defines the nature of the cuticular transition from earless to eared in the Dictyoptera. The single, midline ear of mantids comprises an auditory chamber with complex walls that contain the tympana and chordotonal transduction elements. The corresponding area in cockroaches, between the furcasternum and coxae, has many socketed hairs arranged in discrete fields and the Nerve 7 chordotonal organ, the homolog of the mantis tympanal organ. The Nerve 7 chordotonal organ attaches at the apex of the lateral ventropleurite (LVp), which has the same shape and general structure as an auditory chamber wall. High-speed video shows that when the coxa moves toward the midline, the LVp rotates medially to stimulate socketed hairs, and also moves like a triangular hinge giving the chordotonal organ maximal in-out stimulation. Formation of the mantis auditory chamber from the LVp and adjacent structures would involve only enlargement, a shift toward the midline, and a mild rotation. Almost all proprioceptive function would be lost, which may constitute the major cost of building and maintaining the mantis ear. Isolation from leg movement dictates the position of the mantis ear in the midline and the rigid frame, formed by the cuticular knobs, which protects the chordotonal organs.

Habitat-dependent transmission of male advertisement calls in bladder grasshoppers (Orthoptera; Pneumoridae)

It has been hypothesized that the physical properties of the environment exert selection pressure on long-range acoustic communication signals to match the local habitat by promoting signal characteristics that minimize excess attenuation and distortion. We tested this in a unique family of bladder grasshoppers notable for producing a signal with a 2 km maximum transmission distance. In direct performance comparisons, male advertisement calls of seven species were broadcast through four vegetation biomes--forest, fynbos, savanna and succulent karoo. The calls of species native to forest and fynbos biomes propagated with lower levels of distortion over distance in their respective habitats relative to those of non-native species, while fynbos species also performed best in the remaining two habitats. In addition, both forest and fynbos species had low levels of signal attenuation over distance in all environments. The fynbos biome was characterized by high inconsistency in signal degradation, while the forest biome had the highest levels of environmental noise. Innate habitat characteristics, leading to comparatively limited acoustic communication distances in the forest and fynbos relative to the savanna and succulent karoo, may therefore explain the need for a higher quality of signal transmission in grasshoppers inhabiting the former two environments.

The physiology of insect auditory afferents

This review presents an overview of the physiology of primary receptors serving tympanal hearing in insects. Auditory receptor responses vary with frequency, intensity, and temporal characteristics of sound stimuli. Various insect species exploit each of these parameters to differing degrees in the neural coding of auditory information, depending on the nature of the relevant stimuli. Frequency analysis depends on selective tuning in individual auditory receptors. In those insect groups that have individually tuned receptors, differences in physiology are correlated with structural differences among receptors and with the anatomical arrangement of receptors within the ear. Intensity coding is through the rate-level characteristics of tonically active auditory receptors and through variation in the absolute sensitivities of individual receptors (range fractionation). Temporal features of acoustic stimuli may be copied directly in the timing of afferent responses. Salient signal characteristics may also be represented by variation in the timing of afferent responses on a finer temporal scale, or by the synchrony of responses across a population of receptors.

The structure and function of auditory chordotonal organs in insects

Insects are capable of detecting a broad range of acoustic signals transmitted through air, water, or solids. Auditory sensory organs are morphologically diverse with respect to their body location, accessory structures, and number of sensilla, but remarkably uniform in that most are innervated by chordotonal organs. Chordotonal organs are structurally complex Type I mechanoreceptors that are distributed throughout the insect body and function to detect a wide range of mechanical stimuli, from gross motor movements to air-borne sounds. At present, little is known about how chordotonal organs in general function to convert mechanical stimuli to nerve impulses, and our limited understanding of this process represents one of the major challenges to the study of insect auditory systems today. This report reviews the literature on chordotonal organs innervating insect ears, with the broad intention of uncovering some common structural specializations of peripheral auditory systems, and identifying new avenues for research. A general overview of chordotonal organ ultrastructure is presented, followed by a summary of the current theories on mechanical coupling and transduction in monodynal, mononematic, Type 1 scolopidia, which characteristically innervate insect ears. Auditory organs of different insect taxa are reviewed, focusing primarily on tympanal organs, and with some consideration to Johnston's and subgenual organs. It is widely accepted that insect hearing organs evolved from pre-existing proprioceptive chordotonal organs. In addition to certain non-neural adaptations for hearing, such as tracheal expansion and cuticular thinning, the chordotonal organs themselves may have intrinsic specializations for sound reception and transduction, and these are discussed. In the future, an integrated approach, using traditional anatomical and physiological techniques in combination with new methodologies in immunohistochemistry, genetics, and biophysics, will assist in refining hypotheses on how chordotonal organs function, and, ultimately, lead to new insights into the peripheral mechanisms underlying hearing in insects.

Serial hearing organs in the atympanate grasshopper Bullacris membracioides (Orthoptera, Pneumoridae)

In different insect taxa, ears can be found on virtually any part of the body. Comparative anatomy and similarities in the embryological development of ears in divergent taxa suggest that they have evolved multiple times from ubiquitous stretch or vibration receptors, but the homology of these structures has not yet been rigorously tested. Here we provide detailed analysis of a novel set of hearing organs in a relatively "primitive" atympanate bladder grasshopper (Bullacris membracioides) that is capable of signaling acoustically over 2 km. We use morphological, physiological, and behavioral experiments to demonstrate that this species has six pairs of serially repeated abdominal ears derived from proprioceptive pleural chordotonal organs (plCOs). We demonstrate continuity in auditory function from the five posterior pairs, which are simple forms comprising 11 sensilla and resembling plCOs in other grasshoppers, to the more complex anterior pair, which contains 2000 sensilla and is homologous to the single pair of tympanate ears found in "modern" grasshoppers. All 12 ears are morphologically differentiated, responsive to airborne sound at frequencies and intensities that are biologically significant (tuned to 1.5 and 4 kHz; 60-98 dB SPL), and capable of mediating behavioral responses of prospective mates. These data provide evidence for the transition in function and selective advantage that must occur during evolutionary development of relatively complex organs from simpler precursors. Our results suggest that ancestral insects with simple atympanate pleural receptors may have had hearing ranges that equal or exceed those of contemporary insects with complex tympanal ears. Moreover, auditory capability may be more prevalent among modern insect taxa than the presence of overt tympana indicates.

Somatotopic mapping of chordotonal organ neurons in a primitive ensiferan, the New Zealand tree weta Hemideina femorata: I. femoral chordotonal organ

The femoral chordotonal organ (FCO) in orthopteran insects comprises several hundred sensory neurons, making it one of the most complex insect proprioceptors. The sensory neurons are suspended from the proximal femur, connecting distally to ligaments and to a needle-like apodeme extending from the proximal tibia. They monitor the position and movement of the tibia. To address how this complexity depends on evolutionary status and function, the morphology of the FCO neurons in the primitive orthopteran Hemideina femorata was investigated by staining small populations of identified afferents. As in crickets, the FCOs in all legs of the weta comprise partly fused ventral and dorsal scoloparia, with the former containing two groups of somata, the ventral group (VG) and the dorsal group (DG). However, the dendrites of the DG insert into thin connective tissue attached to the ventral side of the dorsal ligament, forming a "third scoloparium." The VG afferents terminate mainly in the motor association neuropils, whereas afferents from the dorsal scoloparium neurons terminate exclusively in the vibratory neuropil as do the afferents from the subgenual organ, a substrate vibration detector. Several afferents originating in the DG have extensive terminations in the motor association-, vibratory-, and auditory-processing neuropils, indicating lesser functional specialization than in the other groups. The evolutionary development of the FCO is discussed from a comparative viewpoint.

The zinc finger transcription factor Gfi1, implicated in lymphomagenesis, is required for inner ear hair cell differentiation and survival

Gfi1 was first identified as causing interleukin 2-independent growth in T cells and lymphomagenesis in mice. Much work has shown that Gfi1 and Gfi1b, a second mouse homolog, play pivotal roles in blood cell lineage differentiation. However, neither Gfi1 nor Gfi1b has been implicated in nervous system development, even though their invertebrate homologues, senseless in Drosophila and pag-3 in C. elegans are expressed and required in the nervous system. We show that Gfi1 mRNA is expressed in many areas that give rise to neuronal cells during embryonic development in mouse, and that Gfi1 protein has a more restricted expression pattern. By E12.5 Gfi1 mRNA is expressed in both the CNS and PNS as well as in many sensory epithelia including the developing inner ear epithelia. At later developmental stages, Gfi1 expression in the ear is refined to the hair cells and neurons throughout the inner ear. Gfi1 protein is expressed in a more restricted pattern in specialized sensory cells of the PNS, including the eye, presumptive Merkel cells, the lung and hair cells of the inner ear. Gfi1 mutant mice display behavioral defects that are consistent with inner ear anomalies, as they are ataxic, circle, display head tilting behavior and do not respond to noise. They have a unique inner ear phenotype in that the vestibular and cochlear hair cells are differentially affected. Although Gfi1-deficient mice initially specify inner ear hair cells, these hair cells are disorganized in both the vestibule and cochlea. The outer hair cells of the cochlea are improperly innervated and express neuronal markers that are not normally expressed in these cells. Furthermore, Gfi1 mutant mice lose all cochlear hair cells just prior to and soon after birth through apoptosis. Finally, by five months of age there is also a dramatic reduction in the number of cochlear neurons. Hence, Gfi1 is expressed in the developing nervous system, is required for inner ear hair cell differentiation, and its loss causes programmed cell death.

Tympanal and atympanal 'mouth-ears' in hawkmoths (Sphingidae)

The labral pilifers and the labial palps form ultrasound-sensitive hearing organs in species of two distantly related hawkmoth subtribes, the Choerocampina and the Acherontiina. Biomechanical examination now reveals that their ears represent different types of hearing organs. In hearing species of both subtribes, the labral pilifer picks up vibrations from specialized sound-receiving structures of the labial palp that are absent in non-hearing species. In Choerocampina, a thin area of cuticle serves as an auditory tympanum, whereas overlapping scales functionally replace a tympanum in Acherontiina that can hear. The tympanum of Choerocampina and the scale-plate of Acherontiina both vibrate maximally in response to ultrasonic, behaviourally relevant sounds, with the vibrations of the tympanum exceeding those of the scale plate by ca. 15 dB. This amplitude difference, however, is not reflected in the vibrations of the pilifers and the neural auditory sensitivity is similar in hearing species of both subtribes. Accordingly, morphologically different - tympanal and atympanal - but functionally equivalent hearing organs evolved independently and in parallel within a single family of moths.

Embryonic development of the central projection of auditory afferents (Schistocerca gregaria, Orthoptera, Insecta)

The auditory system of Schistocerca gregaria is a well investigated sensory network in the adult grasshopper. Here we present a first study on the embryonic development of this neuronal network. Focussing on the auditory receptor cells we show that they differentiate axonal processes at around 45% of embryonic development. These axons fasciculate with the intersegmental nerve and enter the central nervous system by 45-50% of development. First collaterals sprout into the major arborization area, the frontal auditory projection area of the metathoracic ganglion by 60%. This projection increases in density until an adult-like morphology is established by 90% of development. Furthermore, by the end of embryogenesis all three types of receptor fiber projections can be distinguished. This development is independent of a hearing ability, which develops much later during postembryonic life. The auditory projection co-develops with the fusion of neuromeres to the metathoracic ganglion, the formation of the target neuropile areas and the expression of the synapse associated molecule synapsin. Fasciclin I and Lachesin, both potential axon-guidance molecules, are expressed strongly on both, peripheral and central auditory pathways and, although much weaker, within the synaptic target area.

Molecular phylogenetic analysis of the Pneumoroidea (Orthoptera, Caelifera): molecular data resolve morphological character conflicts in the basal acridomorpha

A key transition in the evolution of the insect suborder Caelifera (Orthoptera; Insecta) was from predominantly non-angiosperm-feeding basal lineages to the modern acridomorph fauna (grasshoppers and related insects). However, because of conflicts in the distribution of several complex morphological characters, the relationships of the presumed intermediates, and in particular of the superfamily Pneumoroidea, are presently unclear. We undertook a phylogenetic study of representatives of all of the transitional acridomorph families using mitochondrial and nuclear DNA sequences. No support for pneumoroid monophyly was obtained from nonparametric bootstrap analysis. Furthermore, adopting a maximum-likelihood approach, specific hypotheses of relationships within the Pneumoroidea were firmly rejected using parametric bootstrapping and Kishino-Hasegawa tests. The results indicate that the Pneumoroidea are at best a grade. This distinction implies that the evolution of the proposed pneumoroid synapomorphies, femoro-abdominal stridulation and simple male genital structure, might previously have been misinterpreted as cases of single character gains or losses within lineages. Reconstructions of character states for the femoro-abdominal stridulation indicate that, in fact, multiple losses or gains are equally likely. An important implication of our findings is that, in grasshoppers, auditory tympana may have evolved before stridulation, supporting the argument that the original function of tympana may have been related not to conspecific communication but to predator detection. Overall, the results of this study emphasize the high information content of these minor groups (in this case, the four intermediate families under consideration contain only 0.2% of extant orthopteran species diversity). Our analyses also demonstrate the advantages of model-based methods in analyzing systematic problems and, in particular, of the importance of testing specific phylogenetic hypotheses when a priori support for groupings (e.g., from nonparametric bootstrapping) is marginal.

Synaptic input from serial chordotonal organs onto segmentally homologous interneurons in the grasshopper Schistocerca gregaria

We investigated the synaptic inputs from the serially homologous pleural, tympanal and wing-hinge chordotonal organs onto a set of identified homologous interneurons (714, 539, 529) in the ventral nerve cord of the grasshopper Schistocerca gregaria. Cobalt backfills show that afferents from all chordotonal organs project into stereotypic tracts in the central nervous system in which intracellular staining reveals the interneurons to have dendritic arborizations. Neuron 714 was found to receive excitatory bilateral synaptic input from all the serial chordotonal organs tested, from the second thoracic segment down to the seventh abdominal segment. Neuron 531, by contrast, only receives input from the chordotonal afferents on the first abdominal segment; those on the axon side are excitatory, while those on the soma side are inhibitory. The pattern of chordotonal input onto neuron 529 is similar to that seen for neuron 714, with the exception that neuron 529 receives no input from the forewing chordotonal organs. The pattern of afferent connectivities onto neurons 714, 531 and 529 differs with respect to those afferents which synapse directly or indirectly with the respective neuron. The synaptic inputs demonstrate a segmental specialization in the chordotonal system and thereby offer an insight into information processing in a modular sensory system.

Functional conservation of atonal and Math1 in the CNS and PNS

To determine the extent to which atonal and its mouse homolog Math1 exhibit functional conservation, we inserted (beta)-galactosidase (lacZ) into the Math1 locus and analyzed its expression, evaluated consequences of loss of Math1 function, and expressed Math1 in atonal mutant flies. lacZ under the control of Math1 regulatory elements duplicated the previously known expression pattern of Math1 in the CNS (i.e., the neural tube, dorsal spinal cord, brainstem, and cerebellar external granule neurons) but also revealed new sites of expression: PNS mechanoreceptors (inner ear hair cells and Merkel cells) and articular chondrocytes. Expressing Math1 induced ectopic chordotonal organs (CHOs) in wild-type flies and partially rescued CHO loss in atonal mutant embryos. These data demonstrate that both the mouse and fly homologs encode lineage identity information and, more interestingly, that some of the cells dependent on this information serve similar mechanoreceptor functions.

Structure, development, and evolution of insect auditory systems

This paper provides an overview of insect peripheral auditory systems focusing on tympanate ears (pressure detectors) and emphasizing research during the last 15 years. The theme throughout is the evolution of hearing in insects. Ears have appeared independently no fewer than 19 times in the class Insecta and are located on various thoracic and abdominal body segments, on legs, on wings, and on mouth parts. All have fundamentally similar structures-a tympanum backed by a tracheal sac and a tympanal chordotonal organ-though they vary widely in size, ancillary structures, and number of chordotonal sensilla. Novel ears have recently been discovered in praying mantids, two families of beetles, and two families of flies. The tachinid flies are especially notable because they use a previously unknown mechanism for sound localization. Developmental and comparative studies have identified the evolutionary precursors of the tympanal chordotonal organs in several insects; they are uniformly chordotonal proprioceptors. Tympanate species fall into clusters determined by which of the embryologically defined chordotonal organ groups in each body segment served as precursor for the tympanal organ. This suggests that the many appearances of hearing could arise from changes in a small number of developmental modules. The nature of those developmental changes that lead to a functional insect ear is not yet known.

Responses of movement-sensitive visual interneurons to prey-like stimuli in the praying mantis Sphodromantis lineola (Burmeister)

Previous behavioral work using both mechanical and computer-generated visual stimuli has demonstrated that mantids use a computational algorithm to recognize prey similar to that used by some amphibian predators: A stimulus elicits prey capture behavior if it falls within a perceptual envelope defined by five fundamental stimulus parameters: (1) overall size, (2) length of the leading edge, (3) contrast to the background, (4) location in the visual field, and (5) apparent speed. In this study, we recorded simultaneously from both cervical nerve cords of monocular Sphodromantis lineola while they viewed the same visual stimuli successfully used in the behavioral studies. Extracellular recordings showed three consistently proportioned amplitude classes of movement-elicited spikes in each cord and these were repeatedly and reliably identifiable across mantids. Overall, the movement-elicited activity in both cords was dominated by very large spikes suggesting the existence of several large, descending movement-sensitive interneurons projecting both ipsilaterally and contralaterally from the optic lobes. However, only the largest contralateral spikes occurred preferentially to prey-like stimuli, mirrored the behavioral response curves generated by S. lineola to the same visual stimuli, and displayed activity peaks that were correlated with the times at which the mantid emitted predatory strikes.

The evolution of neuronal circuits underlying species-specific behavior

The nervous system is evolutionarily conservative compared to the peripheral appendages that it controls. However, species-specific behaviors may have arisen from very small changes in neuronal circuits. In particular, changes in neuromodulatory systems may allow multifunctional circuits to produce different sets of behaviors in closely related species. Recently, it was demonstrated that even species differences in complex social behavior may be attributed to a change in the promoter region of a single gene regulating a neuromodulatory action.