The structure and function of auditory chordotonal organs in insects

Innexin expression and localization in the Drosophila antenna indicate gap junction or hemichannel involvement in antennal chemosensory sensilla

Odor detection in insects is largely mediated by structures on antennae called sensilla, which feature a strongly conserved architecture and repertoire of olfactory sensory neurons (OSNs) and various support cell types. In Drosophila, OSNs are tightly apposed to supporting cells, whose connection with neurons and functional roles in odor detection remain unclear. Coupling mechanisms between these neuronal and non-neuronal cell types have been suggested based on morphological observations, concomitant physiological activity during odor stimulation, and known interactions that occur in other chemosensory systems. For instance, it is not known whether cell-cell coupling via gap junctions between OSNs and neighboring cells exists, or whether hemichannels interconnect cellular and extracellular sensillum compartments. Here, we show that innexins, which form hemichannels and gap junctions in invertebrates, are abundantly expressed in adult drosophilid antennae. By surveying antennal transcriptomes and performing various immunohistochemical stainings in antennal tissues, we discover innexin-specific patterns of expression and localization, with a majority of innexins strongly localizing to glial and non-neuronal cells, likely support and epithelial cells. Finally, by injecting gap junction-permeable dye into a pre-identified sensillum, we observe no dye coupling between neuronal and non-neuronal cells. Together with evidence of non-neuronal innexin localization, we conclude that innexins likely do not conjoin neurons to support cells, but that junctions and hemichannels may instead couple support cells among each other or to their shared sensillum lymph to achieve synchronous activity. We discuss how coupling of sensillum microenvironments or compartments may potentially contribute to facilitate chemosensory functions of odor sensing and sensillum homeostasis.

Perspectives on the manipulation of mosquito hearing

Vector control is essential for preventing mosquito-borne diseases. However, different challenges associated with the development of insecticide resistance and behavioural adaptations across mosquito populations means novel control strategies are urgently needed. In recent years, disrupting mosquito mating has emerged as an alternative target of control tools because of its potential to reduce mosquito population numbers. Mosquito mating relies on sophisticated auditory processing for mate finding in many medically important species. Manipulating this key process could provide novel methods for mosquito control.

A burden shared: The evolutionary case for studying human deafness in Drosophila

Hearing impairment is the most prevalent sensory disease in humans and can have dramatic effects on the development, and preservation, of our cognitive abilities and social interactions. Currently 20 % of the world's population suffer from a form of hearing impairment; this is predicted to rise to 25 % by 2050. Despite this staggering disease load, and the vast damage it inflicts on the social, medical and economic fabric of humankind, our ability to predict, or prevent, the loss of hearing is very poor indeed. We here make the case for a paradigm shift in our approach to studying deafness. By exploiting more forcefully the molecular-genetic conservation between human hearing and hearing in morphologically distinct models, such as the fruit fly Drosophila melanogaster, we believe, a deeper understanding of hearing and deafness can be achieved. An understanding that moves beyond the surface of the 'deafness genes' to probe the underlying bedrock of hearing, which is shared across taxa, and partly shared across modalities. When it comes to understanding the workings (and failings) of human sensory function, a simple fruit fly has a lot to offer and a fly eye might sometimes be a powerful model for a human ear. Particularly the use of fly avatars, in which specific molecular (genetic or proteomic) states of humans (e.g. specific patients) are experimentally reproduced, in order to study the corresponding molecular mechanisms (e.g. specific diseases) in a controlled yet naturalistic environment, is a tool that promises multiple unprecedented insights. The use of the fly - and fly avatars - would benefit humans and will help enhance the power of other scientific models, such as the mouse.

Coupled membranes: a mechanism of frequency filtering and transmission in the field cricket ear evidenced by micro-computed tomography, laser Doppler vibrometry and finite element analysis

Many animals employ a second frequency filter beyond the initial filtering of the eardrum (or tympanal membrane). In the field cricket ear, both the filtering mechanism and the transmission path from the posterior tympanal membrane (PTM) have remained unclear. A mismatch between PTM vibrations and sensilla tuning has prompted speculations of a second filter. PTM coupling to the tracheal branches is suggested to support a transmission pathway. Here, we present three independent lines of evidence converging on the same conclusion: the existence of a series of linked membranes with distinct resonant frequencies serving both filtering and transmission functions. Micro-computed tomography (µ-CT) highlighted the 'dividing membrane (DivM)', separating the tracheal branches and connected to the PTM via the dorsal membrane of the posterior tracheal branch (DM-PTB). Thickness analysis showed the DivM to share significant thinness similarity with the PTM. Laser Doppler vibrometry indicated the first of two PTM vibrational peaks, at 6 and 14 kHz, originates not from the PTM but from the coupled DM-PTB. This result was corroborated by µ-CT-based finite element analysis. These findings clarify further the biophysical source of neuroethological pathways in what is an important model of behavioural neuroscience. Tuned microscale coupled membranes may also hold biomimetic relevance.

Functional Morphology of Leg Mechanosensory Organs in Early Postembryonic Development in the Stick Insect (Sipyloidea chlorotica)

The subgenual organ complex of stick insects has a unique neuroanatomical organisation with two elaborate chordotonal organs, the subgenual organ and the distal organ. These organs are present in all leg pairs and are already developed in newly hatched stick insects. The present study analyses for the first time the morphology of sensory organs in the subgenual organ complex for a membrane connecting the two sensory organs in newly hatched insects (Sipyloidea chlorotica (Audinet-Serville 1838)). The stick insect legs were analysed following hatching by axonal tracing and light microscopy. The subgenual organ complex in first juvenile instars shows the sensory organs and a thin membrane connecting the sensory organs resembling the morphology of adult animals. Rarely was this membrane not detected, where it is assumed as not developed during embryogenesis. The connection appears to influence the shape of the subgenual organ, with one end extending towards the distal organ as under tension. These findings are discussed for the following functional implications: (1) the physiological responses of the subgenual organ complex to mechanical stimuli after hatching, (2) the influence of the membrane on the displacement of the sensory organs, and (3) the connection between the subgenual organ and distal organ as a possible functional coupling.

Defensive fungal symbiosis on insect hindlegs

Tympanal organs as "insect ears" have evolved repeatedly. Dinidorid stinkbugs were reported to possess a conspicuous tympanal organ on female's hindlegs. Here we report an unexpected discovery that the stinkbug's "tympanal organ" is actually a novel symbiotic organ. The stinkbug's "tympanum" is not membranous but a porous cuticle, where each pore connects to glandular secretory cells. In reproductive females, the hindleg organ is covered with fungal hyphae growing out of the pores. Upon oviposition, the females skillfully transfer the fungi from the organ to the eggs. The eggs are quickly covered with hyphae and physically protected against wasp parasitism. The fungi are mostly benign Cordycipitaceae entomopathogens and show considerable diversity among insect individuals and populations, indicating environmental acquisition of specific fungal associates. These results uncover a novel external fungal symbiosis in which host's elaborate morphological, physiological and behavioral specializations underpin the selective recruitment of benign entomopathogens for a defensive purpose.

Vibration receptor organs in the insect leg: neuroanatomical diversity and functional principles

Detecting substrate vibrations is essential for insects in different behavioural contexts. These vibrational behaviours are mediated by mechanoreceptor organs detecting and processing vibrational stimuli transmitted in the environment. We discuss recently gained insights about the functional principles of insect vibration receptors, mainly leg chordotonal organs highly sensitive to vibrational stimuli, and the mechanisms of their diversification in neuroanatomy and functional morphology, in relation to the attachment structures and mechanical coupling. The two main input pathways for vibration stimuli transferred by the insect legs to vibrosensory organs via the cuticle and via the hemolymph are fundamental for explaining sensory specialisations. The vibroreceptor organs can diversify in their neuroanatomy and morphology in several key aspects. This provides the structural basis for complex adaptations in sensory evolution.

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.

Grasshoppers do not flee from their alarmed predators but from non-alarmed ones

Alarm calls produced by basal prey have a well-known informative value. In multi-predator communities, mesopredators, when faced with top predators, may emit alarm calls that could inform basal prey about their lowered predation risk. To test this unexplored possibility, we conducted one field and one mesocosm experiment in which we simulated alarm and non-alarm calls from little owls (Athene noctua) as mesopredators and measured their effects on grasshoppers as prey of little owls but not of top predators. In the field experiment, we found that grasshopper species were significantly more abundant in patches where we simulated either the presence of scared little owls (alarm treatment) or no owls (control treatment) compared to patches where the presence of non-scared little owls (non-alarm treatment) was simulated. In the mesocosm experiment, locusts (Locusta migratoria) moved significantly more to exposed areas when we simulated the presence of scared little owls (alarm treatment) or of a granivorous bird (control treatment), while they moved to sheltered areas when we simulated the presence of non-scared owls (non-alarm treatment). These results show that prey could cue on predators' calls to assess their predation risk and make decisions, revealing unprecedented potential ecological consequences of alarm calls in invertebrate communities.

Effects of dimpropyridaz on feeding behavior, locomotivity and biological parameters of Aphis gossypii

Aphis gossypii is a worldwide agricultural pest insect that has developed resistance to multiple pesticides. Dimpropyridaz is a new chordotonal organ regulator and has been registered for control of sap-sucking insects including A. gossypii. For the aim to effectively apply dimpropyridaz for A. gossypii control, it is necessary to clarify the toxic effects of dimpropyridaz on cotton aphids. In the present study, the effects of dimpropyridaz on feeding behavior, locomotivity and biological parameters of A. gossypii were investigated. The bioassay results showed that dimpropyridaz had good insecticidal activity against A. gossypii, with LC50 as 1.91 mg/L at 72 h post exposure. Moreover, the dimpropyridaz treated A. gossypii showed obvious poisoning symptoms of dehydration and shrivel. Through the gentle-touch experiment and feeding experiment, it was found that dimpropyridaz treatment had significant adverse impacts on the locomotivity and feeding behavior of A. gossypii. Compared with the control group, the coordinated movement ability of the treated A. gossypii attenuated, moreover the feeding behavior of A. gossypii was inhibited. The feeding rate decreased by 62.00%, 64.00% and 71.67% after treatment with 50.33 mg/L dimpropyridaz for 24 h, 48 h and 72 h, respectively. Especially, EPG recordings showed that the number of intracellular stylet puncture and the total duration of phloem sap ingestion and concurrent salivation decreased substantially, while the total duration of non-probing increased after exposure to dimpropyridaz. Furthermore, the treatments with LC10 and LC30 of dimpropyridaz significantly reduced the longevity and fecundity of F0, and led to a decrease of the relative fitness of F0 to 0.48 and 0.32, respectively. The net reproductive rate (R0) and mean generation time (T) of F1 generation were also significantly reduced, moreover the duration of reproduction was significantly shortened. In addition, at 72 h post treatment with LC30 dimpropyridaz, the gene expression levels of JHEH and USP of cotton aphids significantly increased, while the expression of FOXO, INR, EcR and INRS decreased. These results provide basis for clarifying the toxicology of dimpropyridaz to cotton aphids, and also are beneficial for effective control of cotton aphid using dimpropyridaz.

Spider dynamics under vertical vibration and its implications for biological vibration sensing

Often overlooked, vibration transmission through the entire body of an animal is an important factor in understanding vibration sensing in animals. To investigate the role of dynamic properties and vibration transmission through the body, we used a modal test and lumped parameter modelling for a spider. The modal test used laser vibrometry data on a tarantula, and revealed five modes of the spider in the frequency range of 20-200 Hz. Our developed and calibrated model took into account the bounce, pitch and roll of the spider body and bounce of all the eight legs. We then performed a parametric study using this calibrated model, varying factors such as mass, inertia, leg stiffness, damping, angle and span to study what effect they had on vibration transmission. The results support that some biomechanical parameters can act as physical constraints on vibration sensing. But also, that the spider may actively control some biomechanical parameters to change the signal intensity it can sense. Furthermore, our analysis shows that the parameter changes in front and back legs have a greater influence on whole system dynamics, so may be of particular importance for active control mechanisms to facilitate biological sensing functions.

Review of the applications of principles of insect hearing to microscale acoustic engineering challenges

When looking for novel, simple, and energy-efficient solutions to engineering problems, nature has proved to be an incredibly valuable source of inspiration. The development of acoustic sensors has been a prolific field for bioinspired solutions. With a diverse array of evolutionary approaches to the problem of hearing at small scales (some widely different to the traditional concept of 'ear'), insects in particular have served as a starting point for several designs. From locusts to moths, through crickets and mosquitoes among many others, the mechanisms found in nature to deal with small-scale acoustic detection and the engineering solutions they have inspired are reviewed. The present article is comprised of three main sections corresponding to the principal problems faced by insects, namely frequency discrimination, which is addressed by tonotopy, whether performed by a specific organ or directly on the tympana; directionality, with solutions including diverse adaptations to tympanal structure; and detection of weak signals, through what is known as active hearing. The three aforementioned problems concern tiny animals as much as human-manufactured microphones and have therefore been widely investigated. Even though bioinspired systems may not always provide perfect performance, they are sure to give us solutions with clever use of resources and minimal post-processing, being serious contenders for the best alternative depending on the requisites of the problem.

The chemistry of an insect ear: ionic composition of a liquid-filled ear and haemolymphs of Neotropical katydids

The purpose of this study is to examine and to compare the ionic composition of the haemolymph and the ear fluid of seven species of katydids (Orthoptera: Tettigoniidae) with the aim of providing from a biochemical perspective a preliminary assessment for an insect liquid contained in the auditory organ of katydids with a hearing mechanism reminiscent of that found in vertebrates. A multi-element trace analysis by inductively coupled plasma optical-emission spectrometry was run for 16 elements for the ear liquid of seven species and the haemolymph of six of them. Based on the obtained results, it can be recognized that the ionic composition is variable among the studied insects, but sodium (Na+), potassium (K+), calcium (Ca2+) and magnesium (Mg2+) are the most prominent of the dissolved inorganic cations. However, the ion concentrations between the two fluids are considerably different and the absence or low concentration of Ca2+ is a noticeable feature in the inner ear liquid. A potential relationship between the male courtship song peak frequency and the total ion (Na+, K+, Mg2+ and Ca2+) concentration of the inner ear liquid is also reported.

3D escape: an alternative paradigm for spatial orientation studies in insects

Arthropods and in particular insects show a great variety of different exoskeletal sensors. For most arthropods, spatial orientation and gravity perception is not fully understood. In particular, the interaction of the different sensors is still a subject of ongoing research. A disadvantage of most of the experimental methods used to date to study the spatial orientation of arthropods in behavioral experiments is that the body or individual body parts are fixed partly in a non-natural manner. Therefore, often only the movement of individual body segments can be used to evaluate the experiments. We here present a novel experimental method to easily study 3D-escape movements in insects and analyze whole-body reaction. The animals are placed in a transparent container, filled with a lightweight substrate and rotating around two axes. To verify our setup, house crickets (Acheta domesticus) with selectively manipulated gravity-perceiving structures were analyzed. The spatial orientation behavior was quantified by measuring the time individuals took to escape toward the surface and the angular deviation toward the gravitational vector. These experiments confirm earlier results and therefore validated our experimental setup. Our new approach thus allows to investigate several comprehensive questions regarding the spatial orientation of insects and other animals.

The auditory efferent system in mosquitoes

Whilst acoustic communication forms an integral component of the mating behavior of many insect species, it is particularly crucial for disease-transmitting mosquitoes; swarming males rely on hearing the faint sounds of flying females for courtship initiation. That males can hear females within the din of a swarm is testament to their fabulous auditory systems. Mosquito hearing is highly frequency-selective, remarkably sensitive and, most strikingly, supported by an elaborate system of auditory efferent neurons that modulate the auditory function - the only documented example amongst insects. Peripheral release of octopamine, serotonin and GABA appears to differentially modulate hearing across major disease-carrying mosquito species, with receptors from other neurotransmitter families also identified in their ears. Because mosquito mating relies on hearing the flight tones of mating partners, the auditory efferent system offers new potential targets for mosquito control. It also represents a unique insect model for studying auditory efferent networks. Here we review current knowledge of the mosquito auditory efferent system, briefly compare it with its counterparts in other species and highlight future research directions to unravel its contribution to mosquito auditory perception.

Micro-CT and deep learning: Modern techniques and applications in insect morphology and neuroscience

Advances in modern imaging and computer technologies have led to a steady rise in the use of micro-computed tomography (µCT) in many biological areas. In zoological research, this fast and non-destructive method for producing high-resolution, two- and three-dimensional images is increasingly being used for the functional analysis of the external and internal anatomy of animals. µCT is hereby no longer limited to the analysis of specific biological tissues in a medical or preclinical context but can be combined with a variety of contrast agents to study form and function of all kinds of tissues and species, from mammals and reptiles to fish and microscopic invertebrates. Concurrently, advances in the field of artificial intelligence, especially in deep learning, have revolutionised computer vision and facilitated the automatic, fast and ever more accurate analysis of two- and three-dimensional image datasets. Here, I want to give a brief overview of both micro-computed tomography and deep learning and present their recent applications, especially within the field of insect science. Furthermore, the combination of both approaches to investigate neural tissues and the resulting potential for the analysis of insect sensory systems, from receptor structures via neuronal pathways to the brain, are discussed.

The role of learning-walk related multisensory experience in rewiring visual circuits in the desert ant brain

Efficient spatial orientation in the natural environment is crucial for the survival of most animal species. Cataglyphis desert ants possess excellent navigational skills. After far-ranging foraging excursions, the ants return to their inconspicuous nest entrance using celestial and panoramic cues. This review focuses on the question about how naïve ants acquire the necessary spatial information and adjust their visual compass systems. Naïve ants perform structured learning walks during their transition from the dark nest interior to foraging under bright sunlight. During initial learning walks, the ants perform rotational movements with nest-directed views using the earth’s magnetic field as an earthbound compass reference. Experimental manipulations demonstrate that specific sky compass cues trigger structural neuronal plasticity in visual circuits to integration centers in the central complex and mushroom bodies. During learning walks, rotation of the sky-polarization pattern is required for an increase in volume and synaptic complexes in both integration centers. In contrast, passive light exposure triggers light-spectrum (especially UV light) dependent changes in synaptic complexes upstream of the central complex. We discuss a multisensory circuit model in the ant brain for pathways mediating structural neuroplasticity at different levels following passive light exposure and multisensory experience during the performance of learning walks.

Early embryonic development of Johnston's organ in the antenna of the desert locust Schistocerca gregaria

Johnston's organ has been shown to act as an antennal auditory organ across a spectrum of insect species. In the hemimetabolous desert locust Schistocerca gregaria, Johnston's organ must be functional on hatching and so develops in the pedicellar segment of the antenna during embryogenesis. Here, we employ the epithelial cell marker Lachesin to identify the pedicellar domain of the early embryonic antenna and then triple-label against Lachesin, the mitosis marker phosphohistone-3, and neuron-specific horseradish peroxidase to reveal the sense-organ precursors for Johnston's organ and their lineages. Beginning with a single progenitor at approximately a third of embryogenesis, additional precursors subsequently appear in both the ventral and dorsal pedicellar domains, each generating a lineage or clone. Lineage locations are remarkably conserved across preparations and ages, consistent with the epithelium possessing an underlying topographic coordinate system that determines the cellular organization of Johnston's organ. By mid-embryogenesis, twelve lineages are arranged circumferentially in the pedicel as in the adult structure. Each sense-organ precursor is associated with a smaller mitotically active cell from which the neuronal complement of each clone may derive. Neuron numbers within a clone increase in discrete steps with age and are invariant between clones and across preparations of a given age. At mid-embryogenesis, each clone comprises five cells consolidated into a tightly bound cartridge. A long scolopale extends apically from each cartridge to an insertion point in the epithelium, and bundled axons project basally toward the brain. Comparative data suggest mechanisms that might also regulate the developmental program of Johnston's organ in the locust.

The 3D ultrastructure of the chordotonal organs in the antenna of a microwasp remains complex although simplified

Insect antennae are astonishingly versatile and have multiple sensory modalities. Audition, detection of airflow, and graviception are combined in the antennal chordotonal organs. The miniaturization of these complex multisensory organs has never been investigated. Here we present a comprehensive study of the structure and scaling of the antennal chordotonal organs of the extremely miniaturized parasitoid wasp Megaphragma viggianii based on 3D electron microscopy. Johnston's organ of M. viggianii consists of 19 amphinematic scolopidia (95 cells); the central organ consists of five scolopidia (20 cells). Plesiomorphic composition includes one accessory cell per scolopidium, but in M. viggianii this ratio is only 0.3. Scolopale rods in Johnston's organ have a unique structure. Allometric analyses demonstrate the effects of scaling on the antennal chordotonal organs in insects. Our results not only shed light on the universal principles of miniaturization of sense organs, but also provide context for future interpretation of the M. viggianii connectome.

Low radiodensity μCT scans to reveal detailed morphology of the termite leg and its subgenual organ

Termites sense tiny substrate-borne vibrations through subgenual organs (SGOs) located within their legs' tibiae. Little is known about the SGOs' structure and physical properties. We applied high-resolution (voxel size 0.45 μm) micro-computed tomography (μCT) to Australian termites, Coptotermes lacteus and Nasutitermes exitiosus (Hill) to test two staining techniques. We compared the effectiveness of a single stain of Lugol's iodine solution (LS) to LS followed by Phosphotungstic acid (PTA) solutions (1% and 2%). We then present results of a soldier of Nasutitermes exitiosus combining μCT with LS + 2%PTS stains and scanning electron microscopy to exemplify the visualisation of their SGOs. The termite's SGO due to its approximately oval shape was shown to have a maximum diameter of 60 μm and a minimum of 48 μm, covering 60 ± 4% of the leg's cross-section and 90.4 ± 5% of the residual haemolymph channel. Additionally, the leg and residual haemolymph channel cross-sectional area decreased around the SGO by 33% and 73%, respectively. We hypothesise that this change in cross-sectional area amplifies the vibrations for the SGO. Since SGOs are directly connected to the cuticle, their mechanical properties and the geometric details identified here may enable new approaches to determine how termites sense micro-vibrations.

A numerical approach to investigating the mechanisms behind tonotopy in the bush-cricket inner-ear

Bush-crickets (or katydids) have sophisticated and ultrasonic ears located in the tibia of their forelegs, with a working mechanism analogous to the mammalian auditory system. Their inner-ears are endowed with an easily accessible hearing organ, the crista acustica (CA), possessing a spatial organisation that allows for different frequencies to be processed at specific graded locations within the structure. Similar to the basilar membrane in the mammalian ear, the CA contains mechanosensory receptors which are activated through the frequency dependent displacement of the CA. While this tonotopical arrangement is generally attributed to the gradual stiffness and mass changes along the hearing organ, the mechanisms behind it have not been analysed in detail. In this study, we take a numerical approach to investigate this mechanism in the Copiphora gorgonensis ear. In addition, we propose and test the effect of the different vibration transmission mechanisms on the displacement of the CA. The investigation was carried out by conducting finite-element analysis on a three-dimensional, idealised geometry of the C. gorgonensis inner-ear, which was based on precise measurements. The numerical results suggested that (i) even the mildest assumptions about stiffness and mass gradients allow for tonotopy to emerge, and (ii) the loading area and location for the transmission of the acoustic vibrations play a major role in the formation of tonotopy.

Reviving the sound of a 150-year-old insect: The bioacoustics of Prophalangopsis obscura (Ensifera: Hagloidea)

Determining the acoustic ecology of extinct or rare species is challenging due to the inability to record their acoustic signals or hearing thresholds. Katydids and their relatives (Orthoptera: Ensifera) offer a model for inferring acoustic ecology of extinct and rare species, due to allometric parameters of their sound production organs. Here, the bioacoustics of the orthopteran Prophalangopsis obscura are investigated. This species is one of only eight remaining members of an ancient family with over 90 extinct species that dominated the acoustic landscape of the Jurassic. The species is known from only a single confirmed specimen–the 150-year-old holotype material housed at the London Natural History Museum. Using Laser-Doppler Vibrometry, 3D surface scanning microscopy, and known scaling relationships, it is shown that P. obscura produces a pure-tone song at a frequency of ~4.7 kHz. This frequency range is distinct but comparable to the calls of Jurassic relatives, suggesting a limitation of early acoustic signals in insects to sonic frequencies (<20 kHz). The acoustic ecology and importance of this species in understanding ensiferan evolution, is discussed.

Vibration detection in arthropods: Signal transfer, biomechanics and sensory adaptations

In arthropods, the detection of vibrational signals and stimuli is essential in several behaviours, including mate recognition and pair formation, prey detection, and predator evasion. These behaviours have been studied in several species of insects, arachnids, and crustaceans for vibration production and propagation in the environment. Vibration stimuli are transferred over the animals' appendages and the body to vibrosensory organs. Ultimately, the stimuli are transferred to act on the dendrites of the mechanosensitive sensilla. We refer to these two different levels of transfer as macromechanics and micromechanics, respectively. These biomechanical processes have important roles in filtering and pre-processing of stimuli, which are not carried out by neuronal components of sensory organs. Also, the macromechanical transfer is posture-dependent and enables behavioural control of vibration detection. Diverse sensory organs respond to vibrations, including cuticular sensilla (slit sensilla, campaniform sensilla) and internal chordotonal organs. These organs provide various adaptations, as they occur at diverse body positions with different mechanical couplings as input pathways. Macromechanics likely facilitated evolution of vibrosensory organs at specific body locations. Thus, vibration detection is a highly complex sensory capacity, which employs body and sensory mechanics for signal filtering, amplification, and analysis of frequency, intensity and directionality.

Large abdominal mechanoreceptive sense organs in small plant-dwelling insects

The Hemiptera, with approximately 98 000 species, is one of the largest insect orders. Most species feed by sucking sap from plant tissues and are thus often vectors for economically important phytopathogens. Well known within this group are the large cicadas (Cicadomorpha: Cicadoidea: Cicadidae) because they produce extremely loud airborne sounds. Less well known are their mostly tiny relatives, the leafhoppers, spittlebugs, treehoppers and planthoppers that communicate by silent vibrational signals. While the generation of these signals has been extensively investigated, the mechanisms of their perception are poorly understood. This study provides a complete description and three-dimensional reconstruction of a large and complex array of mechanoreceptors in the first abdominal segments of the Rhododendron leafhopper Graphocephala fennahi (Cicadomorpha: Membracoidea: Cicadellidae). Further, we identify homologous organs in the spittlebug Philaenus spumarius (Cicadomorpha: Cercopoidea: Aphrophoridae) and the planthopper Issus coleoptratus (Fulgoromorpha: Fulgoroidea: Issidae). Such large abdominal sensory arrays have not been found in any other insect orders studied so far. This indicates that these sense organs, together with the signal-producing tymbal organ, constitute a synapomorphy of the Tymbalia (Hemiptera excl. Sternorrhyncha). Our results contribute to the understanding of the evolution from substrate-borne to airborne communication in insects.

Functional Interaction Between Drosophila Olfactory Sensory Neurons and Their Support Cells

Insects detect volatile chemicals using antennae, which house a vast variety of olfactory sensory neurons (OSNs) that innervate hair-like structures called sensilla where odor detection takes place. In addition to OSNs, the antenna also hosts various support cell types. These include the triad of trichogen, tormogen, and thecogen support cells that lie adjacent to their respective OSNs. The arrangement of OSN supporting cells occurs stereotypically for all sensilla and is widely conserved in evolution. While insect chemosensory neurons have received considerable attention, little is known about the functional significance of the cells that support them. For instance, it remains unknown whether support cells play an active role in odor detection, or only passively contribute to homeostasis, e.g., by maintaining sensillum lymph composition. To investigate the functional interaction between OSNs and support cells, we used optical and electrophysiological approaches in Drosophila. First, we characterized the distribution of various supporting cells using genetic markers. By means of an ex vivo antennal preparation and genetically-encoded Ca²⁺ and K⁺ indicators, we then studied the activation of these auxiliary cells during odor presentation in adult flies. We observed acute responses and distinct differences in Ca²⁺ and K⁺ fluxes between support cell types. Finally, we observed alterations in OSN responses upon thecogen cell ablation in mature adults. Upon inducible ablation of thecogen cells, we notice a gain in mechanical responsiveness to mechanical stimulations during single-sensillum recording, but a lack of change to the neuronal resting activity. Taken together, these results demonstrate that support cells play a more active and responsive role during odor processing than previously thought. Our observations thus reveal that support cells functionally interact with OSNs and may be important for the extraordinary ability of insect olfactory systems to dynamically and sensitively discriminate between odors in the turbulent sensory landscape of insect flight.

The antennal scape organ of Scutigera coleoptrata (Myriapoda) and a new type of arthropod tip-pore sensilla integrating scolopidial components

BACKGROUND

Centipedes are terrestrial, predatory arthropods with specialized sensory organs. However, many aspects of their sensory biology are still unknown. This also concerns hygroreception, which is especially important for centipedes, as their epicuticle is thin and they lose water rapidly at low humidity. Thus, the detection of humid places is vital but to date no definite hygroreceptor was found in centipedes. House centipedes (Scutigeromorpha) possess a peculiar opening at the base of their antenna, termed 'scape organ', that houses up to 15 cone-shaped sensilla in a cavity. Lacking wall and tip-pores, these socket-less sensilla may be hypothesized to function as hygroreceptors similar to those found in hexapods.

RESULTS

The cone-shaped sensilla in the scape organ as well as nearby peg-shaped sensilla are composed of three biciliated receptor cells and three sheath cells. A tip-pore is present but plugged by a highly electron-dense secretion, which also overlays the entire inner surface of the cavity. Several solitary recto-canal epidermal glands produce the secretion. Receptor cell type 1 (two cells in cone-shaped sensilla, one cell in peg-shaped sensilla) possesses two long dendritic outer segments that project to the terminal pore. Receptor cell type 2 (one cell in both sensilla) possesses two shorter dendritic outer segments connected to the first (proximal) sheath cell that establishes a scolopale-like structure, documented for the first time in detail in a myriapod sensillum.

CONCLUSIONS

The nearly identical configuration of receptor cells 1 with their long dendritic outer segments in both sensilla is similar to hexapod hygroreceptors. In Scutigera coleoptrata, however, the mechanism of stimulus transduction is different. Water vapor may lead to swelling and subsequent elongation of the plug pin that enters the terminal pore, thus causing stimulation of the elongated dendritic outer segments. The interconnection of receptor cell 2 with short outer dendritic segments to a scolopale-like structure potentially suits both sensilla for vibration or strain detection. Thus, both sensilla located at the antennal base of scutigeromorph centipedes fulfill a dual function.

Tuned vibration modes in a miniature hearing organ: Insights from the bushcricket

Most hearing organs contain an array of sensory cells that act as miniature microphones, each tuned to its own frequency like piano strings. Acoustically communicating insects like bushcrickets have evolved miniscule hearing organs, typically smaller than 1 mm, in their forelegs. It is still unknown how the sensory structures inside the leg vibrate in response to sound. Using advanced imaging techniques, we meticulously mapped the nanovibrations in the bushcricket ear. We discovered a complex motion pattern in which structures separated by only 1/50 mm showed systematic tuning differences. Despite the insect ear’s tiny dimensions, its mode of operation strikingly resembled that of vertebrate ears. Apparently, evolution has provided similar solutions to the spectral processing of sounds.

Bushcrickets (katydids) rely on only 20 to 120 sensory units located in their forelegs to sense sound. Situated in tiny hearing organs less than 1 mm long (40× shorter than the human cochlea), they cover a wide frequency range from 1 kHz up to ultrasounds, in tonotopic order. The underlying mechanisms of this miniaturized frequency-place map are unknown. Sensory dendrites in the hearing organ (crista acustica [CA]) are hypothesized to stretch, thereby driving mechanostransduction and frequency tuning. However, this has not been experimentally confirmed. Using optical coherence tomography (OCT) vibrometry, we measured the relative motion of structures within and adjacent to the CA of the bushcricket Mecopoda elongata. We found different modes of nanovibration in the CA that have not been previously described. The two tympana and the adjacent septum of the foreleg that enclose the CA were recorded simultaneously, revealing an antiphasic lever motion strikingly reminiscent of vertebrate middle ears. Over the entire length of the CA, we were able to separate and compare vibrations of the top (cap cells) and base (dorsal wall) of the sensory tissue. The tuning of these two structures, only 15 to 60 μm (micrometer) apart, differed systematically in sharpness and best frequency, revealing a tuned periodic deformation of the CA. The relative motion of the two structures, a potential drive of transduction, demonstrated sharper tuning than either of them. The micromechanical complexity indicates that the bushcricket ear invokes multiple degrees of freedom to achieve frequency separation with a limited number of sensory cells.

Bridging the Gap Between Mammal and Insect Ears – A Comparative and Evolutionary View of Sound-Reception

Insects must wonder why mammals have ears only in their head and why they evolved only one common principle of ear design—the cochlea. Ears independently evolved at least 19 times in different insect groups and therefore can be found in completely different body parts. The morphologies and functional characteristics of insect ears are as wildly diverse as the ecological niches they exploit. In both, insects and mammals, hearing organs are constrained by the same biophysical principles and their respective molecular processes for mechanotransduction are thought to share a common evolutionary origin. Due to this, comparative knowledge of hearing across animal phyla provides crucial insight into fundamental processes of auditory transduction, especially at the biomechanical and molecular level. This review will start by comparing hearing between insects and mammals in an evolutionary context. It will then discuss current findings about sound reception will help to bridge the gap between both research fields.

A narrow ear canal reduces sound velocity to create additional acoustic inputs in a microscale insect ear

The katydid tympanal ears have outer, middle, and inner ear components analogous to mammalian ears. Unlike mammals, each ear has two tympana exposed to sound both externally and internally, with a delayed internal version arriving via the gas-filled ear canal (EC). The two combined inputs in each ear play a significant role in directional hearing. Here, we demonstrate that the major factor causing the internal delay is the EC geometry. The EC bifurcates asymmetrically, producing two additional internal paths that impose different sound velocities for each tympanum. Therefore, various versions of the same signal reach the ears at various times, increasing the chance to pinpoint the sound source. Findings could inspire algorithms for accurate acoustic triangulation in detection sensors.

Located in the forelegs, katydid ears are unique among arthropods in having outer, middle, and inner components, analogous to the mammalian ear. Unlike mammals, sound is received externally via two tympanic membranes in each ear and internally via a narrow ear canal (EC) derived from the respiratory tracheal system. Inside the EC, sound travels slower than in free air, causing temporal and pressure differences between external and internal inputs. The delay was suspected to arise as a consequence of the narrowing EC geometry. If true, a reduction in sound velocity should persist independently of the gas composition in the EC (e.g., air, CO2). Integrating laser Doppler vibrometry, microcomputed tomography, and numerical analysis on precise three-dimensional geometries of each experimental animal EC, we demonstrate that the narrowing radius of the EC is the main factor reducing sound velocity. Both experimental and numerical data also show that sound velocity is reduced further when excess CO2 fills the EC. Likewise, the EC bifurcates at the tympanal level (one branch for each tympanic membrane), creating two additional narrow internal sound paths and imposing different sound velocities for each tympanic membrane. Therefore, external and internal inputs total to four sound paths for each ear (only one for the human ear). Research paths and implication of findings in avian directional hearing 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.

Control vs. Constraint: Understanding the Mechanisms of Vibration Transmission During Material-Bound Information Transfer

Material-bound vibrations are ubiquitous in the environment and are widely used as an information source by animals, whether they are generated by biotic or abiotic sources. The process of vibration information transfer is subject to a wide range of physical constraints, especially during the vibration transmission phase. This is because vibrations must travel through materials in the environment and body of the animal before reaching embedded mechanosensors. Morphology therefore plays a key and often overlooked role in shaping information flow. Web-building spiders are ideal organisms for studying vibration information transfer due to the level of control they have over morphological traits, both within the web (environment) and body, which can give insights for bioinspired design. Here we investigate the mechanisms governing vibration information transfer, including the relative roles of constraints and control mechanisms. We review the known and theoretical contributions of morphological and behavioral traits to vibration transmission in these spiders, and propose an interdisciplinary framework for considering the effects of these traits from a biomechanical perspective. Whereas morphological traits act as a series of springs, dampers and masses arranged in a specific geometry to influence vibration transmission, behavioral traits influence these morphologies often over small timescales in response to changing conditions. We then explore the relative roles of constraints and control mechanisms in shaping the variation of these traits at various taxonomic levels. This analysis reveals the importance of morphology modification to gain control over vibration transmission to mitigate constraints and essentially promote information transfer. In particular, we hypothesize that morphological computation is used by spiders during vibration information transfer to reduce the amount of processing required by the central nervous system (CNS); a hypothesis that can be tested experimentally in the future. We can take inspiration from how spiders control vibration transmission and apply these insights to bioinspired engineering. In particular, the role of morphological computation for vibration control could open up potential developments for soft robots, which could use multi-scale vibration sensory systems inspired by spiders to quickly and efficiently adapt to changing environments.

Directional hearing in insects: biophysical, physiological and ecological challenges

Sound localisation is a fundamental attribute of the way that animals perceive their external world. It enables them to locate mates or prey, determine the direction from which a predator is approaching and initiate adaptive behaviours. Evidence from different biological disciplines that has accumulated over the last two decades indicates how small insects with body sizes much smaller than the wavelength of the sound of interest achieve a localisation performance that is similar to that of mammals. This Review starts by describing the distinction between tympanal ears (as in grasshoppers, crickets, cicadas, moths or mantids) and flagellar ears (specifically antennae in mosquitoes and fruit flies). The challenges faced by insects when receiving directional cues differ depending on whether they have tympanal or flagellar years, because the latter respond to the particle velocity component (a vector quantity) of the sound field, whereas the former respond to the pressure component (a scalar quantity). Insects have evolved sophisticated biophysical solutions to meet these challenges, which provide binaural cues for directional hearing. The physiological challenge is to reliably encode these cues in the neuronal activity of the afferent auditory system, a non-trivial problem in particular for those insect systems composed of only few nerve cells which exhibit a considerable amount of intrinsic and extrinsic response variability. To provide an integrative view of directional hearing, I complement the description of these biophysical and physiological solutions by presenting findings on localisation in real-world situations, including evidence for localisation in the vertical plane.

What Does an Insect Hear? Reassessing the Role of Hearing in Predator Avoidance with Insights from Vertebrate Prey

Insects have a diversity of hearing organs known to function in a variety of contexts, including reproduction, locating food, and defense. While the role of hearing in predator avoidance has been extensively researched over the past several decades, this research has focused on the detection of one type of predator-echolocating bats. Here we reassess the role of hearing in antipredator defense by considering how insects use their ears to detect and avoid the wide range of predators that consume them. To identify the types of sounds that could be relevant to insect prey, we first review the topic of hearing-mediated predator avoidance in vertebrates. Sounds used by vertebrate prey to assess predation risk include incidental sound cues (e.g., flight sounds, rustling vegetation, and splashing) produced by an approaching predator or another escaping prey, as well as communication signals produced by a predator (e.g., echolocation calls, songs) or nonpredator (e.g., alarm calls). We then review what is known, and what is not known, about such sounds made by the main predators and parasitoids of insects (i.e., birds, bats, terrestrial vertebrates, and invertebrates) and how insects respond to them. Three key insights emerged from our review. First, there is a lack of information on how both vertebrate and insect prey use passive sound cues produced by predators to avoid being captured. Second, while there are numerous examples of vertebrate prey eavesdropping on the calls and songs of predators and nonpredators to assess risk, there are currently no such examples for eared insect prey. Third, the hearing sensitivity of many insects, including those with ears considered to be dedicated to detecting bats or mates, overlaps with both sound cues and signals generated by nonbat predators. Sounds of particular relevance to insect prey include the flight sounds and calls of insectivorous birds, the flight sounds of insect predators and parasitoids, and rustling vegetation sounds of birds and terrestrial predators. We conclude that research on the role of insect hearing in predator avoidance has been disproportionally focused on bat-detection, and that acoustically-mediated responses to other predators may have been overlooked because the responses of prey may be subtle (e.g., ceasing activity, increasing vigilance). We recommend that researchers expand their testing of hearing-mediated risk assessment in insects by considering the wide range of sounds generated by predators, and the varied responses exhibited by prey to these sounds.

Centrioles and Ciliary Structures during Male Gametogenesis in Hexapoda: Discovery of New Models

Centrioles are-widely conserved barrel-shaped organelles present in most organisms. They are indirectly involved in the organization of the cytoplasmic microtubules both in interphase and during the cell division by recruiting the molecules needed for microtubule nucleation. Moreover, the centrioles are required to assemble cilia and flagella by the direct elongation of their microtubule wall. Due to the importance of the cytoplasmic microtubules in several aspects of the cell life, any defect in centriole structure can lead to cell abnormalities that in humans may result in significant diseases. Many aspects of the centriole dynamics and function have been clarified in the last years, but little attention has been paid to the exceptions in centriole structure that occasionally appeared within the animal kingdom. Here, we focused our attention on non-canonical aspects of centriole architecture within the Hexapoda. The Hexapoda is one of the major animal groups and represents a good laboratory in which to examine the evolution and the organization of the centrioles. Although these findings represent obvious exceptions to the established rules of centriole organization, they may contribute to advance our understanding of the formation and the function of these organelles.

Neural representation of bat predation risk and evasive flight in moths: A modelling approach

Most animals are at risk from multiple predators and can vary anti-predator behaviour based on the level of threat posed by each predator. Animals use sensory systems to detect predator cues, but the relationship between the tuning of sensory systems and the sensory cues related to predator threat are not well-studied at the community level. Noctuid moths have ultrasound-sensitive ears to detect the echolocation calls of predatory bats. Here, combining empirical data and mathematical modelling, we show that moth hearing is adapted to provide information about the threat posed by different sympatric bat species. First, we found that multiple characteristics related to the threat posed by bats to moths correlate with bat echolocation call frequency. Second, the frequency tuning of the most sensitive auditory receptor in noctuid moth ears provides information allowing moths to escape detection by all sympatric bats with similar safety margin distances. Third, the least sensitive auditory receptor usually responds to bat echolocation calls at a similar distance across all moth species for a given bat species. If this neuron triggers last-ditch evasive flight, it suggests that there is an ideal reaction distance for each bat species, regardless of moth size. This study shows that even a very simple sensory system can adapt to deliver information suitable for triggering appropriate defensive reactions to each predator in a multiple predator community.

Multimodal cue integration in the dung beetle compass

South African ball-rolling dung beetles exhibit a unique orientation behavior to avoid competition for food: after forming a piece of dung into a ball, they efficiently escape with it from the dung pile along a straight-line path. To keep track of their heading, these animals use celestial cues, such as the sun, as an orientation reference. Here we show that wind can also be used as a guiding cue for the ball-rolling beetles. We demonstrate that this mechanosensory compass cue is only used when skylight cues are difficult to read, i.e., when the sun is close to the zenith. This raises the question of how the beetles combine multimodal orientation input to obtain a robust heading estimate. To study this, we performed behavioral experiments in a tightly controlled indoor arena. This revealed that the beetles register directional information provided by the sun and the wind and can use them in a weighted manner. Moreover, the directional information can be transferred between these 2 sensory modalities, suggesting that they are combined in the spatial memory network in the beetle's brain. This flexible use of compass cue preferences relative to the prevailing visual and mechanosensory scenery provides a simple, yet effective, mechanism for enabling precise compass orientation at any time of the day.

Material stiffness variation in mosquito antennae

The antennae of mosquitoes are model systems for acoustic sensation, in that they obey general principles for sound detection, using both active feedback mechanisms and passive structural adaptations. However, the biomechanical aspect of the antennal structure is much less understood than the mechano-electrical transduction. Using confocal laser scanning microscopy, we measured the fluorescent properties of the antennae of two species of mosquito- Toxorhynchites brevipalpis and Anopheles arabiensis-and, noting that fluorescence is correlated with material stiffness, we found that the structure of the antenna is not a simple beam of homogeneous material, but is in fact a rather more complex structure with spatially distributed discrete changes in material properties. These present as bands or rings of different material in each subunit of the antenna, which repeat along its length. While these structures may simply be required for structural robustness of the antennae, we found that in FEM simulation, these banded structures can strongly affect the resonant frequencies of cantilever-beam systems, and therefore taken together our results suggest that modulating the material properties along the length of the antenna could constitute an additional mechanism for resonant tuning in these species.

Cricket tympanal organ revisited: morphology, development and possible functions of the adult-specific chitin core beneath the anterior tympanal membrane

Vertebrates and insects are phylogenetically separated by millions of years but have commonly developed tympanal membranes for efficiently converting airborne sound to mechanical oscillation in hearing. The tympanal organ of the field cricket Gryllus bimaculatus, spanning 200 μm, is one of the smallest auditory organs among animals. It indirectly links to two tympana in the prothoracic tibia via tracheal vesicles. The anterior tympanal membrane is smaller and thicker than the posterior tympanal membrane and it is thought to have minor function as a sound receiver. Using differential labeling of sensory neurons/surrounding structures and three-dimensional reconstructions, we revealed that a shell-shaped chitin mass and associated tissues are hidden behind the anterior tympanal membrane. The mass, termed the epithelial core, is progressively enlarged by discharge of cylindrical chitin from epithelial cells that start to aggregate immediately after the final molt and it reaches a plateau in size after 6 days. The core, bridging between the anterior tracheal vesicle and the fluid-filled chamber containing sensory neurons, is supported by a taut membrane, suggesting the possibility that anterior displacements of the anterior tracheal vesicle are converted into fluid motion via a lever action of the core. The epithelial core did not exist in tympanal organ homologs of meso- and metathoracic legs or of nymphal legs. Taken together, the findings suggest that the epithelial core, a potential functional homolog to mammalian ossicles, underlies fine sound frequency discrimination required for adult-specific sound communications.

The NompC channel regulates Nilaparvata lugens proprioception and gentle-touch response

NompC channel is a member of the transient receptor potential (TRP) ion channel superfamily. It can regulate gentle-touch, locomotion, hearing and food texture detection in Drosophila. We cloned the NompC gene of Nilaparvata lugens (NlNompC). The full length NlNompC possessed similar structure as DmNompC, which belongs to TRPN subfamily. The expression pattern analysis of different developmental stages and body parts showed that the transcription of NlNompC was more abundant in adult stage and in the abdomen. Injection of double-stranded RNA (dsRNA) of NlNompC in the third-instar nymphs successfully knocked down the target gene with 75% suppression. At nine days after injection, the survival rate of dsRNA injected nymphs was as low as 9.84%. Behavioral observation revealed that the locomotion of the dsRNA injected nymphs was defective with much less movement compared to the negative control. Feeding and honeydew excretion of the dsRNA injected insects also decreased significantly. These results suggested that NlNompC is a classical mechanotransduction channel that plays important roles in proprioception and locomotion, and is essential for the survival of N. lugens. The results also contribute to the understanding of how TRP channels regulate proprioception.

Prolonged Bat Call Exposure Induces a Broad Transcriptional Response in the Male Fall Armyworm (Spodoptera frugiperda; Lepidoptera: Noctuidae) Brain

Predation risk induces broad behavioral and physiological responses that have traditionally been considered acute and transitory. However, prolonged or frequent exposure to predators and the sensory cues of their presence they broadcast to the environment impact long-term prey physiology and demographics. Though several studies have assessed acute and chronic stress responses in varied taxa, these attempts have often involved a priori expectations of the molecular pathways involved in physiological responses, such as glucocorticoid pathways and neurohormone production in vertebrates. While relatively little is known about physiological and molecular predator-induced stress in insects, many dramatic insect defensive behaviors have evolved to combat selection by predators. For instance, several moth families, such as Noctuidae, include members equipped with tympanic organs that allow the perception of ultrasonic bat calls and facilitate predation avoidance by eliciting evasive aerial flight maneuvers. In this study, we exposed adult male fall armyworm (Spodoptera frugiperda) moths to recorded ultrasonic bat foraging and attack calls for a prolonged period and constructed a de novo transcriptome based on brain tissue from predator cue-exposed relative to control moths kept in silence. Differential expression analysis revealed that 290 transcripts were highly up- or down-regulated among treatment tissues, with many annotating to noteworthy proteins, including a heat shock protein and an antioxidant enzyme involved in cellular stress. Though nearly 50% of differentially expressed transcripts were unannotated, those that were are implied in a broad range of cellular functions within the insect brain, including neurotransmitter metabolism, ionotropic receptor expression, mitochondrial metabolism, heat shock protein activity, antioxidant enzyme activity, actin cytoskeleton dynamics, chromatin binding, methylation, axonal guidance, cilia development, and several signaling pathways. The five most significantly overrepresented Gene Ontology terms included chromatin binding, macromolecular complex binding, glutamate synthase activity, glutamate metabolic process, and glutamate biosynthetic process. As a first assessment of transcriptional responses to ecologically relevant auditory predator cues in the brain of moth prey, this study lays the foundation for examining the influence of these differentially expressed transcripts on insect behavior, physiology, and life history within the framework of predation risk, as observed in ultrasound-sensitive Lepidoptera and other 'eared' insects.

Pymetrozine activates TRPV channels of brown planthopper Nilaparvata lugens

The commercial insecticide pymetrozine has been extensively used for brown planthopper control in East Asia. The transient receptor potential vanilloid (TRPV) channel, which consists of two proteins, Nanchung (Nan) and Inactive (Iav), has recently been shown to be the molecular target of pymetrozine in the fruit fly (Drosophila melanogaster) and pea aphid (Acyrthosiphon pisum). In this study, we characterized the Nan and Iav TRPV channel subunits of N. lugens and measured the action of pymetrozine on them. NlNan and NlIav are structurally similar to homologs from other insects. The expression pattern analysis of various body parts showed that NlNan and NlIav were both more abundantly expressed in antennae. When NlNan and NlIav were co-expressed in Xenopus laevis oocytes, they formed channels with high sensitivity to pymetrozine (EC₅₀ = 5.5 × 10^-8 M). Behavioral observation revealed that the gravitaxis defect in the fruit fly nan^36a mutant was rescued by ectopically expressed NlNan and the rescued behavior could be abolished by pymetrozine. Our results confirm that NlNan and NlIav co-expressed complexes can be activated by pymetrozine both in vitro and in vivo and provide useful information for future resistance mechanism studies.