Active auditory mechanics in mosquitoes

Multisensory integration in Anopheles mosquito swarms: The role of visual and acoustic information in mate tracking and collision avoidance

Male mosquitoes form aerial aggregations, known as swarms, to attract females and maximize their chances of finding a mate. Within these swarms, individuals must be able to recognize potential mates and navigate the dynamic social environment to successfully intercept a mating partner. Prior research has almost exclusively focused on the role of acoustic cues in mediating the male mosquito's ability to recognize and pursue flying females. However, the role of other sensory modalities in this behavior has not been explored. Moreover, how males avoid collisions with one another in the dense swarm while pursuing females remains poorly understood. In this study, we combined free-flight and tethered flight simulator experiments to demonstrate that swarming Anopheles coluzzii mosquitoes integrate visual and acoustic information to track conspecifics and avoid collisions. Our tethered experiments revealed that acoustic stimuli gated mosquito steering responses to visual objects simulating nearby mosquitoes, especially in males that exhibited attraction to visual objects in the presence of female flight tones. Additionally, we observed that visual cues alone could trigger changes in mosquitoes' wingbeat amplitude and frequency. These findings were corroborated by our free-flight experiments, which revealed that mosquitoes modulate their flight responses to nearby conspecifics in a similar manner to tethered animals, allowing for collision avoidance within swarms. Together, these results demonstrate that both males and females integrate multiple sensory inputs to mediate swarming behavior, and for males, the change in flight kinematics in response to multimodal cues allows them to simultaneously track females while avoiding collisions.

From bristle to brain: embryonic development of topographic projections from basiconic sensilla in the antennal nervous system of the locust Schistocerca gregaria

The antennal flagellum of the locust S. gregaria is an articulated structure bearing a spectrum of sensilla that responds to sensory stimuli. In this study, we focus on the basiconic-type bristles as a model for sensory system development in the antenna. At the end of embryogenesis, these bristles are found at fixed locations and then on only the most distal six articulations of the antenna. They are innervated by a dendrite from a sensory cell cluster in the underlying epithelium, with each cluster directing fused axons topographically to an antennal tract running to the brain. We employ confocal imaging and immunolabeling to (a) identify mitotically active sense organ precursors for sensory cell clusters in the most distal annuli of the early embryonic antenna; (b) observe the subsequent spatial appearance of their neuronal progeny; and (c) map the spatial and temporal organization of axon projections from such clusters into the antennal tracts. We show that early in embryogenesis, proliferative precursors are localized circumferentially within discrete epithelial domains of the flagellum. Progeny first appear distally at the antennal tip and then sequentially in a proximal direction so that sensory neuron populations are distributed in an age-dependent manner along the antenna. Autotracing reveals that axon fasciculation with a tract is also sequential and reflects the location and age of the cell cluster along the most distal annuli. Cell cluster location and bristle location are therefore represented topographically and temporally within the axon profile of the tract and its projection to the brain.

Effects of age and noise on tympanal displacement in the Desert Locust

Insect cuticle is an evolutionary-malleable exoskeleton that has specialised for various functions. Insects that detect the pressure component of sound bear specialised sound-capturing tympani evolved from cuticular thinning. Whilst the outer layer of insect cuticle is composed of non-living chitin, its mechanical properties change during development and aging. Here, we measured the displacements of the tympanum of the desert Locust, Schistocerca gregaria, to understand biomechanical changes as a function of age and noise-exposure. We found that the stiffness of the tympanum decreases within 12 h of noise-exposure and increases as a function of age, independent of noise-exposure. Noise-induced changes were dynamic with an increased tympanum displacement to sound within 12 h post noise-exposure. Within 24 h, however, the tone-evoked displacement of the tympanum decreased below that of control Locusts. After 48 h, the tone-evoked displacement of the tympanum was not significantly different to Locusts not exposed to noise. Tympanal displacements reduced predictably with age and repeatably noise-exposed Locusts (every three days) did not differ from their non-noise-exposed counterparts. Changes in the biomechanics of the tympanum may explain an age-dependent decrease in auditory detection in tympanal insects.

Central projections from Johnston's organ in the locust: Axogenesis and brain neuroarchitecture

Johnston's organ (Jo) acts as an antennal wind-sensitive and/or auditory organ across a spectrum of insect species and its axons universally project to the brain. In the locust, this pathway is already present at mid-embryogenesis but the process of fasciculation involved in its construction has not been investigated. Terminal projections into the fine neuropilar organization of the brain also remain unresolved, information essential not only for understanding the neural circuitry mediating Jo-mediated behavior but also for providing comparative data offering insights into its evolution. In our study here, we employ neuron-specific, axon-specific, and epithelial domain labels to show that the pathway to the brain of the locust is built in a stepwise manner during early embryogenesis as processes from Jo cell clusters in the pedicel fasciculate first with one another, and then with the two tracts constituting the pioneer axon scaffold of the antenna. A comparison of fasciculation patterns confirms that projections from cell clusters of Jo stereotypically associate with only one axon tract according to their location in the pedicellar epithelium, consistent with a topographic plan. At the molecular level, all neuronal elements of the Jo pathway to the brain express the lipocalin Lazarillo, a cell surface epitope that regulates axogenesis in the primary axon scaffold itself, and putatively during fasciculation of the Jo projections to the brain. Central projections from Jo first contact the primary axon scaffold of the deutocerebral brain at mid-embryogenesis, and in the adult traverse mechanosensory/motor neuropils similar to those in Drosophila. These axons then terminate among protocerebral commissures containing premotor interneurons known to regulate flight behavior.

Mapping the Auditory Space of Culex pipiens Female Mosquitoes in 3D

The task of directional hearing faces most animals that possess ears. They approach this task in different ways, but a common trait is the use of binaural cues to find the direction to the source of sound. In insects, the task is further complicated by their small size and, hence, minute temporal and level differences between two ears. A single symmetric flagellar particle velocity receiver, such as the antenna of a mosquito, should not be able to discriminate between the two opposite directions along the vector of the sound wave. Paired antennae of mosquitoes presume the usage of binaural hearing, but its mechanisms are expected to be significantly different from the ones typical for the pressure receivers. However, the directionality of flagellar auditory organs has received little attention. Here, we measured the in-flight orientation of antennae in female Culex pipiens pipiens mosquitoes and obtained a detailed physiological mapping of the Johnston's organ directionality at the level of individual sensory units. By combining these data, we created a three-dimensional model of the mosquito's auditory space. The orientation of the antennae was found to be coordinated with the neuronal asymmetry of the Johnston's organs to maintain a uniformly shaped auditory space, symmetric relative to a flying mosquito. The overlap of the directional characteristics of the left and right sensory units was found to be optimal for binaural hearing focused primarily in front of, above and below a flying mosquito.

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.

Hearing of malaria mosquitoes is modulated by a beta-adrenergic-like octopamine receptor which serves as insecticide target

Malaria mosquitoes acoustically detect their mating partners within large swarms that form transiently at dusk. Indeed, male malaria mosquitoes preferably respond to female flight tones during swarm time. This phenomenon implies a sophisticated context- and time-dependent modulation of mosquito audition, the mechanisms of which are largely unknown. Using transcriptomics, we identify a complex network of candidate neuromodulators regulating mosquito hearing in the species Anopheles gambiae. Among them, octopamine stands out as an auditory modulator during swarm time. In-depth analysis of octopamine auditory function shows that it affects the mosquito ear on multiple levels: it modulates the tuning and stiffness of the flagellar sound receiver and controls the erection of antennal fibrillae. We show that two α- and β-adrenergic-like octopamine receptors drive octopamine's auditory roles and demonstrate that the octopaminergic auditory control system can be targeted by insecticides. Our findings highlight octopamine as key for mosquito hearing and mating partner detection and as a potential novel target for mosquito control.

Effect of temperature on mosquito olfaction

Mosquitoes use a wide range of cues to find a host to feed on, eventually leading to the transmission of pathogens. Among them, olfactory cues ( e.g. , host emitted odors, including CO 2 , and skin volatiles) play a central role in mediating host seeking behaviors. While mosquito olfaction can be impacted by many factors, such as the physiological state of the insect ( e.g. , age, reproductive state), the impact of environmental temperature on the olfactory system remains unknown. In this study, we quantified the behavioral responses of Aedes aegypti mosquitoes, vectors of dengue, yellow fever and Zika viruses, to host and plant related odors under different environmental temperatures.

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.

Acoustic-Related Mating Behavior in Tethered and Free-Flying Mosquitoes

Acoustics play an essential role in mosquito communication, particularly during courtship and mating. Mosquito mating occurs in flight and is coordinated by the perception of wingbeat tones. Flight tone frequencies have been shown to mediate sex recognition in Aedes, Anopheles, Culex, and Toxorhynchites genera and are thus a conserved feature of mating across the mosquito family (Culicidae). Upon recognizing a flying female, males respond phonotactically by lunging toward the female and initiating a precopulatory courtship flight interaction. During this interaction, males and females often harmonize their flight tones in a behavior known as harmonic convergence, and male acoustics display rapid frequency modulation. These acoustic phenomena have been characterized both in tethered and free-flying mosquitoes using similar audio recording and analysis methods. Further, the manipulation of mosquito acoustic-related mating behavior shows great promise as a tool for reproductive control strategies. In this brief methodological introduction, we provide an overview of the biological and technical concepts necessary for understanding the recording and analysis of mosquito mating acoustics.

Serotonin modulation in the male Aedes aegypti ear influences hearing

Male Aedes aegypti (Ae. aegypti) mosquitoes rely on hearing to identify conspecific females for mating, with the male attraction to the sound of flying females (“phonotaxis”) an important behavior in the initial courtship stage. Hearing thus represents a promising target for novel methods of mosquito control, and hearing behaviors (such as male phonotaxis) can be targeted via the use of sound traps. These traps unfortunately have proven to be relatively ineffective during field deployment. Shifting the target from hearing behavior to hearing function could therefore offer a novel method of interfering with Ae. aegypti mating. Numerous neurotransmitters, including serotonin (5-hydroxytryptamine, or 5-HT) and octopamine, are expressed in the male ear, with modulation of the latter proven to influence the mechanical responses of the ear to sound. The effect of serotonin modulation however remains underexplored despite its significant role in determining many key behaviors and biological processes of animals. Here we investigated the influence of serotonin on the Ae. aegypti hearing function and behaviors. Using immunohistochemistry, we found significant expression of serotonin in the male and female Ae. aegypti ears. In the male ear, presynaptic sites identified via antibody labelling showed only partial overlap with serotonin. Next, we used RT-qPCR to identify and quantify the expression levels of three different serotonin receptor families (5-HT1, 5-HT2, and 5-HT7) in the mosquito heads and ears. Although all receptors were identified in the ears of both sexes, those from the 5-HT7 family were significantly more expressed in the ears relative to the heads. We then thoracically injected serotonin-related compounds into the mosquitoes and found a significant, reversible effect of serotonin exposure on the male ear mechanical tuning frequency. Finally, oral administration of a serotonin-synthesis inhibitor altered male phonotaxis. The mosquito serotonergic system and its receptors thus represent interesting targets for novel methods of mosquito, and thus disease, control.

The Antenna Base Plays a Crucial Role in Mosquito Courtship Behavior

Mosquitoes are vectors of pathogens that cause diseases like malaria, dengue fever, yellow fever, chikungunya and Zika. For mosquito control it is crucial to understand their hearing system, as mosquitoes’ courting behavior is mostly auditory. Many nonlinear characteristics of the mosquito hearing organ have been observed through behavioral studies and neural measurements. These enable mosquitoes to detect and synchronize to other mosquitoes. Many hypotheses concerning the role of the flagellum and the fibrillae of the antenna in mosquito hearing have been made, and neural processes have been considered as the origin of the nonlinearities. In this study we introduce a geometric model based on the morphology of the mosquito antenna base. The model produces many of the observed nonlinear characteristics, providing evidence that the base of the antenna plays a crucial role in mosquito hearing. Even without neural processing, the antenna response to sound produces behaviorally relevant cues that can inform about the presence, location, and sex of other mosquitoes.

Behavioural analysis of swarming mosquitoes reveals higher hearing sensitivity than previously measured with electrophysiology methods

Mosquitoes of many species mate in station-keeping swarms. Mating chases ensue as soon as a male detects the flight tones of a female with his auditory organs. Previous studies of hearing thresholds have mainly used electrophysiological methods that prevent the mosquito from flying naturally. The main aim of this study was to quantify behaviourally the sound-level threshold at which males can hear females. Free-flying male Anopheles coluzzii were released in a large arena (∼2 m high x 2 m x 1 m) with a conspicuous object on the ground that stimulates swarming behaviour. Males were exposed to a range of natural and synthetic played-back sounds of female flight. We monitored the responses of males and their distance to the speaker by recording changes in their wingbeat frequency and angular speed. We show that the mean male behavioural threshold of particle-velocity hearing lies between 13-20 dB SVL (95%-CI). A conservative estimate of 20 dB SVL (i.e.,<0.5 µm/s particle velocity) is already 12 to 26 dB lower than most of the published electrophysiological measurements from the Johnston's organ. In addition, we suggest that 1) the first harmonic of female flight-sound is sufficient for males to detect her presence, 2) males respond with a greater amplitude to single-female sounds than to the sound of a group of females and 3) the response of males to the playback of the flight sound of a live female is the same as that of a recorded sound of constant frequency and amplitude.

Hitting the right note at the right time: Circadian control of audibility in Anopheles mosquito mating swarms is mediated by flight tones

Mating swarms of malaria mosquitoes form every day at sunset throughout the tropical world. They typically last less than 30 minutes. Activity must thus be highly synchronized between the sexes. Moreover, males must identify the few sporadically entering females by detecting the females’ faint flight tones. We show that the Anopheles circadian clock not only ensures a tight synchrony of male and female activity but also helps sharpen the males’ acoustic detection system: By raising their flight tones to 1.5 times the female flight tone, males enhance the audibility of females, specifically at swarm time. Previously reported “harmonic convergence” events are only a random by-product of the mosquitoes’ flight tone variance and not a signature of acoustic interaction between males and females. The flight tones of individual mosquitoes occupy narrow, partly non-overlapping frequency ranges, suggesting that the audibility of individual females varies across males.

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.

Harmonic convergence coordinates swarm mating by enhancing mate detection in the malaria mosquito Anopheles gambiae

The mosquito Anopheles gambiae is a major African malaria vector, transmitting parasites responsible for significant mortality and disease burden. Although flight acoustics are essential to mosquito mating and present promising alternatives to insecticide-based vector control strategies, there is limited data on mosquito flight tones during swarming. Here, for the first time, we present detailed analyses of free-flying male and female An. gambiae flight tones and their harmonization (harmonic convergence) over a complete swarm sequence. Audio analysis of single-sex swarms showed synchronized elevation of male and female flight tones during swarming. Analysis of mixed-sex swarms revealed additional 50 Hz increases in male and female flight tones due to mating activity. Furthermore, harmonic differences between male and female swarm tones in mixed-sex swarms and in single-sex male swarms with artificial female swarm audio playback indicate that frequency differences of approximately 50 Hz or less at the male second and female third harmonics (M2:F3) are maintained both before and during mating interactions. This harmonization likely coordinates male scramble competition by maintaining ideal acoustic recognition within mating pairs while acoustically masking phonotactic responses of nearby swarming males to mating females. These findings advance our knowledge of mosquito swarm acoustics and provide vital information for reproductive control strategies.

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.

Mosquitoes buzz and fruit flies don't-a comparative aeroacoustic analysis of wing-tone generation

Crepuscular mosquitoes, which swarm in low light conditions, exhibit a range of adaptations including large aspect-ratio wings, high flapping frequencies and small stroke amplitudes that taken together, facilitate the generation of wing-tones that are well-suited for acoustic communication. In the current study, we employ computational aeroacoustic modeling to conduct a comparative study of wing-tone and flight efficiency in a mosquito (maleCulex) and a similar sized flying insect: a fruit fly (Drosophila). Based on this analysis, we show that pound-for-pound, a mosquito generates wing-tones that are a factor of about 3.4 times more intense than a fruit fly, and the mosquito is more efficient by a factor of about 3.7 in converting mechanical power into acoustic power. The wing-tones for the mosquito are also more tilted in the forward direction, a characteristic that would be more conducive for acoustic signaling during a mate chase. The simulation data also shows that the specific power (mechanical power over mean lift) of the mosquito is nearly equal to that of the fruit fly, indicating that the adaptations that facilitate wing-tone based communication in mosquitoes, do not seem to compromise their flight efficiency.

Buzzkill: targeting the mosquito auditory system

Sound plays an important role in mosquito sensory ecology. Acoustic perception and acoustically driven behaviours therefore represent potentially effective control targets. Previous scientific efforts around acoustic-based control and surveillance have not been systematic and ambiguity around the exact role of acoustic communication in conspecific interactions remains. Here, we briefly review recent advances in mosquito auditory physiology and behavioural ecology as well as ongoing activities to incorporate sound into control and surveillance tools. We highlight areas where increased collaboration between physiologists, molecular biologists, behavioural ecologists and control experts is needed to capitalize on this progress and realize the potential of sound-based technologies and strategies.

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.

Bumblebee hairs as electric and air motion sensors: theoretical analysis of an isolated hair

Foraging bumblebees are electrically charged. Charge accumulation has been proposed to enable their ability to detect and react to electrical cues. One mechanism suggested for bumblebee electro-sensing is the interaction between external electric fields and electric charges accumulating on fine hairs on the cuticular body. Such hairs exhibit several functional adaptations, for example, thermal insulation, pollen capture and notably, the sensing of air motion such as flow currents or low frequency sound particle velocity. Both air motion and electric fields are ubiquitous in the sensory ecology of terrestrial arthropods, raising the question as to whether cuticular hairs respond to both stimuli. Here, a model-theoretical approach is taken to investigate the capacity of bumblebee filiform hairs as electric sensors and compare it to their response to air motion. We find that oscillating air motion and electric fields generate different mechanical responses, depending on stimulus frequency and body geometry. Further, hair morphology can enhance one sensing mode over the other; specifically, higher surface area favours electric sensitivity. Assuming a maximum stable charge on the hair that is limited only by electric breakdown of air, it is expected that an applied oscillating electric field strength of approximately 300 V m⁻¹ produces comparable mechanical response on the hair as a 35 mm s⁻¹ air flow oscillating at 130 Hz—an air disturbance signal similar to that produced by wingbeats of insects within a few bodylengths of the bumblebee. This analysis reveals that bumblebee filiform hairs can operate as bi-modal sensors, responding to both oscillating electric and air motion stimuli in the context of ecologically relevant scenarios.

Aerodynamic imaging by mosquitoes inspires a surface detector for autonomous flying vehicles

Some flying animals use active sensing to perceive and avoid obstacles. Nocturnal mosquitoes exhibit a behavioral response to divert away from surfaces when vision is unavailable, indicating a short-range, mechanosensory collision-avoidance mechanism. We suggest that this behavior is mediated by perceiving modulations of their self-induced airflow patterns as they enter a ground or wall effect. We used computational fluid dynamics simulations of low-altitude and near-wall flights based on in vivo high-speed kinematic measurements to quantify changes in the self-generated pressure and velocity cues at the sensitive mechanosensory antennae. We validated the principle that encoding aerodynamic information can enable collision avoidance by developing a quadcopter with a sensory system inspired by the mosquito. Such low-power sensing systems have major potential for future use in safer rotorcraft control systems.

Physiological Basis of Noise-Induced Hearing Loss in a Tympanal Ear

Acoustic overexposure, such as listening to loud music too often, results in noise-induced hearing loss. The pathologies of this prevalent sensory disorder begin within the ear at synapses of the primary auditory receptors, their postsynaptic partners and their supporting cells. The extent of noise-induced damage, however, is determined by overstimulation of primary auditory receptors, upstream of where the pathologies manifest. A systematic characterization of the electrophysiological function of the upstream primary auditory receptors is warranted to understand how noise exposure impacts on downstream targets, where the pathologies of hearing loss begin. Here, we used the experimentally-accessible locust ear (male, Schistocerca gregaria) to characterize a decrease in the auditory receptor's ability to respond to sound after noise exposure. Surprisingly, after noise exposure, the electrophysiological properties of the auditory receptors remain unchanged, despite a decrease in the ability to transduce sound. This auditory deficit stems from changes in a specialized receptor lymph that bathes the auditory receptors, revealing striking parallels with the mammalian auditory system.SIGNIFICANCE STATEMENT Noise exposure is the largest preventable cause of hearing loss. It is the auditory receptors that bear the initial brunt of excessive acoustic stimulation, because they must convert excessive sound-induced movements into electrical signals, but remain functional afterward. Here we use the accessible ear of an invertebrate to, for the first time in any animal, characterize changes in auditory receptors after noise overexposure. We find that their decreased ability to transduce sound into electrical signals is, most probably, due to changes in supporting (scolopale) cells that maintain the ionic composition of the ear. An emerging doctrine in hearing research is that vertebrate primary auditory receptors are surprisingly robust, something that we show rings true for invertebrate ears too.

Physiological Basis of Noise-Induced Hearing Loss in a Tympanal Ear

Acoustic overexposure, such as listening to loud music too often, results in noise-induced hearing loss. The pathologies of this prevalent sensory disorder begin within the ear at synapses of the primary auditory receptors, their postsynaptic partners and their supporting cells. The extent of noise-induced damage, however, is determined by overstimulation of primary auditory receptors, upstream of where the pathologies manifest. A systematic characterization of the electrophysiological function of the upstream primary auditory receptors is warranted to understand how noise exposure impacts on downstream targets, where the pathologies of hearing loss begin. Here, we used the experimentally-accessible locust ear (male, Schistocerca gregaria) to characterize a decrease in the auditory receptor's ability to respond to sound after noise exposure. Surprisingly, after noise exposure, the electrophysiological properties of the auditory receptors remain unchanged, despite a decrease in the ability to transduce sound. This auditory deficit stems from changes in a specialized receptor lymph that bathes the auditory receptors, revealing striking parallels with the mammalian auditory system.SIGNIFICANCE STATEMENT Noise exposure is the largest preventable cause of hearing loss. It is the auditory receptors that bear the initial brunt of excessive acoustic stimulation, because they must convert excessive sound-induced movements into electrical signals, but remain functional afterward. Here we use the accessible ear of an invertebrate to, for the first time in any animal, characterize changes in auditory receptors after noise overexposure. We find that their decreased ability to transduce sound into electrical signals is, most probably, due to changes in supporting (scolopale) cells that maintain the ionic composition of the ear. An emerging doctrine in hearing research is that vertebrate primary auditory receptors are surprisingly robust, something that we show rings true for invertebrate ears too.

Mechanism and scaling of wing tone generation in mosquitoes

The generation of sound from flapping (i.e. wing tones) of mosquito (Culex) wings is investigated using computational modeling. The flow field around the wing is simulated by solving the incompressible Navier-Stokes equations using a sharp-interface immersed boundary method, and the aeroacoustic sound is predicted by the Ffowcs Williams and Hawkings equation using data from the aerodynamic simulations. In addition to the aerodynamics, the characteristics of mosquito's wing tone, spectral directivity patterns, and generation mechanisms are investigated. The effects of wing-beat frequency and stroke amplitude are also studied, and scaling analysis for the mean lift, mechanical power, and overall wing tone sound power are performed to understand the effects of the wing shape and kinematics parameters. The analysis shows that the high wing aspect-ratio, high wing beat frequency, and small stroke amplitude adopted by mosquitoes enable efficient generation of high-intensity wing-tones for acoustic communications. The present findings may also apply to the optimized noise control in the flapping-wing micro air vehicles (FWMAV).

Anthropogenic noise and the bioacoustics of terrestrial invertebrates

Anthropogenic noise is an important issue of environmental concern owing to its wide-ranging effects on the physiology, behavior and ecology of animals. To date, research has focused on the impacts of far-field airborne noise (i.e. pressure waves) on vertebrates, with few exceptions. However, invertebrates and the other acoustic modalities they rely on, primarily near-field airborne and substrate-borne sound (i.e. particle motion and vibrations, respectively) have received little attention. Here, we review the literature on the impacts of different types of anthropogenic noise (airborne far-field, airborne near-field, substrate-borne) on terrestrial invertebrates. Using literature on invertebrate bioacoustics, we propose a framework for understanding the potential impact of anthropogenic noise on invertebrates and outline predictions of possible constraints and adaptations for invertebrates in responding to anthropogenic noise. We argue that understanding the impacts of anthropogenic noise requires us to consider multiple modalities of sound and to cultivate a broader understanding of invertebrate bioacoustics.

Directional and frequency characteristics of auditory neurons in Culex male mosquitoes

The paired auditory organ of mosquito, the Johnston's organ (JO), being the receiver of particle velocity component of sound, is directional by its structure. However, to date almost no physiological measurements of its directionality was done. In addition, the recent finding on the grouping of the JO auditory neurons into the antiphase pairs demanded confirmation by different methods. Using the vector superposition of the signals produced by two orthogonally oriented speakers, we measured the directional characteristics of individual units as well as their relations in physiologically distinguishable groups – pairs or triplets. The feedback stimulation method allowed to discriminate responses of the two simultaneously recorded units, and to show that they indeed responded in antiphase. Units of different frequency tuning as well as high-sensitive units (thresholds of 27 dB SPVL and below) were found in every angular sector of the JO, providing the mosquito with the ability to produce complex auditory behaviors.

Sex and species specific hearing mechanisms in mosquito flagellar ears

Hearing is essential for the courtship of one of the major carriers of human disease, the mosquito. Males locate females through flight-tone recognition and both sexes engage in mid-air acoustic communications, which can take place within swarms containing thousands of individuals. Despite the importance of hearing for mosquitoes, its mechanisms are still largely unclear. We here report a multilevel analysis of auditory function across three disease-transmitting mosquitoes (Aedes aegypti, Anopheles gambiae and Culex quinquefasciatus). All ears tested display transduction-dependent power gain. Quantitative analyses of mechanotransducer function reveal sex-specific and species-specific variations, including male-specific, highly sensitive transducer populations. Systemic blocks of neurotransmission result in large-amplitude oscillations only in male flagellar receivers, indicating sexually dimorphic auditory gain control mechanisms. Our findings identify modifications of auditory function as a key feature in mosquito evolution. We propose that intra-swarm communication has been a driving force behind the observed sex-specific and species-specific diversity.

Locomotor activity in males of Aedes aegypti can shift in response to females' presence

BACKGROUND

The study of physiological and behavioral traits of mosquito vectors has been of growing relevance for the proposition of alternative methods for controlling vector-borne diseases. Despite this, most studies focus on the female's traits, including the behavior of host seeking, the physiology of disease transmission and the site-choice for oviposition. However, understanding the factors that lead to males' reproductive success is of utmost importance, since it can help building new strategies for constraining population growth. Male behavior towards mating varies widely among species and the communication between males and females is the first aspect securing a successful encounter. Here we used an automated monitoring system to study the profile of locomotor activity of Aedes aegypti males in response to female's presence in an adapted confinement tube. We propose a new method to quantify male response to the presence of females, which can be potentially tested as an indicator of the success of one male in recognizing a female for mating.

RESULTS

Locomotor activity varies in daily cycles regulated by an endogenous clock and synchronized by external factors, such as light and temperature. Our results show the previously described startle response to light, which is displayed as a steep morning activity peak immediately when lights are on. Activity drops during the day and begins to rise again right before evening, happening about 1.5 h earlier in males than in females. Most interestingly, males' activity shows a double peak, and the second peak is very subtle when males are alone and relatively more pronounced when females are present in the confinement tubes. The switch in the peak of activity, measured by the herein suggested Peak Matching Index (PMI), was significantly different between males with and without females.

CONCLUSIONS

The adapted monitoring system used here allowed us to quantify the response of individual males to nearby females in terms of the extent of the activity peak displacement. In this direction, we created the peak matching index (PMI), a new parameter that we anticipate could be interpreted as the inclination of males to respond to females' presence, and further tested as an indicator of the potential for finding females for mating.

Masking of an auditory behaviour reveals how male mosquitoes use distortion to detect females

The mating behaviour of many mosquito species is mediated essentially by sound: males follow and mate with a female mid-flight by detecting and tracking the whine of her flight-tones. The stereotypical rapid frequency modulation (RFM) male behaviour, initiated in response to the detection of the female's flight-tones, has provided a means of investigating these auditory mechanisms while males are free-flying. Mosquitoes hear with their antennae, which vibrate to near-field acoustic excitation. The antennae generate nonlinear vibrations (distortion products, DPs) at frequencies that are equal to the difference between the two simultaneously presented tones, e.g. the male and female flight-tones, which are detected by mechanoreceptors in the auditory Johnston's organ (JO) at the base of the antenna. Recent studies indicated the male mosquito's JO is tuned not to the female flight-tone, but to the frequency difference between the male and female flight-tones. To test the hypothesis that mosquitoes detect this frequency difference, Culex quinquefasciatus males were presented simultaneously with a female flight-tone and a masking tone, which should suppress the male's RFM response to sound. The free-flight behavioural and in vivo electrophysiological experiments revealed that acoustic masking suppresses the RFM response to the female's flight-tones by attenuating the DPs generated in the nonlinear vibration of the antennae. These findings provide direct evidence in support of the hypothesis that male mosquitoes detect females when both are in flight through difference tones generated in the vibrations of their antennae owing to the interaction between their own flight-tones and those of a female.

Frequency doubling by active in vivo motility of mechanosensory neurons in the mosquito ear

Across vertebrate and invertebrate species, nonlinear active mechanisms are employed to increase the sensitivity and acuity of hearing. In mosquitoes, the antennal hearing organs are known to use active force feedback to enhance auditory acuity to female generated sounds. This sophisticated form of signal processing involves active nonlinear events that are proposed to rely on the motile properties of mechanoreceptor neurons. The fundamental physical mechanism for active auditory mechanics is theorized to rely on a synchronization of motile neurons, with a characteristic frequency doubling of the force generated by an ensemble of motile mechanoreceptors. There is however no direct biomechanical evidence at the mechanoreceptor level, hindering further understanding of the fundamental mechanisms of sensitive hearing. Here, using in situ and in vivo atomic force microscopy, we measure and characterize the mechanical response of mechanosensory neuron units during forced oscillations of the hearing organ. Mechanoreceptor responses exhibit the hallmark of nonlinear feedback for force generation, with movements at twice the stimulus frequency, associated with auditory amplification. Simultaneous electrophysiological recordings exhibit similar response features, notably a frequency doubling of the firing rate. This evidence points to the nature of the mechanism, whereby active hearing in mosquitoes emerges from the double-frequency response of the auditory neurons. These results open up the opportunity to directly investigate active cellular mechanics in auditory systems, and they also reveal a pathway to study the nanoscale biomechanics and its dynamics of cells beyond the sense of hearing.

Comparative Aspects of Hearing in Vertebrates and Insects with Antennal Ears

The evolution of hearing in terrestrial animals has resulted in remarkable adaptations enabling exquisitely sensitive sound detection by the ear and sophisticated sound analysis by the brain. In this review, we examine several such characteristics, using examples from insects and vertebrates. We focus on two strong and interdependent forces that have been shaping the auditory systems across taxa: the physical environment of auditory transducers on the small, subcellular scale, and the sensory-ecological environment within which hearing happens, on a larger, evolutionary scale. We briefly discuss acoustical feature selectivity and invariance in the central auditory system, highlighting a major difference between insects and vertebrates as well as a major similarity. Through such comparisons within a sensory ecological framework, we aim to emphasize general principles underlying acute sensitivity to airborne sounds.

Pre-copula acoustic behaviour of males in the malarial mosquitoes Anopheles coluzzii and Anopheles gambiae s.s. does not contribute to reproductive isolation

We reveal that males of two members of the Anopheles gambiae s.l. species complex, Anopheles coluzzii and Anopheles gambiae s.s. (hereafter A. gambiae), which are both malaria vectors, perform a stereotypical acoustic behaviour in response to pure tones at frequencies that encompass the frequency range of the female's flight-tones. This behaviour resembles that described for Culex quinquefasciatus and consists of phonotactic flight initiated by a steep increase in wing-beat frequency (WBF) followed by rapid frequency modulation (RFM) of WBF when in close proximity to the sound source. RFM was elicited without acoustic feedback or the presence of a live female, but it appears to be a stereotypic behaviour in the immediate lead up to copula formation. RFM is an independent and different behavioural process from harmonic convergence interactions used by male-female pairs for mate recognition at earlier stages of mating. Acoustic threshold for RFM was used to plot behavioural audiograms from free-flying A coluzzii and A gambiae males. These audiograms were almost identical (minima ∼400 Hz) and encompassed the WBF ranges of A coluzzii (378-601 Hz) and A gambiae (373-590 Hz) females, indicating that males of the two species share similar frequency tuning and range. Furthermore, no differences were found between the two species in their WBFs, RFM behaviour or harmonic convergence ratios. These results indicate that assortative mating between A coluzzii and A gambiae is unlikely to be based on male-specific acoustic behaviours during RFM. The significance of these findings in relation to possible mechanisms for assortative mating is discussed.

Stay tuned: active amplification tunes tree cricket ears to track temperature-dependent song frequency

Tree cricket males produce tonal songs, used for mate attraction and male-male interactions. Active mechanics tunes hearing to conspecific song frequency. However, tree cricket song frequency increases with temperature, presenting a problem for tuned listeners. We show that the actively amplified frequency increases with temperature, thus shifting mechanical and neuronal auditory tuning to maintain a match with conspecific song frequency. Active auditory processes are known from several taxa, but their adaptive function has rarely been demonstrated. We show that tree crickets harness active processes to ensure that auditory tuning remains matched to conspecific song frequency, despite changing environmental conditions and signal characteristics. Adaptive tuning allows tree crickets to selectively detect potential mates or rivals over large distances and is likely to bestow a strong selective advantage by reducing mate-finding effort and facilitating intermale interactions.

Frequency organization of the Johnston organ in male mosquitoes (Diptera, Culicidae)

The Johnston's organs (JO) of mosquitoes are the most complex mechanosensitive organs yet found in insects. Previous findings on behavior of mosquitoes suggest that, together with exceptional sensitivity, their auditory system can discriminate frequencies. Analysis of compound responses of the JO did not provide unambiguous evidence of such discrimination, even less did it help to find its mechanism. Using the feedback stimulation method, we measured the tuning frequencies of the JO sensory neurons. Here we present electrophysiological evidence that male mosquitoes of Culex pipiens possess at least eight groups of auditory neurons which are distinct in their frequency tuning, with individual frequencies ranging from 85 to 470 Hz. Most of the neurons are tuned to 190 – 270 Hz, which corresponds to the difference between a male and a female flight tones. Axons of the JO sensory units propagate graded amplified receptor potentials rather than all-or-none action potentials, are grouped into pairs or triplets and often respond in anti-phase to each other. Some features of the mosquito auditory system suggest an analogy to the retinal mechanisms. Together with our previous findings on frequency tuning in female mosquitoes of different species, this study presents evidence in favor of sophisticated frequency analysis of sound in mosquitoes.

Quantitative analysis of harmonic convergence in mosquito auditory interactions

This article analyses the hearing and behaviour of mosquitoes in the context of inter-individual acoustic interactions. The acoustic interactions of tethered live pairs of Aedes aegypti mosquitoes, from same and opposite sex mosquitoes of the species, are recorded on independent and unique audio channels, together with the response of tethered individual mosquitoes to playbacks of pre-recorded flight tones of lone or paired individuals. A time-dependent representation of each mosquito's non-stationary wing beat frequency signature is constructed, based on Hilbert spectral analysis. A range of algorithmic tools is developed to automatically analyse these data, and used to perform a robust quantitative identification of the ‘harmonic convergence’ phenomenon. The results suggest that harmonic convergence is an active phenomenon, which does not occur by chance. It occurs for live pairs, as well as for lone individuals responding to playback recordings, whether from the same or opposite sex. Male–female behaviour is dominated by frequency convergence at a wider range of harmonic combinations than previously reported, and requires participation from both partners in the duet. New evidence is found to show that male–male interactions are more varied than strict frequency avoidance. Rather, they can be divided into two groups: convergent pairs, typified by tightly bound wing beat frequencies, and divergent pairs, that remain widely spaced in the frequency domain. Overall, the results reveal that mosquito acoustic interaction is a delicate and intricate time-dependent active process that involves both individuals, takes place at many different frequencies, and which merits further enquiry.

Mechanosensory hairs in bumblebees (Bombus terrestris) detect weak electric fields

Bumblebees (Bombus terrestris) use information from surrounding electric fields to make foraging decisions. Electroreception in air, a nonconductive medium, is a recently discovered sensory capacity of insects, yet the sensory mechanisms remain elusive. Here, we investigate two putative electric field sensors: antennae and mechanosensory hairs. Examining their mechanical and neural response, we show that electric fields cause deflections in both antennae and hairs. Hairs respond with a greater median velocity, displacement, and angular displacement than antennae. Extracellular recordings from the antennae do not show any electrophysiological correlates to these mechanical deflections. In contrast, hair deflections in response to an electric field elicited neural activity. Mechanical deflections of both hairs and antennae increase with the electric charge carried by the bumblebee. From this evidence, we conclude that sensory hairs are a site of electroreception in the bumblebee.

A role for acoustic distortion in novel rapid frequency modulation behaviour in free-flying male mosquitoes

We describe a new stereotypical acoustic behaviour by male mosquitoes in response to the fundamental frequency of female flight tones during mating sequences. This male-specific free-flight behaviour consists of phonotactic flight beginning with a steep increase in wing-beat frequency (WBF) followed by rapid frequency modulation (RFM) of WBF in the lead up to copula formation. Male RFM behaviour involves remarkably fast changes in WBF and can be elicited without acoustic feedback or physical presence of the female. RFM features are highly consistent, even in response to artificial tones that do not carry the multi-harmonic components of natural female flight tones. Comparison between audiograms of the robust RFM behaviour and the electrical responses of the auditory Johnston's organ (JO) reveals that the male JO is tuned not to the female WBF per se but, remarkably, to the difference between the male and female WBFs. This difference is generated in the JO responses as a result of intermodulation distortion products (DPs) caused by non-linear interaction between male-female flight tones in the vibrations of the antenna. We propose that male mosquitoes rely on their own flight tones in making use of DPs to acoustically detect, locate and orientate towards flying females. We argue that the previously documented flight-tone harmonic convergence of flying male and female mosquitoes could be a consequence of WBF adjustments so that DPs generated through flight-tone interaction fall within the optimal frequency ranges for JO detection.

Concept of an Active Amplification Mechanism in the Infrared Organ of Pyrophilous Melanophila Beetles

Jewel beetles of the genus Melanophila possess a pair of metathoracic infrared (IR) organs. These organs are used for forest fire detection because Melanophila larvae can only develop in fire killed trees. Several reports in the literature and a modeling of a historic oil tank fire suggest that beetles may be able to detect large fires by means of their IR organs from distances of more than 100 km. In contrast, the highest sensitivity of the IR organs, so far determined by behavioral and physiological experiments, allows a detection of large fires from distances up to 12 km only. Sensitivity thresholds, however, have always been determined in non-flying beetles. Therefore, the complete micromechanical environment of the IR organs in flying beetles has not been taken into consideration. Because the so-called photomechanic sensilla housed in the IR organs respond bimodally to mechanical as well as to IR stimuli, it is proposed that flying beetles make use of muscular energy coupled out of the flight motor to considerably increase the sensitivity of their IR sensilla during intermittent search flight sequences. In a search flight the beetle performs signal scanning with wing beat frequency while the inputs of the IR organs on both body sides are compared. By this procedure the detection of weak IR signals could be possible even if the signals are hidden in the thermal noise. If this proposed mechanism really exists in Melanophila beetles, their IR organs could even compete with cooled IR quantum detectors. The theoretical concept of an active amplification mechanism in a photon receptor innervated by highly sensitive mechanoreceptors is presented in this article.

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.

Active amplification in insect ears: mechanics, models and molecules

Active amplification in auditory systems is a unique and sophisticated mechanism that expends energy in amplifying the mechanical input to the auditory system, to increase its sensitivity and acuity. Although known for decades from vertebrates, active auditory amplification was only discovered in insects relatively recently. It was first discovered from two dipterans, mosquitoes and flies, who hear with their light and compliant antennae; only recently has it been observed in the stiffer and heavier tympanal ears of an orthopteran. The discovery of active amplification in two distinct insect lineages with independently evolved ears, suggests that the trait may be ancestral, and other insects may possess it as well. This opens up extensive research possibilities in the field of acoustic communication, not just in auditory biophysics, but also in behaviour and neurobiology. The scope of this review is to establish benchmarks for identifying the presence of active amplification in an auditory system and to review the evidence we currently have from different insect ears. I also review some of the models that have been posited to explain the mechanism, both from vertebrates and insects and then review the current mechanical, neurobiological and genetic evidence for each of these models.

Influence of Substrate Color on Oviposition Behavior, Egg Hatchability, and Substance of Egg Origin in the Mosquito Anopheles sinensis (Wiedemann) (Diptera: Culicidae)

Understanding the factors that influence the choice of substrate for oviposition by the malaria mosquito is critical to efforts directed to the management of the disease and vector control measures aimed at modifications of larval habitat. The influence of black and white substrates on Anopheles sinensis (Wiedemann) (Culicidae: Anophelinae) female oviposition behavior and egg-hatching rate and the presence of substances associated with egg laying were studied. Results from the no-choice tests showed that the number of eggs laid on black substrate was significantly greater than that laid on white substrate. Results from the dual-choice tests revealed that gravid females showed stronger preference for the black substrate than that for the white substrate. Furthermore, the egg-hatching rate on white substrate was significantly lower than that observed on black substrate. Results from the three-choice tests showed that substance of egg origin was associated with the black substrates (UBS) that were attractive for and stimulated oviposition. The results of this study suggest that there might be some compounds in the black substrates which play a positive role in the oviposition behavior of female mosquitoes and in the development of eggs and that eggs might produce and release active substances that attract females and stimulate oviposition. These results could be important as regards to the optimization of mosquitoes raised for experimental purposes and the control of malaria mosquitoes by altering the oviposition behavior of gravid females.

Genome analysis of a major urban malaria vector mosquito, Anopheles stephensi

Background

Anopheles stephensi is the key vector of malaria throughout the Indian subcontinent and Middle East and an emerging model for molecular and genetic studies of mosquito-parasite interactions. The type form of the species is responsible for the majority of urban malaria transmission across its range.

Results

Here, we report the genome sequence and annotation of the Indian strain of the type form of An. stephensi. The 221 Mb genome assembly represents more than 92% of the entire genome and was produced using a combination of 454, Illumina, and PacBio sequencing. Physical mapping assigned 62% of the genome onto chromosomes, enabling chromosome-based analysis. Comparisons between An. stephensi and An. gambiae reveal that the rate of gene order reshuffling on the X chromosome was three times higher than that on the autosomes. An. stephensi has more heterochromatin in pericentric regions but less repetitive DNA in chromosome arms than An. gambiae. We also identify a number of Y-chromosome contigs and BACs. Interspersed repeats constitute 7.1% of the assembled genome while LTR retrotransposons alone comprise more than 49% of the Y contigs. RNA-seq analyses provide new insights into mosquito innate immunity, development, and sexual dimorphism.

Conclusions

The genome analysis described in this manuscript provides a resource and platform for fundamental and translational research into a major urban malaria vector. Chromosome-based investigations provide unique perspectives on Anopheles chromosome evolution. RNA-seq analysis and studies of immunity genes offer new insights into mosquito biology and mosquito-parasite interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0459-2) contains supplementary material, which is available to authorized users.

The importance of invertebrates when considering the impacts of anthropogenic noise

Anthropogenic noise is now recognized as a major global pollutant. Rapidly burgeoning research has identified impacts on individual behaviour and physiology through to community disruption. To date, however, there has been an almost exclusive focus on vertebrates. Not only does their central role in food webs and in fulfilling ecosystem services make imperative our understanding of how invertebrates are impacted by all aspects of environmental change, but also many of their inherent characteristics provide opportunities to overcome common issues with the current anthropogenic noise literature. Here, we begin by explaining why invertebrates are likely to be affected by anthropogenic noise, briefly reviewing their capacity for hearing and providing evidence that they are capable of evolutionary adaptation and behavioural plasticity in response to natural noise sources. We then discuss the importance of quantifying accurately and fully both auditory ability and noise content, emphasizing considerations of direct relevance to how invertebrates detect sounds. We showcase how studying invertebrates can help with the behavioural bias in the literature, the difficulties in drawing strong, ecologically valid conclusions and the need for studies on fitness impacts. Finally, we suggest avenues of future research using invertebrates that would advance our understanding of the impact of anthropogenic noise.

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

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