Deaf moths employ acoustic Müllerian mimicry against bats using wingbeat-powered tymbals

Buckling-induced sound production in the aeroelastic tymbals of Yponomeuta

The loss of elastic stability (buckling) can lead to catastrophic failure in the context of traditional engineering structures. Conversely, in nature, buckling often serves a desirable function, such as in the prey-trapping mechanism of the Venus fly trap (Dionaea muscipula). This paper investigates the buckling-enabled sound production in the wingbeat-powered (aeroelastic) tymbals of Yponomeuta moths. The hindwings of Yponomeuta possess a striated band of ridges that snap through sequentially during the up- and downstroke of the wingbeat cycle-a process reminiscent of cellular buckling in compressed slender shells. As a result, bursts of ultrasonic clicks are produced that deter predators (i.e. bats). Using various biological and mechanical characterization techniques, we show that wing camber changes during the wingbeat cycle act as the single actuation mechanism that causes buckling to propagate sequentially through each stria on the tymbal. The snap-through of each stria excites a bald patch of the wing's membrane, thereby amplifying sound pressure levels and radiating sound at the resonant frequencies of the patch. In addition, the interaction of phased tymbal clicks from the two wings enhances the directivity of the acoustic signal strength, suggesting an improvement in acoustic protection. These findings unveil the acousto-mechanics of Yponomeuta tymbals and uncover their buckling-driven evolutionary origin. We anticipate that through bioinspiration, aeroelastic tymbals will encourage novel developments in the context of multi-stable morphing structures, acoustic structural monitoring, and soft robotics.

Behavioural Repeatability and Behavioural Syndrome in the Dung Beetle Copris umbilicatus (Coleoptera, Scarabaeidae)

Although personality studies have primarily focused on vertebrates, the evidence showing invertebrates to be capable of displaying personalities has been steadily growing in recent years. In this study, we investigated the behavioural repeatability (repetition of a behaviour over time) and behavioural syndromes (a set of correlated behaviours) in Copris umbilicatus, which is a dung beetle species showing complex sub-social behaviour. We analysed three behaviours (activity, thanatosis and distress call emission) by measuring seven distinct behavioural traits (i.e., three activity-, one thanatosis- and three distress call-related traits). We found moderate to high levels of individual repeatability in all behavioural traits considered. The duration of thanatosis was inversely correlated with two activity traits, hinting a behavioural syndrome for thanatosis and activity, with bolder individuals exhibiting shorter thanatosis and higher locomotor activity in contrast with fearful individuals, which display longer thanatosis and poor locomotor activity. No relationships were found between the behavioural traits and body size or sex. Results of the principal component analysis (PCA) suggested personality differences among individuals. Dung beetles provide an impressive variety of ecosystem services. Since the provision of these services may depend on the personalities represented in local populations and communities, studies on the ecology of personality in dung beetles should be encouraged in future research.

Sexual selection and predation drive the repeated evolution of stridulation in Heteroptera and other arthropods

Acoustic and substrate-borne vibrations are among the most widely used signalling modalities in animals. Arthropods display a staggering diversity of vibroacoustic organs generating acoustic sound and/or substrate-borne vibrations, and are fundamental to our broader understanding of the evolution of animal signalling. The primary mechanism that arthropods use to generate vibroacoustic signals is stridulation, which involves the rubbing together of opposing body parts. Although stridulation is common, its behavioural context and evolutionary drivers are often hard to pinpoint, owing to limited synthesis of empirical observations on stridulatory species. This is exacerbated by the diversity of mechanisms involved and the sparsity of their description in the literature, which renders their documentation a challenging task. Here, we present the most comprehensive review to date on the systematic distribution and behavioural context of stridulation. We use the megadiverse heteropteran insects as a model, together with multiple arthropod outgroups (arachnids, myriapods, and selected pancrustaceans). We find that stridulatory vibroacoustic signalling has evolved independently at least 84 times and is present in roughly 20% of Heteroptera, representing a remarkable case of convergent evolution. By studying the behavioural context of stridulation across Heteroptera and 189 outgroup lineages, we find that predation pressure and sexual selection are the main behaviours associated with stridulation across arthropods, adding further evidence for their role as drivers of large-scale signalling and morphological innovation in animals. Remarkably, the absence of tympanal ears in most Heteroptera suggests that they typically cannot detect the acoustic component of their stridulatory signals. This demonstrates that the adoption of new signalling modalities is not always correlated with the ability to perceive those signals, especially when these signals are directed towards interspecific receivers in defensive contexts. Furthermore, by mapping their morphology and systematic distribution, we show that stridulatory organs tend to evolve in specific body parts, likely originating from cleaning motions and pre-copulatory displays that are common to most arthropods. By synthesising our understanding of stridulation and stridulatory organs across major arthropod groups, we create the necessary framework for future studies to explore their systematic and behavioural significance, their potential role in sensory evolution and innovation, and the biomechanics of this mode of signalling.

The genome sequence of the Grey Ermine, Yponomeuta sedella (Treitschke, 1832)

We present a genome assembly from an individual male Yponomeuta sedella (the Grey Ermine; Arthropoda; Insecta; Lepidoptera; Yponomeutidae). The genome sequence is 658 megabases in span. Most of the assembly is scaffolded into 31 chromosomal pseudomolecules, including the assembled Z sex chromosome. The mitochondrial genome has also been assembled and is 16.4 kilobases in length. Gene annotation of this assembly on Ensembl has identified 13,010 protein coding genes.

Anti-bat ultrasound production in moths is globally and phylogenetically widespread

Warning signals are well known in the visual system, but rare in other modalities. Some moths produce ultrasonic sounds to warn bats of noxious taste or to mimic unpalatable models. Here, we report results from a long-term study across the globe, assaying moth response to playback of bat echolocation. We tested 252 genera, spanning most families of large-bodied moths, and document anti-bat ultrasound production in 52 genera, with eight subfamily origins described. Based on acoustic analysis of ultrasonic emissions and palatability experiments with bats, it seems that acoustic warning and mimicry are the raison d'être for sound production in most moths. However, some moths use high-duty-cycle ultrasound capable of jamming bat sonar. In fact, we find preliminary evidence of independent origins of sonar jamming in at least six subfamilies. Palatability data indicate that jamming and warning are not mutually exclusive strategies. To explore the possible organization of anti-bat warning sounds into acoustic mimicry rings, we intensively studied a community of moths in Ecuador and, using machine-learning approaches, found five distinct acoustic clusters. While these data represent an early understanding of acoustic aposematism and mimicry across this megadiverse insect order, it is likely that ultrasonically signaling moths comprise one of the largest mimicry complexes on earth.

Wingtip folds and ripples on saturniid moths create decoy echoes against bat biosonar

Sensory coevolution has equipped certain moth species with passive acoustic defenses to counter predation by echolocating bats.^1,2 Some large silkmoths (Saturniidae) possess curved and twisted biosonar decoys at the tip of elongated hindwing tails.^3,4 These are thought to create strong echoes that deflect biosonar-guided bat attacks away from the moth's body to less essential parts of their anatomy. We found that closely related silkmoths lacking such hindwing decoys instead often possess intriguing ripples and folds on the conspicuously lobed tips of their forewings. The striking analogy of twisted shapes displayed far from the body suggests these forewing structures might function as alternative acoustic decoys. Here we reveal that acoustic reflectivity and hence detectability of such wingtips is higher than that of the body at ultrasonic frequencies used by hunting bats. Wingtip reflectivity is higher the more elaborate the structure and the further from the body. Importantly, wingtip reflectivity is often considerably higher than in a well-studied functional hindwing decoy. Such increased reflectivity would misdirect the bat's sonar-guided attack toward the wingtip, resulting in similar fitness benefits to hindwing acoustic decoys. Structurally, folded wingtips present echo-generating surfaces to many directions, and folds and ripples can act as retroreflectors that together create conspicuous targets. Phylogenetically, folds and ripples at wingtips have evolved multiple times independently within silkmoths and always as alternatives to hindwing decoys. We conclude that they function as acoustic wingtip decoys against bat biosonar. VIDEO ABSTRACT.

Why and how to apply Weber's Law to coevolution and mimicry

In mimicry systems, receivers discriminate between the stimuli of models and mimics. Weber's Law of proportional processing states that receiver discrimination is based on proportional, not absolute, differences between stimuli. Weber's Law operates in a variety of taxa and modalities, yet it has largely been ignored in the context of mimicry, despite its potential relevance to whether receivers can discriminate models from mimics. Specifically, Weber's Law implies that for a given difference in stimulus magnitude between a model and mimic, as stimulus magnitudes increase, the mimic will be less discriminable from their model. This implies that mimics should benefit when stimulus magnitudes are high, and that high stimulus magnitudes will reduce selection for mimetic fidelity. Whether models benefit from high stimulus magnitudes depends on whether mimicry is honest or deceptive. We present four testable predictions about evolutionary trajectories of models and mimics based on this logic. We then provide a framework for testing whether receiver discrimination adheres to Weber's Law and illustrate it using coevolutionary examples and case studies from avian brood parasitism. We conclude that, when studying mimicry systems, researchers should consider whether receiver perception conforms to Weber's Law, because it could drive stimulus evolution in counterintuitive directions.

Convergent Evolution of Wingbeat-Powered Anti-Bat Ultrasound in the Microlepidoptera

Bats and moths provide a textbook example of predator-prey evolutionary arms races, demonstrating adaptations, and counter adaptations on both sides. The evolutionary responses of moths to the biosonar-led hunting strategies of insectivorous bats include convergently evolved hearing structures tuned to detect bat echolocation frequencies. These allow many moths to detect hunting bats and manoeuvre to safety, or in the case of some taxa, respond by emitting sounds which startle bats, jam their biosonar, and/or warn them of distastefulness. Until now, research has focused on the larger macrolepidoptera, but the recent discovery of wingbeat-powered anti-bat sounds in a genus of deaf microlepidoptera (Yponomeuta), suggests that the speciose but understudied microlepidoptera possess further and more widespread anti-bat defences. Here we demonstrate that wingbeat-powered ultrasound production, likely providing an anti-bat function, appears to indeed be spread widely in the microlepidoptera; showing that acoustically active structures (aeroelastic tymbals, ATs) have evolved in at least three, and likely four different regions of the wing. Two of these tymbals are found in multiple microlepidopteran superfamilies, and remarkably, three were found in a single subfamily. We document and characterise sound production from four microlepidopteran taxa previously considered silent. Our findings demonstrate that the microlepidoptera contribute their own unwritten chapters to the textbook bat-moth coevolutionary arms race.

Fireflies produce ultrasonic clicks during flight as a potential aposematic anti-bat signal

Fireflies are known for emitting light signals for intraspecific communication. However, in doing so, they reveal themselves to many potential nocturnal predators from a large distance. Therefore, many fireflies evolved unpalatable compounds and probably use their light signals as anti-predator aposematic signals. Fireflies are occasionally attacked by predators despite their warning flashes. Bats are among the most substantial potential firefly predators. Using their echolocation, bats might detect a firefly from a short distance and attack it in between two flashes. We thus aimed to examine whether fireflies use additional measures of warning, specifically focusing on sound signals. We recorded four species from different genera of fireflies in Vietnam and Israel and found that all of them generated ultrasonic clicks centered around bats' hearing range. Clicks were synchronized with the wingbeat and are probably produced by the wings. We hypothesize that ultrasonic clicks can serve as part of a multimodal aposematic display.

Ways that Animal Wings Produce Sound

There are at least eight ways that wings potentially produce sound. Five mechanisms are aerodynamic sounds, created by airflow, and three are structural sound created by interactions of solid surfaces. Animal flight is low Mach (M), meaning all animals move at <30% of the speed of sound. Thus in aerodynamic mechanisms the effects of air compressibility can be ignored, except in mechanism #1. Mechanism #1 is trapped air, in which air approaches or exceeds Mach 1 as it escapes a constriction. This mechanism is hypothetical but likely. #2 is Gutin sound, the aerodynamic reaction to lift and drag. This mechanism is ubiquitous in flight, and generates low frequency sound such as the humming of hummingbirds or insect wing tones. #3 is turbulence-generated atonal whooshing sounds, which are also widespread in animal flight. #4 are whistles, tonal sounds generated by geometry-induced flow feedback. This mechanism is hypothetical. #5 is aeroelastic flutter, sound generated by elasticity-induced feedback that is usually but not always tonal. This is widespread in birds (feathers are predisposed to flutter) but apparently not bats or insects. Mechanism #6 is rubbing sound (including stridulation), created when bird feathers or insect wings slide past each other. Atonal rubbing sounds are widespread in bird flight and insects; tonal stridulation is widespread in insects. #7 is percussion, created when two stiff elements collide and vibrate, and is present in some birds and insects. Mechanism #8 are tymbals and other bistable conformations. These are stiff elements that snap back and forth between two conformations, producing impulsive, atonal sound. Tymbals are widespread in insects but not birds or bats; insect cuticle appears predisposed to form tymbals. There are few examples of bat wing sounds: are bats intrinsically quiet, or just under-studied? These mechanisms, especially Gutin sound, whooshes, and rubbing (#2, #3, and #6) are prominent cues in ordinary flight of all flying animals, and are the "acoustic substrate" available to be converted from an adventitious sound (cue) into a communication signal. For instance, wing sounds have many times evolved into signals that are incorporated into courtship displays. Conversely, these are the sounds selected to be suppressed if quiet flight is selected for. The physical mechanisms that underlie animal sounds provide context for understanding the ways in which signals and cues may evolve.

Survival Sounds in Insects: Diversity, Function, and Evolution

Insect defense sounds have been reported for centuries. Yet, aside from the well-studied anti-bat sounds of tiger moths, little is understood about the occurrence, function, and evolution of these sounds. We define a defense sound as an acoustic signal (air- or solid-borne vibration) produced in response to attack or threat of attack by a predator or parasitoid and that promotes survival. Defense sounds have been described in 12 insect orders, across different developmental stages, and between sexes. The mechanisms of defensive sound production include stridulation, percussion, tymbalation, tremulation, and forced air. Signal characteristics vary between species, and we discuss how morphology, the intended receiver, and specific functions of the sounds could explain this variation. Sounds can be directed at predators or non-predators, and proposed functions include startle, aposematism, jamming, and alarm, although experimental evidence for these hypotheses remains scant for many insects. The evolutionary origins of defense sounds in insects have not been rigorously investigated using phylogenetic methodology, but in most cases it is hypothesized that they evolved from incidental sounds associated with non-signaling behaviors such as flight or ventilatory movements. Compared to our understanding of visual defenses in insects, sonic defenses are poorly understood. We recommend that future investigations focus on testing hypotheses explaining the functions and evolution of these survival sounds using predator-prey experiments and comparative phylogenetics.

The Ander's organ: a mechanism for anti-predator ultrasound in a relict orthopteran

The use of acoustics in predator evasion is a widely reported phenomenon amongst invertebrate taxa, but the study of ultrasonic anti-predator acoustics is often limited to the prey of bats. Here, we describe the acoustic function and morphology of a unique stridulatory structure - the Ander's organ - in the relict orthopteran Cyphoderris monstrosa (Ensifera, Hagloidea). This species is one of just eight remaining members of the family Prophalangopsidae, a group with a fossil record of over 90 extinct species widespread during the Jurassic period. We reveal that the sound produced by this organ has the characteristics of a broadband ultrasonic anti-predator defence, with a peak frequency of 58±15.5 kHz and a bandwidth of 50 kHz (at 10 dB below peak). Evidence from sexual dimorphism, knowledge on hearing capabilities and assessment of local predators, suggests that the signal likely targets ground-dwelling predators. Additionally, we reveal a previously undescribed series of cavities underneath the organ that probably function as a mechanism for ultrasound amplification. Morphological structures homologous in both appearance and anatomical location to the Ander's organ are observed to varying degrees in 4 of the 7 other extant members of this family, with the remaining 3 yet to be assessed. Therefore, we suggest that such structures may either be more widely present in this ancient family than previously assumed, or have evolved to serve a key function in the long-term survival of these few species, allowing them to outlive their extinct counterparts.

Moth wings are acoustic metamaterials

Metamaterials assemble multiple subwavelength elements to create structures with extraordinary physical properties (1-4). Optical metamaterials are rare in nature and no natural acoustic metamaterials are known. Here, we reveal that the intricate scale layer on moth wings forms a metamaterial ultrasound absorber (peak absorption = 72% of sound intensity at 78 kHz) that is 111 times thinner than the longest absorbed wavelength. Individual scales act as resonant (5) unit cells that are linked via a shared wing membrane to form this metamaterial, and collectively they generate hard-to-attain broadband deep-subwavelength absorption. Their collective absorption exceeds the sum of their individual contributions. This sound absorber provides moth wings with acoustic camouflage (6) against echolocating bats. It combines broadband absorption of all frequencies used by bats with light and ultrathin structures that meet aerodynamic constraints on wing weight and thickness. The morphological implementation seen in this evolved acoustic metamaterial reveals enticing ways to design high-performance noise mitigation devices.

Thoracic scales of moths as a stealth coating against bat biosonar

Many moths are endowed with ultrasound-sensitive ears that serve the detection and evasion of echolocating bats. Moths lacking such ears could still gain protection from bat biosonar by using stealth acoustic camouflage, absorbing sound waves rather than reflecting them back as echoes. The thorax of a moth is bulky and hence acoustically highly reflective. This renders it an obvious target for any bat. Much of the thorax of moths is covered in hair-like scales, the layout of which is remarkably similar in structure and arrangement to natural fibrous materials commonly used in sound insulation. Despite this structural similarity, the effect of thorax scales on moth echoes has never been characterized. Here, we test whether and how moth thorax scales function as an acoustic absorber. From tomographic echo images, we find that the thin layer of thoracic scales of diurnal butterflies affects the strength of ultrasound echoes from the thorax very little, while the thorax scales of earless moths absorbs an average of 67 ± 9% of impinging ultrasonic sound energy. We show that the thorax scales of moths provide acoustic camouflage by acting as broadband (20-160 kHz) stealth coating. Modelling results suggest the scales are acting as a porous sound absorber; however, the thorax scales of moths achieve a considerably higher absorption than technical fibrous porous absorbers with the same structural parameters. Such scales, despite being thin and lightweight, constitute a broadband, multidirectional and efficient ultrasound absorber that reduces the moths' detectability to hunting bats and gives them a survival advantage.