Insects inhabit complex vibroscapes shaped by substrate-borne vibrations from multiple biotic and abiotic sources. One underappreciated topic is how vibrations function in predator-prey interactions. Tiny warty birch caterpillars (Falcaria bilineata) are known to produce complex vibratory signals to defend leaf-tip territories against conspecifics, raising the question of whether vibratory signalling and sensing also play roles in predator-prey interactions. We staged encounters between resident neonate caterpillars and three natural intruders: conspecifics, ladybird beetle larvae and adult ladybird beetles, while simultaneously recording behaviour and substrate-borne vibrations. Resident caterpillars showed three key responses - vibratory signalling, freezing and dropping - but these responses varied strongly with intruder identity and stage of encounter. Residents signalled vigorously toward conspecifics, with rates escalating as intruders approached their territories. In contrast, encounters with predators evoked predator-specific defensive strategies including freezing and dropping. Adult ladybird beetles, which caused high mortality (43%), elicited rapid escape responses and subsequent territory abandonment, while ladybird beetle larvae, which caused no mortality, triggered slower responses with initial signalling that ceased upon closer approach. Critically, vibrations generated by the 'footsteps' of each approaching intruder type produced distinct vibratory 'signatures', differing in amplitude, spectral and temporal characteristics. Resident caterpillars also initiated defensive responses before physical contact, often when intruders were still centimetres away. Together, these findings demonstrate that these miniature larvae, no larger than ∼1-2 mm, thrive in complex vibroscapes where vibrations not only function to advertise territory ownership against conspecifics but also provide essential early-warning cues enabling sophisticated threat assessment and context-appropriate defensive responses in predator-rich environments.

Drepanidae caterpillars exhibit vibratory communication and variability in social behaviors within and between species, offering an excellent opportunity to test hypotheses on the function and evolution of vibratory communication and sociality in juvenile insects using comparative methods. Critical for conducting such studies is understanding morphological and behavioral traits across ontogeny. Here, we examine Falcaria bilineata to identify the number of developmental stages and their durations, as well as to characterize and distinguish their morphological and behavioral changes throughout development. Seven immature stages were identified: egg, five larval instars, and pupa, with a development time of 45 days from egg to pupation. Five instars were confirmed by counting head capsules. Head width showed a geometric increase between instars, consistent with Dyar's rule, thus confirming its reliability as an indicator of the instar. Morphological traits in head capsule shape and tubercles also help to distinguish instars. Feeding patterns transitioned from early instar leaf skeletonization to later instar leaf cutting. No group formation was observed, suggesting a lack of sociality. All instars lay silk mats, but no shelters were observed. Caterpillars rested in an arched position. Notably, all five instars generated vibratory signals. Fifth instars construct silk cocoons and remain inside until pupation. Pupae are covered by powdery white bloom in their cocoons and may emerge in two weeks or after overwintering. Our findings advance the understanding of life history and ontogenetic changes in F. bilineata, laying the groundwork for comparative studies into the mechanisms of social behavior and communication in larval insects.

Territorial displays include some of the most elaborate behaviours in the animal kingdom. In this study, we investigated the territorial behaviour and vibratory signalling of neonate warty birch caterpillars (Falcaria bilineata Lepidoptera: Drepanidae), which reside solitarily on birch leaves and defend the leaf tip. Upon hatching, these tiny caterpillars - no larger than 2 mm - seek out and establish a small solitary territory (∼1 cm wide) at the leaf tip, where they lay silk mats, feed, and advertise their presence by producing multicomponent vibratory signals – Buzz Scrapes and Drums. When a conspecific neonate (intruder) is introduced to a leaf occupied by a resident, the resident increases its signalling rate up to four times than when undisturbed, and even more - up to 14 times - if the intruder enters the territory. Intruders rarely manage to take over the resident's defended space, with most confrontations (71%) ending in the resident maintaining control. Residents signal significantly more than intruders at all stages of the contest. If physical contact occurs, residents flee by dropping from the leaf by a silk thread. This results in territorial contests that involve no physical aggression, relying entirely on vibratory communication. These vibratory displays most likely function to establish spacing between conspecifics on a tree branch, but these complex signals may also function to exclude other members of the vibratory community by mimicking something dangerous, like a jumping spider.

For over a century, the role of acoustic communication in the sensory ecology of bark beetles (Scolytinae) has been recognized. However, their 'world of sound' remains largely unexplored. Here, we review 153 years of bark beetle bioacoustics publications to summarize current knowledge, identify gaps and suggest future research directions. Our survey identified 117 publications covering 170 species. Morphological reports revealed five stridulatory organs across 125 species, with elytro-tergal, gular-prosternal and vertex-pronotum mechanisms being the most prevalent for sound production. However, confirmed sound recordings exist for only 40 species. Acoustic signalling in adults is proposed to function in avoiding enemies, pair formation, sexual selection and spacing, while in juveniles, vibratory communication is proposed for gallery spacing. However, experimental evidence supporting these functions is lacking. Acoustic sensory organs remain unidentified, and comprehension of signal transmission-whether through airborne sounds or solid-borne vibrations (or both)-is limited. Bioacoustic technologies have emerged as tools for potential management practices and are also discussed. Based on these findings, we recommend three directions for future research: (1) characterize acoustic morphology and behaviours in more species, particularly unrepresented taxa, with recordings in various contexts, preferably under natural conditions; (2) test hypotheses to explain the functions of acoustic communication through experimental and comparative phylogenetic methods and (3) investigate how sounds or vibrations are transmitted and received through behavioural and neurophysiological experiments. Advancements in bark beetle acoustic sensing and communication research will enhance our understanding of their sensory ecology and facilitate potential control measures of these fascinating insects. We review publications spanning 153 years of research on bark beetle bioacoustics-we summarize existing knowledge, identify gaps and propose avenues for future research in this field. At least five sound production mechanisms have been reported across 125 examined species of bark beetles, with elytro-tergal, gular-prosternal and vertex-pronotum mechanisms being the most common types of stridulatory organs. Acoustic communication in adult bark beetles is proposed to function in antipredator defence, species identification, sexual selection, territoriality, colonization, gallery construction and spacing, while in juveniles, vibratory communication is proposed for gallery spacing. image

Silk and hawk moth caterpillars produce a variety of defence sounds described as clicking, chirping, whistling, and vocalising. Such diversity provides opportunity to test hypotheses on the functions and evolution of insect defence sounds using comparative analyses. A key step to conducting comparative analyses is to categorise different phenotypes. Here, we describe mandible sounds of 20 species of late instar Bombycoidea caterpillars and establish objective criteria for differentiating between sound-producing mechanisms. First, we assess how the two mandibular mechanisms – clicking and chirping – differ from one another using two reference species: Antheraea polyphemus (clicker) and Saturnia pyri (chirper). In these references, clicks are produced by ridged and serrated mandibles and have short duration units with few pulses, whereas chirps are produced by scalloped mandibles and have longer duration units with more pulses. Second, we characterise acoustic and morphological traits of 18 additional species. These are categorised as clickers (13 species), chirpers (4 species), or other (1 species) using diagnostic features identified from the reference species. Third, these categorisations are tested using a predictive logistic regression model. The results of this study contribute to our understanding of acoustically mediated defences in caterpillars and provide necessary criteria for conducting further comparative studies.

Objective Group-living plays a key role in the success of many insects, but the mechanisms underlying group formation and maintenance are poorly understood. Here we use the masked birch caterpillar, Drepana arcuata, to explore genetic influences on social grouping. These larvae predictably transition from living in social groups to living solitarily during the 3rd instar of development. Our previous study showed a notable shift in the D. arcuata transcriptome that correlates with the transition from grouping to solitary behavior. We noted that one differentially regulated gene, octopamine receptor gene (DaOAR), is a prominent ‘social’ gene in other insect species, prompting us to test the hypothesis that DaOAR influences grouping behavior in D. arcuata . This was done using RNA interference (RNAi) methods by feeding second instar larvae synthetic dsRNAs. Results RT–qPCR analysis confirmed a significant reduction in DaOAR transcript abundance in dsRNA-fed larvae compared to controls. Behavioral trials showed that caterpillars with reduced transcript abundance of DaOAR remained solitary throughout the observation period compared to controls. These results provide evidence that regulation of the octopamine receptor gene influences social grouping in D. arcuata , and that specifically, a decrease in octopamine receptor expression triggers the larval transition from social to solitary.

Deimatic behaviours, also referred to as startle behaviours, are used against predators and rivals. Although many are spectacular, their proximate and ultimate causes remain unclear. In this review we aim to synthesise what is known about deimatic behaviour and identify knowledge gaps. We propose a working hypothesis for deimatic behaviour, and discuss the available evidence for the evolution, ontogeny, causation, and survival value of deimatic behaviour using Tinbergen's Four Questions as a framework. Our overarching aim is to direct future research by suggesting ways to address the most pressing questions in this field.

Research on insect biotremology has resulted in a burgeoning body of literature over the past few decades. Despite this, several biases and knowledge gaps have been proposed, but not quantified. Therefore, a systematic literature review and meta-analyses were carried out to summarize the temporal trends and test for biases regarding taxa, develop-mental stages, and research topics reported in scientific papers spanning 75 years. The survey tracked 831 papers, which exhibited exponential growth since the 1990s and covered 17 insect orders. Among these studies, 70.4% were associated with adaptive behaviors, while the remaining (29.6%) focused on applied entomology and sensory organs. Three main biases were detected: (i) a prevalence of studies on Hemiptera, Hymenoptera, and Coleoptera, (ii) a focus on adults, and (iii) a preponderance of studies on reproductive behaviors. Considering only adaptive behaviors, the likelihood of studies with adults was 3?? higher than for juveniles. Studies documenting receiver response were 2?? higher than not. Still, few insect orders (9 of 17) included reports on vibrations used in an adaptive context, while studies reported in the remain-ing orders focused on mechanisms of vibration production or vibration characteristics. The results of this study highlight knowledge gaps worthy of future investigations. In particular, further research is necessary on the role of vibratory sensing and communication in juveniles (eggs, larvae, pupae, and nymphs), testing hypotheses on the adaptive roles of vibrations in a broader range of taxa, characterization of vibratory landscapes, and research on sensory receptors.

Leaf-borne vibrations are predicted to be significant for caterpillar communication and risk assessment, but the caterpillar’s vibratory landscape remains largely unknown. To address this, we used the fall armyworm Spodoptera frugiperda , as a model in our study with two main goals: (1) to characterize the vibratory landscape on a leaf in the presence of abiotic (wind and rain) and biotic (conspecifics and invertebrate predator) stimuli; and (2) to assess whether different larval instars detect and respond to those vibrations. Our findings show that abiotic and biotic vibrations were distinct from background noise, except for those produced by 1st instar larvae. Wind-induced leaf movement produced vibrations with a low-frequency and high-amplitude (< 100 Hz and 2.97 mm s −1 ), in contrast with raindrops (> 174 Hz; 3.25 mm s −1 ). The 2nd to 5th instar larvae and predatory stinkbugs moving on leaves produced vibrations with dominant frequencies ranging from 140 to 326 Hz and amplitudes from 1.42 to 2.95 mm s −1 . Furthermore, the spatial distribution of vibrations across bean leaves revealed that abiotic vibrations were more widely spread across leaves, unlike the more concentrated biotic vibrations. Regarding the caterpillar response to vibratory stimuli, caterpillars exposed to abiotic stimuli behaved differently from undisturbed caterpillars, regardless of instar. By contrast, caterpillars exposed to biotic stimuli do not respond consistently. Our findings contribute insights into a caterpillar’s vibroscape and support the hypothesis that armyworms can perceive and respond to both abiotic and biotic vibrations, filling a knowledge gap about this economically important pest species' sensory ecology.

Over the past few decades, scientists have discovered that insects live in complex vibratory environments that they both create and exploit for their survival. Yet, this growing body of literature is focused predominantly on adults, leaving a significant knowledge gap on vibratory sensing and communication in juveniles. There is burgeoning yet scattered evidence indicating that juveniles use vibrations in a wide range of contexts. In this chapter, we endeavored to synthesize the literature on vibratory sensing and communication in caterpillars (larval Lepidoptera). Caterpillars are obligate substrate-bound insects that are exposed to a wide range of vibrations generated by biotic and abiotic sources. This chapter shows that caterpillars across diverse taxonomic groups use vibrations in a variety of contexts, including predator detection and risk assessment, detection of abiotic events such as wind and rain, recruitment and spacing, territorial defense, and maintaining relationships with ants. While it is clear that many caterpillars are capable of detecting and discriminating between vibration sources, vibration receptors have not yet been identified in caterpillars, or any other holometabolous insect larvae. We discuss potential vibration receptors in larvae based on our knowledge of adult receptors and larval morphology and physiology. The vibratory landscapes of juvenile insects, including eggs, larvae, nymphs, and pupae, remain poorly understood. Yet, most juveniles are substrate-bound, and therefore, it is likely that vibrations play an important role in their survival. We recommend further investigations on vibratory sensing and communication in juveniles, from documenting the species using vibrations to discovering the sensory organs involved in detecting and processing vibratory information.

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.

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 batdetection, 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.

The masked birch caterpillar, Drepana arcuata Walker (Lepidoptera: Drepanidae), and other Drepanoidea (Lepidoptera) species are excellent organisms for investigating the function and evolution of vibratory communication and sociality in caterpillars. We present a de novo assembled draft genome and functional annotation for D. arcuata, using a combination of short and long sequencing reads generated by Illumina HiSeq X and Oxford Nanopore Technologies (ONT) MinION sequencing platforms, respectively. A total of 460,694,612 150bp paired-end Illumina and 395,890 ONT raw reads were assembled into 11,493 scaffolds spanning a genome size of 270.5Mb. The resulting D. arcuata genome has a GC content of 38.79%, repeat content of 8.26%, is 86.5% complete based on Benchmarking Universal Single-Copy Orthologs (BUSCO) assessment, and comprises 10,398 predicted protein-coding genes. These data represent the first genomic resources for the lepidopteran superfamily Drepanoidea. Although the order Lepidoptera comprises numerous ecologically and economically important species, assembled genomes and annotations are available for < 1% of the total species. These data can be further utilized for research on Lepidoptera genomics as well as on the function and evolution of vibratory communication and sociality in larval insects.

The masked birch caterpillar, Drepana arcuata, provides an excellent opportunity to study mechanisms mediating developmental changes in social behaviour. Larvae transition from being social to solitary during the 3rd instar, concomitant with shifts in their use of acoustic communication. In this study we characterize the transcriptome of D. arcuata to initiate sociogenomic research of this lepidopteran insect. We assembled and annotated the combined larval transcriptome of "social" early and "solitary" late instars using next generation Illumina sequencing, and used this transcriptome to conduct differential gene expression analysis of the two behavioural phenotypes. A total of 211,012,294 reads generated by RNA sequencing were assembled into 231,348 transcripts and 116,079 unigenes for the functional annotation of the transcriptome. Expression analysis revealed 3300 transcripts that were differentially expressed between early and late instars, with a large proportion associated with development and metabolic processes. We independently validated differential expression patterns of selected transcripts using RT-qPCR. The expression profiles of social and solitary larvae revealed differentially expressed transcripts coding for gene products that have been previously reported to influence social behaviour in other insects (e.g. cGMP- and cAMP- dependent kinases, and bioamine receptors). This study provides the first transcriptomic resources for a lepidopteran species belonging to the superfamily Drepanoidea, and gives insight into genetic factors mediating grouping behaviour in insects.

Many insects vary their song patterns to communicate different messages, but the underlying biomechanisms are often poorly understood. Here, we report on the mechanics of sound production and variation in an elytro-tergal stridulator, male Dendroctonus valens bark beetles. Using ablation experiments coupled with high-speed video and audio recordings, we show that: (1) chirps are produced using a stridulatory file on the left elytron (forewing) and a protrusion (plectrum) on the seventh abdominal segment; (2) chirps are produced by 'spring stridulation', a catch-and-release mechanism whereby the plectrum catches on a file tooth and, upon release, springs forward along the file; and (3) variability in chirp types is caused by introducing multiple catch-and-release events along the file to create regular interruptions. These results provide experimental evidence for the mechanics of elytro-tergal stridulation, and provide insight into how an insect can incorporate variability into its acoustic repertoire using a spring-loaded mechanism.

Butterflies and moths (Lepidoptera) are one of the major superradiations of insects, comprising nearly 160,000 described extant species. As herbivores, pollinators, and prey, Lepidoptera play a fundamental role in almost every terrestrial ecosystem. Lepidoptera are also indicators of environmental change and serve as models for research on mimicry and genetics. They have been central to the development of coevolutionary hypotheses, such as butterflies with flowering plants and moths' evolutionary arms race with echolocating bats. However, these hypotheses have not been rigorously tested, because a robust lepidopteran phylogeny and timing of evolutionary novelties are lacking. To address these issues, we inferred a comprehensive phylogeny of Lepidoptera, using the largest dataset assembled for the order (2,098 orthologous protein-coding genes from transcriptomes of 186 species, representing nearly all superfamilies), and dated it with carefully evaluated synapomorphy-based fossils. The oldest members of the Lepidoptera crown group appeared in the Late Carboniferous (∼300 Ma) and fed on nonvascular land plants. Lepidoptera evolved the tube-like proboscis in the Middle Triassic (∼241 Ma), which allowed them to acquire nectar from flowering plants. This morphological innovation, along with other traits, likely promoted the extraordinary diversification of superfamily-level lepidopteran crown groups. The ancestor of butterflies was likely nocturnal, and our results indicate that butterflies became day-flying in the Late Cretaceous (∼98 Ma). Moth hearing organs arose multiple times before the evolutionary arms race between moths and bats, perhaps initially detecting a wide range of sound frequencies before being co-opted to specifically detect bat sonar. Our study provides an essential framework for future comparative studies on butterfly and moth evolution.

Many species of caterpillars have been reported to respond to sound, but there has been limited formal study of what sounds they hear, how they hear them and how they respond to them. Here, we report on hearing in caterpillars of the monarch butterfly (Danaus plexippus). Fourth and fifth instar caterpillars respond to sounds by freezing, contracting, and flicking their thorax in a vertical direction. Behavioural responses were evoked by sound frequencies between 50 and 900 Hz, with best sensitivity at 100-200 Hz. The lowest mean threshold was 79 dB SPL (particle velocity 605 mu m s(-1)) at 150 Hz. When presented with a repeated 200 Hz sound tone, caterpillars habituate by no longer responding. A series of ablation experiments confirmed that the primary sensory receptors are a pair of long hairs, called trichoid sensilla, located on the upper prothorax. These sensilla are similar to 450 mu m long, rest in a socket and are innervated by a single bipolar sensory neuron. Removal of these setae reduced responses significantly compared with controls. Other setae contributed minimally to hearing in response to 200 Hz tones, and tubercles and prothoracic shields played no apparent role in sound reception. We propose that hearing functions to prevent attacks by aerial insect predators and parasitoids, which produce flight sounds in the frequency range to which the caterpillars are sensitive. This research lays the foundation for further investigations on the function and evolution of hearing in caterpillars, and has significance for the conservation of threatened monarch butterfly larvae living near noisy urban environments and roadways.

Acoustic communication is taxonomically widespread in bark beetles and is proposed to play an important role in a variety of social and defensive behavioural contexts. Yet our understanding of how signals vary between contexts is currently limited. The present study tests the hypothesis that acoustic signals vary between behavioural contexts in the female pine engraver beetle Ips pini (Say) (Coleoptera: Curculionidae: Scolytinae). Female Ips pini produce acoustic chirps using a vertex-pronotal stridulatory organ. Randomly sampled chirps generated under three contexts (i.e. distress, predation and premating) are compared for their duration, number of pulses, interpulse intervals, pulse rate and amplitude envelope shapes. The results obtained show that, during premating events, chirps are significantly longer in duration and tend to have a higher proportion of descending amplitude envelopes than chirps occurring during distress and predation events. Chirps produced during distress and predation conditions are indistinguishable from one another. By contrast to the results from previous bark beetle studies, no support is found for categorizing chirps as interrupted' or uninterrupted' types based on temporal patterns. The functional significance of context-dependent variation in chirp characteristics is discussed. Previous studies on acoustic communication in bark beetles are limited as a result of a general lack of objective sampling and measurement criteria for characterizing signals. Recommendations are outlined for future studies on the functions and evolution of acoustic communication in bark beetles.