Hearing in hooktip moths (Drepanidae: Lepidoptera)

Determining the complete mitochondrial genome of Tethea albicostata (Lepidoptera: Drepanidae) and phylogenetic analysis

Tethea albicostata is a widely distributed insect species in northern and central China. To date, few studies have been conducted on this species, with the exception of morphological taxonomy studies. Here, we report the complete mitochondrial genome of T. albicostata collected in China. The circular-mapping mitogenome is 15,308 bp in length, with an overall A + T content of 80.52%, encoding 2 ribosomal RNA genes, 22 transfer RNA genes, and 13 protein-coding genes. The gene arrangement and components of T. albicostata are identical to those of most other Lepidopteran insects. Phylogenetic analysis based on mitogenomes showed that T. albicostata is grouped with Drepana pallida, which belongs to the same family as Drepanidae. The family Drepanidae formed a separate branch from other families in the phylogenetic tree. This study determined the second mitochondrial genome of the Drepanidae species.

Potential of Ultrasound to Control Sesamia cretica (Lepidoptera: Noctuidae)

The pink stalk borer, Sesamia cretica Led. (Lepidoptera: Noctuidae), is one of the most important sugarcane pests in many regions of the world, causing severe damage to sugarcane every year. This insect has a specialized form of the auditory organ called the tympanal organ, and ultrasound can be employed as a potential tactic employed in physical control strategy against the pest. The present study evaluates the efficacy of ultrasound in controlling the pest in laboratory conditions. For this purpose, the repellent properties of various ultrasonic frequencies ranging from 21 to 100 kHz with 0.5 kHz intervals and wave shapes, including Sin(x), Cos(x) square, and sawtooth, were studied in choice experiments on the moths. The repellent effects of ultrasonic waves at frequencies 39.5 and 37.5 kHz were more significant than other frequencies in male and female moths, respectively. Furthermore, there was no significant difference between the repellent properties of different wave shapes. In non-choice experiments, the effects of the most repellent ultrasonic treatment, at frequency 37.5 kHz, on biological characteristics of various life stages and distribution patterns of the moths were investigated. The results showed that the ultrasonic treatment causes substantial reductions in many biological parameters of the immature life stages of pests, including longevity, weight, survival rate, and fecundity. Moreover, the pattern indicated that the moths tended to escape from the ultrasound. The findings of this study can be employed for manufacturing the ultrasonic repeller to be used in sugarcane fields.

Light might suppress both types of sound-evoked antipredator flight in moths

Urbanization exposes wild animals to increased levels of light, affecting particularly nocturnal animals. Artificial light at night might shift the balance of predator-prey interactions, for example, of nocturnal echolocating bats and eared moths. Moths exposed to light show less last-ditch maneuvers in response to attacking close-by bats. In contrast, the extent to which negative phonotaxis, moths' first line of defense against distant bats, is affected by light is unclear. Here, we aimed to quantify the overall effect of light on both types of sound-evoked antipredator flight, last-ditch maneuvers and negative phonotaxis. We caught moths at two light traps, which were alternately equipped with loudspeakers that presented ultrasonic playbacks to simulate hunting bats. The light field was omnidirectional to attract moths equally from all directions. In contrast, the sound field was directional and thus, depending on the moth's approach direction, elicited either only negative phonotaxis, or negative phonotaxis and last-ditch maneuvers. We did not observe an effect of sound playback on the number of caught moths, suggesting that light might suppress both types of antipredator flight, as either type would have caused a decline in the number of caught moths. As control, we confirmed that our playback was able to elicit evasive flight in moths in a dark flight room. Showing no effect of a treatment, however, is difficult. We discuss potential alternative explanations for our results, and call for further studies to investigate how light interferes with animal behavior.

Transcriptome analysis of a social caterpillar, Drepana arcuata: De novo assembly, functional annotation and developmental analysis

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 3^rd 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.

Discrepancies in the spiking threshold and frequency sensitivity of nocturnal moths explainable by biases in the canonical auditory stimulation method

The auditory stimulation method used in experiments on moth A cell(s) is generally believed to be adequate to characterize the encoding of bat echolocation signals. The stimulation method hosts, though, several biases. Their compounded effects can explain a range of discrepancies between the reported electrophysiological recordings and significantly alter the current interpretation. To test the hypothesis that the bias may significantly alter our current understanding of the moth's auditory transducer characteristics, papers using the same auditory stimulation method and reporting on either spiking threshold or spiking activity of the moth's A cells were analysed. The consistency of the reported data was assessed. A range of corrections issued from best practices and theoretical background were applied to the data in an attempt to re-interpret the data. We found that it is not possible to apply a posteriori corrections to all data and bias. However the corrected data indicate that the A cell's spiking may (i) be independent of the repetition rate, (ii) be maximum when detecting long and low-intensity pulses and (iii) steadily reduce as the bat closes on the moth. These observations raise the possibility that a fixed action pattern drives the moths' erratic evasive manoeuvres until the final moment. In-depth investigations of the potential bias also suggest that the auditory transducer's response may be constant for a larger frequency range than thought so far, and provide clues to explain the negative taxis in response to the searching bats' calls detection.

Evolutionary escalation: the bat-moth arms race

Echolocation in bats and high-frequency hearing in their insect prey make bats and insects an ideal system for studying the sensory ecology and neuroethology of predator-prey interactions. Here, we review the evolutionary history of bats and eared insects, focusing on the insect order Lepidoptera, and consider the evidence for antipredator adaptations and predator counter-adaptations. Ears evolved in a remarkable number of body locations across insects, with the original selection pressure for ears differing between groups. Although cause and effect are difficult to determine, correlations between hearing and life history strategies in moths provide evidence for how these two variables influence each other. We consider life history variables such as size, sex, circadian and seasonal activity patterns, geographic range and the composition of sympatric bat communities. We also review hypotheses on the neural basis for anti-predator behaviours (such as evasive flight and sound production) in moths. It is assumed that these prey adaptations would select for counter-adaptations in predatory bats. We suggest two levels of support for classifying bat traits as counter-adaptations: traits that allow bats to eat more eared prey than expected based on their availability in the environment provide a low level of support for counter-adaptations, whereas traits that have no other plausible explanation for their origination and maintenance than capturing defended prey constitute a high level of support. Specific predator counter-adaptations include calling at frequencies outside the sensitivity range of most eared prey, changing the pattern and frequency of echolocation calls during prey pursuit, and quiet, or 'stealth', echolocation.

Directional escape behavior in allis shad (Alosa alosa) exposed to ultrasonic clicks mimicking an approaching toothed whale

Toothed whales emit high-powered ultrasonic clicks to echolocate a wide range of prey. It may be hypothesized that some of their prey species have evolved capabilities to detect and respond to such ultrasonic pulses in a way that reduces predation, akin to the situation for many nocturnal insects and echolocating bats. Using high-speed film recordings and controlled exposures, we obtained behavioural evidence that simulated toothed whale biosonar clicks elicit highly directional anti-predator responses in an ultrasound-sensitive allis shad (Alosa alosa). Ten shad were exposed to 192 dB re. 1 μPa (pp) clicks centred at 40 kHz at repetition rates of 1, 20, 50 and 250 clicks s(-1) with summed energy flux density levels of 148, 161, 165 and 172 dB re. 1 μPa(2) s. The exposures mimicked the acoustic exposure from a delphinid toothed whale in different phases of prey search and capture. The response times of allis shad were faster for higher repetition rates of clicks with the same sound pressure level. None of the fish responded to a single click, but had median response times of 182, 93 and 57 ms when exposed to click rates of 20, 50 and 250 clicks s(-1), respectively. This suggests that the ultrasound detector of allis shad is an energy detector and that shad respond faster when exposed to a nearby fast-clicking toothed whale than to a slow-clicking toothed whale far away. The findings are thus consistent with the hypothesis that shad ultrasound detection is used for reducing predation from echolocating toothed whales.

Moths produce extremely quiet ultrasonic courtship songs by rubbing specialized scales

Insects have evolved a marked diversity of mechanisms to produce loud conspicuous sounds for efficient communication. However, the risk of eavesdropping by competitors and predators is high. Here, we describe a mechanism for producing extremely low-intensity ultrasonic songs (46 dB sound pressure level at 1 cm) adapted for private sexual communication in the Asian corn borer moth, Ostrinia furnacalis. During courtship, the male rubs specialized scales on the wing against those on the thorax to produce the songs, with the wing membrane underlying the scales possibly acting as a sound resonator. The male's song suppresses the escape behavior of the female, thereby increasing his mating success. Our discovery of extremely low-intensity ultrasonic communication may point to a whole undiscovered world of private communication, using "quiet" ultrasound.

The structure and function of auditory chordotonal organs in insects

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