Wing movements during cricket stridulation are affected by mechanosensory input from wing hair plates

Differentiation between left and right wing stridulatory files in the field cricket Gryllus bimaculatus (Orthoptera: Gryllidae)

Male crickets produce acoustic signals by wing stridulation, attracting females for mating. A plectrum on the left forewing's (or tegmen) anal margin rapidly strikes along a serrated vein (stridulatory file, SF) on the opposite tegmen as they close, producing vibrations, ending in a tonal sound. The tooth strike rate of the plectrum across file teeth is equal to the sound frequency produced by the cricket (i.e., ∼5k teeth/s for ∼5 kHz in field crickets) and is specific to the forewing's resonant frequency. Sound is subsequently amplified using specialised wing cells. Anatomically, the forewings appear to mirror each other: both tegmina bear a SF and plectrum; however, most cricket species stridulate using right-over-left wing overlap making the stridulatory mechanism asymmetrical by default, rendering the left tegmen's SF unused. Therefore, we hypothesised structural differences between functional and unfunctional SFs. Three-dimensional mapping was used to accurately measure SF structures in Gryllus bimaculatus wings. We found that the left SF shows significantly greater variation in inter-tooth distance than the right, but less variation within the first sixty teeth (the functional part) than the right file. The left SF's slow evolutionary change over millions of years is discussed considering modern molecular phylogenies and fossil records.

Neuroethology of acoustic communication in field crickets - from signal generation to song recognition in an insect brain

Field crickets are best known for the loud calling songs produced by males to attract conspecific females. This review aims to summarize the current knowledge of the neurobiological basis underlying the acoustic communication for mate finding in field crickets with emphasis on the recent research progress to understand the neuronal networks for motor pattern generation and auditory pattern recognition of the calling song in Gryllus bimaculatus. Strong scientific interest into the neural mechanisms underlying intraspecific communication has driven persistently advancing research efforts to study the male singing behaviour and female phonotaxis for mate finding in these insects. The growing neurobiological understanding also inspired many studies testing verifiable hypotheses in sensory ecology, bioacoustics and on the genetics and evolution of behaviour. Over last decades, acoustic communication in field crickets served as a very successful neuroethological model system. It has contributed significantly to the scientific process of establishing, reconsidering and refining fundamental concepts in behavioural neurosciences such as command neurons, central motor pattern generation, corollary discharge processing and pattern recognition by sensory feature detection, which are basic building blocks of our modern understanding on how nervous systems control and generate behaviour in all animals.

Impact of cercal air currents on singing motor pattern generation in the cricket (Gryllus bimaculatus DeGeer)

The cercal system of crickets detects low-frequency air currents produced by approaching predators and self-generated air currents during singing, which may provide sensory feedback to the singing motor network. We analyzed the effect of cercal stimulation on singing motor pattern generation to reveal the response of a singing interneuron to predator-like signals and to elucidate the possible role of self-generated air currents during singing. In fictive singing males, we recorded an interneuron of the singing network while applying air currents to the cerci; additionally, we analyzed the effect of abolishing the cercal system in freely singing males. In fictively singing crickets, the effect of short air stimuli is either to terminate prematurely or to lengthen the interchirp interval, depending on their phase in the chirp cycle. Within our stimulation paradigm, air stimuli of different velocities and durations always elicited an inhibitory postsynaptic potential in the singing interneuron. Current injection in the singing interneuron elicited singing motor activity, even during the air current-evoked inhibitory input from the cercal pathway. The disruptive effects of air stimuli on the fictive singing pattern and the inhibitory response of the singing interneuron point toward the cercal system being involved in initiating avoidance responses in singing crickets, according to the established role of cerci in a predator escape pathway. After abolishing the activity of the cercal system, the timing of natural singing activity was not significantly altered. Our study provides no evidence that self-generated cercal sensory activity has a feedback function for singing motor pattern generation.

Temporal resolution for calling song signals by female crickets, Gryllus bimaculatus

A behavioural gap detection paradigm was used to determine the temporal resolution for song patterns by female crickets, Gryllus bimaculatus. For stimuli with a modulation depth of 100% the critical gap duration was 6-8 ms. A reduction of the modulation depth of gaps to 50% led either to an increase or a decrease of the critical gap duration. In the latter case, the critical gap duration dropped to 3-4 ms indicating a higher sensitivity of auditory processing. The response curve for variation of pulse period was not limited by temporal resolution. However, the reduced response to stimuli with a high duty cycle, and thus short pause durations, was in accordance with the limits of temporal resolution. The critical duration of masking pulses inserted into pauses was 4-6 ms. An analysis of the songs of males revealed that gaps (5.8 ms) and masking pulses (6.9 ms) were at detectable time scales for the auditory pathway of female crickets. However, most of the observed temporal variation of song patterns was tolerated by females. Critical cues such as pulse period and pulse duty cycle provided little basis for inter-individual selection by females.

Stridulatory file and calling song of two populations of the tropical bush cricket Eneoptera surinamensis (Orthoptera, Gryllidae, Eneopterinae)

This study characterizes the calling song and ultramorphology of the stridulatory file of two geographically isolated populations of the tropical bush cricket Eneoptera surinamensis (De Geer, 1773) from city of Foz do Iguaçu, state of Paraná, and town of Rio Claro, state of São Paulo, Brazil, distant 1,000 Km from each other. The teeth are shell-shaped, the larger ones are distributed in the medium region of the file, decreasing gradually in size towards the edges. Specimens from Foz do Iguaçu have a file with 82 ± 9.8 teeth, length=1.89 mm ± 0.15 with 43.76 ± 5.94 teeth per mm (n=15). Specimens from Rio Claro present a file with 87 ± 9.81, length=1.96 ± 0.19 mm with 44.52 ± 4.61 teeth per mm (n=15). Statistical differences found between the two populations are not significant. The calling song is an uninterrupted trill that alternates two sets of notes distinct for its temporal features.

Mechanoreceptors involved in the hindwing-evoked escape behaviour in cricket, Gryllus bimaculatus

Mechanoreceptors involved in the escape jumping evoked by hindwing stimulation have been investigated in the field cricket Gryllus bimaculatus. By partial ablation of the hindwing, we found that a mechanosensory system relevant to the escape behaviour was localized on specific veins of the hindwing tip. Scanning electron microscopy revealed three types of mechanoreceptive sensillae on the corresponding region. Based on their morphology, type I and type III sensillae were judged to be trichoid and chaetic sensillae, respectively. Type II sensillae were newly found in this study, having a twisted shaft with a socket-like structure at its base. They existed almost exclusively on the tip and middle regions of the hindwing. The conduction velocity of type II units was significantly smaller than that of type I units. One cycle of sinusoidal deflection of a single type II sensilla at frequencies in the range of 10-120 Hz caused the sensory unit to discharge a single or a few spikes that were not directly correlated with any specific direction of hair movement nor specific deflection angle. The response probability decreased with the stimulus frequency to be less than 0.1 at 0.2 Hz. The results suggest that type II sensillae would serve as contact mechanoreceptors with a low-cut filter property to obtain general information on the presence of stimuli on the hindwing tip rather than specific information on their precise positioning or movement.

Escape behavior in response to mechanical stimulation of hindwing in cricket, Gryllus bimaculatus

We studied behavioral responses of the cricket, Gryllus bimaculatus, to mechanical stimulation of the hindwing tip using three different kinds of stimuli: touching, bending and pinching. The most characteristic was a sequence of initial jump-like movements and subsequent running steps, that is referred to as escape behavior in this study. Touching stimulus elicited the escape behavior in 52% of resting animals tethered on a treadmill, whereas bending elicited the same behavior in 94% or 98% depending on the bend direction. Pinching was effective in all tested animals. The effectiveness of pinching stimulus in eliciting the escape behavior depends on the ongoing activity in the animal. Video and electromyographic recordings have revealed that, in the initial jump-like movements, forelegs and hindlegs move simultaneously on both sides while midlegs remain on the ground, followed by simultaneous movements of bilateral midlegs. The subsequent stepping was characterized by out-of-phase rhythmical activities of the leg muscles. Touching stimulus evoked tonic afferent responses of small amplitude in the second nerve root of the metathoracic ganglion. Bending stimulus evoked tonic responses of large units that showed rapid habituation and medium units that persisted during repeated stimulation. Pinching stimulus elicited only phasic responses of large and medium amplitude in the R2 afferents. The results suggest that touching, bending and pinching stimuli are transmitted to the metathoracic ganglion via different sensory systems having different effectiveness in activating the escape motor system.