Danger detection and escape behaviour in wood crickets

Persistence of auditory modulation of wind-induced escape behavior in crickets

Animals, including insects, change their innate escape behavior triggered by a specific threat stimulus depending on the environmental context to survive adaptively the predators' attack. This indicates that additional inputs from sensory organs of different modalities indicating surrounding conditions could affect the neuronal circuit responsible for the escape behavior. Field crickets, Gryllus bimaculatus, exhibit an oriented running or jumping escape in response to short air puff detected by the abdominal mechanosensory organ called cerci. Crickets also receive a high-frequency acoustic stimulus by their tympanal organs on their frontal legs, which suggests approaching bats as a predator. We have reported that the crickets modulate their wind-elicited escape running in the moving direction when they are exposed to an acoustic stimulus preceded by the air puff. However, it remains unclear how long the effects of auditory inputs indicating surrounding contexts last after the sound is terminated. In this study, we applied a short pulse (200 ms) of 15-kHz pure tone to the crickets in various intervals before the air-puff stimulus. The sound given 200 or 1000 ms before the air puff biased the wind-elicited escape running backward, like the previous studies using the longer and overlapped sound. But the sounds that started 2000 ms before and simultaneously with the air puff had little effect. In addition, the jumping probability was higher only when the delay of air puff to the sound was 1000 ms. These results suggest that the cricket could retain the auditory memory for at least one second and alter the motion choice and direction of the wind-elicited escape behavior.

Visual threats reduce blood-feeding and trigger escape responses in Aedes aegypti mosquitoes

The diurnal mosquitoes Aedes aegypti are vectors of several arboviruses, including dengue, yellow fever, and Zika viruses. To find a host to feed on, they rely on the sophisticated integration of olfactory, visual, thermal, and gustatory cues emitted by the hosts. If detected by their target, this latter may display defensive behaviors that mosquitoes need to be able to detect and escape in order to survive. In humans, a typical response is a swat of the hand, which generates both mechanical and visual perturbations aimed at a mosquito. Here, we used programmable visual displays to generate expanding objects sharing characteristics with the visual component of an approaching hand and quantified the behavioral response of female mosquitoes. Results show that Ae. aegypti is capable of using visual information to decide whether to feed on an artificial host mimic. Stimulations delivered in a LED flight arena further reveal that landed Ae. aegypti females display a stereotypical escape strategy by taking off at an angle that is a function of the direction of stimulus introduction. Altogether, this study demonstrates that mosquitoes landed on a host mimic can use isolated visual cues to detect and avoid a potential threat.

Roles of neural communication between the brain and thoracic ganglia in the selection and regulation of the cricket escape behavior

To survive a predator's attack, prey animals must exhibit escape responses that are appropriately regulated in terms of their moving speed, distance, and direction. Insect locomotion is considered to be controlled by an interaction between the brain, which is involved in behavioral decision-making, and the thoracic ganglia (TG), which are primary motor centers. However, it remains unknown which descending and ascending signals between these neural centers are involved in the regulation of the escape behavior. We addressed the distinct roles of the brain and TG in the wind-elicited escape behavior of crickets by assessing the effects of partial ablation of the intersegmental communications on escape responses. We unilaterally cut the ventral nerve cord (VNC) at different locations, between the brain and TG, or between the TG and terminal abdominal ganglion (TAG), a primary sensory center of the cercal system. The partial ablation of ascending signals to the brain greatly reduced the jumping response rather than running, indicating that sensory information processing in the brain is essential for the choice of escape responses. The ablation of descending signals from the brain to the TG impaired locomotor performance and directional control of the escape responses, suggesting that locomotion in the escape behavior largely depends on the descending signals from the brain. Finally, the extracellular recording from the cervical VNC indicated a difference in the descending activities preceding the escape responses between running and jumping. Our results demonstrated that the brain sends the descending signals encoding the behavioral choice and locomotor regulation to the TG, while the TG seem to have other specific roles, such as in the preparation of escape movement.

Spatial perception mediated by insect antennal mechanosensory system

Animals perceive their surroundings by using various modalities of sensory inputs to guide their locomotion. Nocturnal insects such as crickets use mechanosensory inputs mediated by their antennae to orient in darkness. Spatial information is acquired via voluntary antennal contacts with surrounding objects, but it remains unclear whether the insects modulate behaviors mediated by other sensory organs based on that information. Crickets exhibit escape behavior in response to a short air-puff, which is detected by the abdominal mechanosensory organs called cerci and is perceived as a “predator approach” signal. We placed objects of different shapes at different locations with which the cricket actively made contact using its antenna. We then examined the effects on wind-elicited escape behavior. The crickets changed their movement trajectory in response to nearby objects like walls so that they could avoid collision with these obstacles even during the cercal-mediated behavior. For instance, when a wall was placed in front of the crickets so that it was detected by one antenna, the escape trajectory in response to a stimulus from behind was significantly biased toward the side opposite the wall. Even when the antenna on the free side without the wall was ablated, this collision avoidance was also observed, suggesting that the mechanosensory inputs from one antenna detecting an object edge would be sufficient to perceive the location of obstacle in front. This study demonstrated that crickets were able to use the spatial information acquired with their antennal system to modify their behavior mediated by other sensory organs.

Action selection based on multiple-stimulus aspects in wind-elicited escape behavior of crickets

Escape behavior is essential for animals to avoid attacks by predators. In some species, multiple escape responses could be employed. However, it remains unknown what aspects of threat stimuli affect the choice of an escape response. We focused on two distinct escape responses (running and jumping) to short airflow in crickets and examined the effects of multiple stimulus aspects including the angle, velocity, and duration on the choice between these responses. The faster and longer the airflow, the more frequently the crickets jumped. This meant that the choice of an escape response depends on both the velocity and duration of the stimulus and suggests that the neural basis for choosing an escape response includes the integration process of multiple stimulus parameters. In addition, the moving speed and distance changed depending on the stimulus velocity and duration for running but not for jumping. Running away would be more adaptive escape behavior.

Descending and Ascending Signals That Maintain Rhythmic Walking Pattern in Crickets

The cricket is one of the model animals used to investigate the neuronal mechanisms underlying adaptive locomotion. An intact cricket walks mostly with a tripod gait, similar to other insects. The motor control center of the leg movements is located in the thoracic ganglia. In this study, we investigated the walking gait patterns of the crickets whose ventral nerve cords were surgically cut to gain an understanding of how the descending signals from the head ganglia and ascending signals from the abdominal nervous system into the thoracic ganglia mediate the initiation and coordination of the walking gait pattern. Crickets whose paired connectives between the brain and subesophageal ganglion (SEG) (circumesophageal connectives) were cut exhibited a tripod gait pattern. However, when one side of the circumesophageal connectives was cut, the crickets continued to turn in the opposite direction to the connective cut. Crickets whose paired connectives between the SEG and prothoracic ganglion were cut did not walk, whereas the crickets exhibited an ordinal tripod gait pattern when one side of the connectives was intact. Crickets whose paired connectives between the metathoracic ganglion and abdominal ganglia were cut initiated walking, although the gait was not a coordinated tripod pattern, whereas the crickets exhibited a tripod gait when one side of the connectives was intact. These results suggest that the brain plays an inhibitory role in initiating leg movements and that both the descending signals from the head ganglia and the ascending signals from the abdominal nervous system are important in initiating and coordinating insect walking gait patterns.

Gone with the wind: Is signal timing in a neotropical katydid an adaptive response to variation in wind-induced vibratory noise?

Abstract

Wind, a major source of environmental noise, forces invertebrates that communicate with plant-borne vibrations to adjust their signaling when communicating in windy conditions. However, the strategies that animals use to reduce the impact of wind noise on communication are not well studied. We investigated the effects of wind on the production of tremulatory signals in the neotropical katydid Copiphora brevirostris. First, we recorded katydid signaling activity and natural wind variation in the field. Additionally, we exposed katydid couples during their most active signaling time period to artificial wind of different levels, and we recorded the number of tremulations produced by the males. We found that wind levels are at their lowest between 2:00 and 5:00 in the morning, which coincides with peak signaling period for male katydids. Furthermore, we found that males produce significantly fewer tremulations when exposed to wind rather than acoustic noise or silence. Wind velocity significantly affected the number of tremulations produced during the wind treatment, with fewer tremulations produced with higher wind velocities. Our results show that katydids can time their vibratory signaling both in the short- and long-term to favorable sensory conditions, either through behavioral flexibility in response to short-term fluctuations in wind or as a result of an evolutionary process in response to predictable periods of low-wind conditions.

Significance statement

Animal communication can be hampered by noise across all sensory modalities. Most research on the effects of noise and the strategies to cope with it has focused on animals that use airborne sounds to communicate. However, although hundreds of thousands of invertebrates communicate with vibrational signals, we know very little about how noise affects this form of communication. For animals that rely on substrate-borne vibrations, wind represents the major source of environmental noise. Wind velocity levels can be predictable at a long-term scale (hours) but rather unpredictable at a short time scale (seconds). Both scales of variation are important for communication. Using a combination of field observations and lab experiments, we investigated the strategies used by a neotropical katydid Copiphora brevirostris to cope with vibrational noise induced by wind. Our results demonstrate that C. brevirostris times its signals at the long- and short-term range. Katydids signaled more at the times at night when wind velocity was lowest. Moreover, when exposed to wind gusts during their peak time of activity, katydids signaled more during the wind-free gaps.

Trade-off between motor performance and behavioural flexibility in the action selection of cricket escape behaviour

To survive a predator’s attack successfully, animals choose appropriate actions from multiple escape responses. The motor performance of escape response governs successful survival, which implies that the action selection in escape behaviour is based on the trade-off between competing behavioural benefits. Thus, quantitative assessment of motor performance will shed light on the biological basis of decision-making. To explore the trade-off underlying the action selection, we focused on two distinct wind-elicited escape responses of crickets, running and jumping. We first hypothesized a trade-off between speed and directional accuracy. This hypothesis was rejected because crickets could control the escape direction in jumping as precisely as in running; further, jumping had advantages with regard to escape speed. Next, we assumed behavioural flexibility, including responsiveness to additional predator’s attacks, as a benefit of running. The double stimulus experiment revealed that crickets running in the first response could respond more frequently to a second stimulus and control the movement direction more precisely compared to when they chose jumping for the first response. These data suggest that not only the motor performance but also the future adaptability of subsequent behaviours are considered as behavioural benefits, which may be used for choosing appropriate escape reactions.

Neural responses from the wind-sensitive interneuron population in four cockroach species

The wind-sensitive insect cercal sensory system is involved in important behaviors including predator detection and initiating terrestrial escape responses as well as flight maintenance. However, not all insects possessing a cercal system exhibit these behaviors. In cockroaches, wind evokes strong terrestrial escape responses in Periplaneta americana and Blattella germanica, but only weak escape responses in Blaberus craniifer and no escape responses in Gromphadorhina portentosa. Both P. americana and B. craniifer possesses pink flight muscles correlated with flight ability while B. germanica possesses white flight muscles that cannot support flight and G. portentosa lacks wings. These different behavioral combinations could correlate with differences in sensory processing of wind information by the cercal system. In this study, we focused on the wind-sensitive interneurons (WSIs) since they provide input to the premotor/motor neurons that influence terrestrial escape and flight behavior. Using extracellular recordings, we characterized the responses from the WSI population by generating stimulus-response (S-R) curves and examining spike firing rates. Using cluster analysis, we also examined the activity of individual units (four per species, though not necessarily homologous) comprising the population response in each species. Our main results were: (1) all four species possessed ascending WSIs in the abdominal connectives; (2) wind elicited the weakest WSI responses (lowest spike counts and spike rates) in G. portentosa; (3) wind elicited WSI responses in B. craniifer that were greater than P. americana or B. germanica; (4) the activity of four individual units comprising the WSI population response in each species was similar across species.

Antennal pointing at a looming object in the cricket Acheta domesticus

Antennal pointing responses to approaching objects were observed in the house cricket Acheta domesticus. In response to a ball approaching from the lateral side, crickets oriented the antenna ipsilateral to the ball towards it. In response to a ball approaching from the front, crickets oriented both antennae forward. Response rates of antennal pointing were higher when the ball was approaching from the front than from behind. The antennal angle ipsilateral to the approaching ball was positively correlated with approaching angle of the ball. Obstructing the cricket's sight decreased the response rate of antennal pointing, suggesting that this response was elicited mainly by visual stimuli. Although the response rates of antennal pointing decreased when the object ceased its approach at a great distance from the cricket, antennal pointing appeared to be resistant to habituation and was not substantially affected by the velocity, size and trajectory of an approaching ball. When presented with computer-generated visual stimuli, crickets frequently showed the antennal pointing response to a darkening stimulus as well as looming and linearly-expanding stimuli. Drifting gratings rarely elicited the antennal pointing. These results suggest that luminance change is sufficient to elicit antennal pointing.

Predator-prey pursuit-evasion games in structurally complex environments

Pursuit and evasion behaviors in many predator-prey encounters occur in a geometrically structured environment. The physical structures in the environment impose strong constraints on the perception and behavioral responses of both antagonists. Nevertheless, no experimental or theoretical study has tackled the issue of quantifying the role of the habitat's architecture on the joint trajectories during a predator-prey encounter. In this study, we report the influence of microtopography of forest leaf litter on the pursuit-evasion trajectories of wolf spiders Pardosa sp. attacking the wood cricket Nemobius sylvestris. Fourteen intact leaf litter samples of 1 m × 0.5 m were extracted from an oak-beech forest floor in summer and winter, with later samples having the most recently fallen leaves. Elevation was mapped at a spatial resolution of 0.5 mm using a laser scanner. Litter structuring patterns were identified by height transects and experimental semi-variograms. Detailed analysis of all visible leaf-fragments of one sample enabled us to relate the observed statistical patterns to the underlying geometry of individual elements. Video recording of pursuit-evasion sequences in arenas with flat paper or leaf litter enabled us to estimate attack and fleeing distances as a function of substrate. The compaction index, the length of contiguous flat surfaces, and the experimental variograms showed that the leaf litter was smoother in summer than in winter. Thus, weathering as well as biotic activities compacted and flattened the litter over time. We found good agreement between the size of the structuring unit of leaf litter and the distance over which attack and escape behaviors both were initiated (both ∼3 cm). There was a four-fold topographical effect on pursuit-escape sequences; compared with a flat surface, leaf litter (1) greatly reduced the likelihood of launching a pursuit, (2) reduced pursuit and escape distances by half, (3) put prey and predator on par in terms of pursuit and escape distances, and (4) reduced the likelihood of secondary pursuits, after initial escape of the prey, to nearly zero. Thus, geometry of the habitat strongly modulates the rules of pursuit-evasion in predator-prey interactions in the wild.

Predator versus prey: locust looming-detector neuron and behavioural responses to stimuli representing attacking bird predators

Many arthropods possess escape-triggering neural mechanisms that help them evade predators. These mechanisms are important neuroethological models, but they are rarely investigated using predator-like stimuli because there is often insufficient information on real predator attacks. Locusts possess uniquely identifiable visual neurons (the descending contralateral movement detectors, DCMDs) that are well-studied looming motion detectors. The DCMDs trigger ‘glides’ in flying locusts, which are hypothesised to be appropriate last-ditch responses to the looms of avian predators. To date it has not been possible to study glides in response to stimuli simulating bird attacks because such attacks have not been characterised. We analyse video of wild black kites attacking flying locusts, and estimate kite attack speeds of 10.8±1.4 m/s. We estimate that the loom of a kite’s thorax towards a locust at these speeds should be characterised by a relatively low ratio of half size to speed (l/|v|) in the range 4–17 ms. Peak DCMD spike rate and gliding response occurrence are known to increase as l/|v| decreases for simple looming shapes. Using simulated looming discs, we investigate these trends and show that both DCMD and behavioural responses are strong to stimuli with kite-like l/|v| ratios. Adding wings to looming discs to produce a more realistic stimulus shape did not disrupt the overall relationships of DCMD and gliding occurrence to stimulus l/|v|. However, adding wings to looming discs did slightly reduce high frequency DCMD spike rates in the final stages of object approach, and slightly delay glide initiation. Looming discs with or without wings triggered glides closer to the time of collision as l/|v| declined, and relatively infrequently before collision at very low l/|v|. However, the performance of this system is in line with expectations for a last-ditch escape response.

Responses of cricket cercal interneurons to realistic naturalistic stimuli in the field

The ability of the insect cercal system to detect approaching predators has been studied extensively in the laboratory and in the field. Some previous studies have assessed the extent to which sensory noise affects the operational characteristics of the cercal system, but these studies have only been carried out in laboratory settings using white noise stimuli of unrealistic nature. Using a piston mimicking the natural airflow of an approaching predator, we recorded the neural activity through the abdominal connectives from the terminal abdominal ganglion of freely moving wood crickets (Nemobius sylvestris) in a semi-field situation. A cluster analysis of spike amplitudes revealed six clusters, or 'units', corresponding to six different subsets of cercal interneurons. No spontaneous activity was recorded for the units of larger amplitude, reinforcing the idea they correspond to the largest giant interneurons. Many of the cercal units are already activated by background noise, sometimes only weakly, and the approach of a predator is signaled by an increase in their activity, in particular for the larger-amplitude units. A scaling law predicts that the cumulative number of spikes is a function of the velocity of the flow perceived at the rear of the cricket, including a multiplicative factor that increases linearly with piston velocity. We discuss the implications of this finding in terms of how the cricket might infer the imminence and nature of a predatory attack.