The cricket cercal system implements delay-line processing

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.

ENCODING OF SMALL-SCALE AIR MOTION DYNAMICS IN THE CRICKET ACHETA DOMESTICUS

The cercal sensory system of the cricket mediates the detection, localization and identification of air current signals generated by predators, mates and competitors. This mechanosensory system has been used extensively for experimental and theoretical studies of sensory coding at the cellular and system levels. It is currently thought that sensory interneurons in the terminal abdominal ganglion extract information about the direction, velocity, and acceleration of the air currents in the animal's immediate environment, and project a coarse-coded representation of those parameters to higher centers. All feature detection is thought to be carried out in higher ganglia by more complex, specialized circuits. We present results that force a substantial revision of current hypotheses. Using multiple extracellular recordings and a special sensory stimulation device, we demonstrate that four well-studied interneurons in this system respond with high sensitivity and selectivity to complex dynamic multi-directional features of air currents which have a spatial scale smaller than the physical dimensions of the cerci. The INs showed much greater sensitivity for these features than for unidirectional bulk-flow stimuli used in previous studies. Thus, in addition to participating in the ensemble encoding of bulk air flow stimulus characteristics, these interneurons are capable of operating as feature detectors for naturalistic stimuli. In this sense, these interneurons are encoding and transmitting information about different aspects of their stimulus environment: they are multiplexing information. Major aspects of the stimulus-response specificity of these interneurons can be understood from the dendritic anatomy and connectivity with the sensory afferent map.

Smart Chemical Engineering-Based Lightweight and Miniaturized Attachable Systems for Advanced Drug Delivery and Diagnostics

Smart attachable systems have attracted much attention owing to their capabilities in terms of body performance evaluation, disease diagnostics, and drug delivery. Recent advances in chemical and engineering techniques provide many opportunities to improve device fabrication and applications owing to the advantages of being lightweight and easy to control as well as their battery absence and functional diversity. This review highlights the latest developments in the field of chemical engineering-based lightweight and miniaturized attachable systems, which are mainly inspired by the natural world. Their applications for real-time monitoring, point-of-care sampling, biomarker detection, and controlled release are discussed thoroughly with respect to specific products/prototypes. The perspectives of the field, including persistence guarantee, burden reduction, and personality improvement, are also discussed. It is believed that chemical engineering-based lightweight and miniaturized attachable systems have good potential in both clinical and industrial fields, indicating a large potential to improve human lives in the near future.

Bio-Inspired Architectures Substantially Reduce the Memory Requirements of Neural Network Models

We propose a neural network model for the jumping escape response behavior observed in the cricket cercal sensory system. This sensory system processes low-intensity air currents in the animal's immediate environment generated by predators, competitors, and mates. Our model is inspired by decades of physiological and anatomical studies. We compare the performance of our model with a model derived through a universal approximation, or a generic deep learning, approach, and demonstrate that, to achieve the same performance, these models required between one and two orders of magnitude more parameters. Furthermore, since the architecture of the bio-inspired model is defined by a set of logical relations between neurons, we find that the model is open to interpretation and can be understood. This work demonstrates the potential of incorporating bio-inspired architectural motifs, which have evolved in animal nervous systems, into memory efficient neural network models.

The morphological heterogeneity of cricket flow-sensing hairs conveys the complex flow signature of predator attacks

Arthropod flow-sensing hair length ranges over more than an order of magnitude, from 0.1 to 5 mm. Previous studies repeatedly identified the longest hairs as the most sensitive, but recent studies identified the shortest hairs as the most responsive. We resolved this apparent conflict by proposing a new model, taking into account both the initial and long-term aspects of the flow pattern produced by a lunging predator. After the estimation of the mechanical parameters of hairs, we measured the flow produced by predator mimics and compared the predicted and observed values of hair displacements in this flow. Short and long hairs respond over different time scales during the course of an attack. By harbouring a canopy of hairs of different lengths, forming a continuum, the insect can fractionize these moments. Short hairs are more agile, but are less able to harvest energy from the air. This may result in longer hairs firing their neurons earlier, despite their slower deflection. The complex interplay between hair agility and sensitivity is also modulated by the predator distance and the attack speed, characteristics defining flow properties. We conclude that the morphological heterogeneity of the hair canopy mirrors the flow complexity of an entire attack, from launch to grasp.

Predator-induced flow disturbances alert prey, from the onset of an attack

Many prey species, from soil arthropods to fish, perceive the approach of predators, allowing them to escape just in time. Thus, prey capture is as important to predators as prey finding. We extend an existing framework for understanding the conjoint trajectories of predator and prey after encounters, by estimating the ratio of predator attack and prey danger perception distances, and apply it to wolf spiders attacking wood crickets. Disturbances to air flow upstream from running spiders, which are sensed by crickets, were assessed by computational fluid dynamics with the finite-elements method for a much simplified spider model: body size, speed and ground effect were all required to obtain a faithful representation of the aerodynamic signature of the spider, with the legs making only a minor contribution. The relationship between attack speed and the maximal distance at which the cricket can perceive the danger is parabolic; it splits the space defined by these two variables into regions differing in their values for this ratio. For this biological interaction, the ratio is no greater than one, implying immediate perception of the danger, from the onset of attack. Particular attention should be paid to the ecomechanical aspects of interactions with such small ratio, because of the high degree of bidirectional coupling of the behaviour of the two protagonists. This conclusion applies to several other predator-prey systems with sensory ecologies based on flow sensing, in air and water.

Developmental and activity-dependent plasticity of filiform hair receptors in the locust

A group of wind sensitive filiform hair receptors on the locust thorax and head makes contact onto a pair of identified interneuron, A4I1. The hair receptors' central nervous projections exhibit pronounced structural dynamics during nymphal development, for example, by gradually eliminating their ipsilateral dendritic field while maintaining the contralateral one. These changes are dependent not only on hormones controlling development but on neuronal activity as well. The hair-to-interneuron system has remarkably high gain (close to 1) and makes contact to flight steering muscles. During stationary flight in front of a wind tunnel, interneuron A4I1 is active in the wing beat rhythm, and in addition it responds strongly to stimulation of sensory hairs in its receptive field. A role of the hair-to-interneuron in flight steering is thus suggested. This system appears suitable for further study of developmental and activity-dependent plasticity in a sensorimotor context with known connectivity patterns.

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.

Structure of the afferent terminals in terminal ganglion of a cricket and persistent homology

We use topological data analysis to investigate the three dimensional spatial structure of the locus of afferent neuron terminals in crickets Acheta domesticus. Each afferent neuron innervates a filiform hair positioned on a cercus: a protruding appendage at the rear of the animal. The hairs transduce air motion to the neuron signal that is used by a cricket to respond to the environment. We stratify the hairs (and the corresponding afferent terminals) into classes depending on hair length, along with position. Our analysis uncovers significant structure in the relative position of these terminal classes and suggests the functional relevance of this structure. Our method is very robust to the presence of significant experimental and developmental noise. It can be used to analyze a wide range of other point cloud data sets.

Active touch in orthopteroid insects: behaviours, multisensory substrates and evolution

Orthopteroid insects (cockroaches, crickets, locusts and related species) allow examination of active sensory processing in a comparative framework. Some orthopteroids possess long, mobile antennae endowed with many chemo- and mechanoreceptors. When the antennae are touched, an animal's response depends upon the identity of the stimulus. For example, contact with a predator may lead to escape, but contact with a conspecific may usually not. Active touch of an approaching object influences the likelihood that a discrimination of identity will be made. Using cockroaches, we have identified specific descending mechanosensory interneurons that trigger antennal-mediated escape. Crucial sensory input to these cells comes from chordotonal organs within the antennal base. However, information from other receptors on the base or the long antennal flagellum allows active touch to modulate escape probability based on stimulus identity. This is conveyed, at least to some extent, by textural information. Guidance of the antennae in active exploration depends on visual information. Some of the visual interneurons and the motor neurons necessary for visuomotor control have been identified. Comparisons across Orthoptera suggest an evolutionary model where subtle changes in the architecture of interneurons, and of sensorimotor control loops, may explain differing levels of vision-touch interaction in the active guidance of behaviour.

Escape behaviors in insects

Escape behaviors are, by necessity, fast and robust, making them excellent systems with which to study the neural basis of behavior. This is especially true in insects, which have comparatively tractable nervous systems and members who are amenable to manipulation with genetic tools. Recent technical developments in high-speed video reveal that, despite their short duration, insect escape behaviors are more complex than previously appreciated. For example, before initiating an escape jump, a fly performs sophisticated posture and stimulus-dependent preparatory leg movements that enable it to jump away from a looming threat. This newfound flexibility raises the question of how the nervous system generates a behavior that is both rapid and flexible. Recordings from the cricket nervous system suggest that synchrony between the activity of specific interneuron pairs may provide a rapid cue for the cricket to detect the direction of an approaching predator and thus which direction it should run. Technical advances make possible wireless recording from neurons while locusts escape from a looming threat, enabling, for the first time, a direct correlation between the activity of multiple neurons and the time-course of an insect escape behavior.

Quantitative characterization of the filiform mechanosensory hair array on the cricket cercus

BACKGROUND

Crickets and other orthopteran insects sense air currents with a pair of abdominal appendages resembling antennae, called cerci. Each cercus in the common house cricket Acheta domesticus is approximately 1 cm long, and is covered with 500 to 750 filiform mechanosensory hairs. The distribution of the hairs on the cerci, as well as the global patterns of their movement vectors, have been characterized semi-quantitatively in studies over the last 40 years, and have been shown to be very stereotypical across different animals in this species. Although the cercal sensory system has been the focus of many studies in the areas of neuroethology, development, biomechanics, sensory function and neural coding, there has not yet been a quantitative study of the functional morphology of the receptor array of this important model system.

METHODOLOGY/PRINCIPAL FINDINGS

We present a quantitative characterization of the structural characteristics and functional morphology of the cercal filiform hair array. We demonstrate that the excitatory direction along each hair's movement plane can be identified by features of its socket that are visible at the light-microscopic level, and that the length of the hair associated with each socket can also be estimated accurately from a structural parameter of the socket. We characterize the length and directionality of all hairs on the basal half of a sample of three cerci, and present statistical analyses of the distributions.

CONCLUSIONS/SIGNIFICANCE

The inter-animal variation of several global organizational features is low, consistent with constraints imposed by functional effectiveness and/or developmental processes. Contrary to previous reports, however, we show that the filiform hairs are not re-identifiable in the strict sense.

Temporal encoding in a nervous system

We examined the extent to which temporal encoding may be implemented by single neurons in the cercal sensory system of the house cricket Acheta domesticus. We found that these neurons exhibit a greater-than-expected coding capacity, due in part to an increased precision in brief patterns of action potentials. We developed linear and non-linear models for decoding the activity of these neurons. We found that the stimuli associated with short-interval patterns of spikes (ISIs of 8 ms or less) could be predicted better by second-order models as compared to linear models. Finally, we characterized the difference between these linear and second-order models in a low-dimensional subspace, and showed that modification of the linear models along only a few dimensions improved their predictive power to parity with the second order models. Together these results show that single neurons are capable of using temporal patterns of spikes as fundamental symbols in their neural code, and that they communicate specific stimulus distributions to subsequent neural structures.

The information coding schemes used within nervous systems have been the focus of an entire field within neuroscience. An unresolved issue within the general coding problem is the determination of the neural “symbols” with which information is encoded in neural spike trains, analogous to the determination of the nucleotide sequences used to represent proteins in molecular biology. The goal of our study was to determine if pairs of consecutive action potentials contain more or different information about the stimuli that elicit them than would be predicted from an analysis of individual action potentials. We developed linear and non-linear coding models and used likelihood analysis to address this question for sensory interneurons in the cricket cercal sensory system. Our results show that these neurons' spike trains can be decomposed into sequences of two neural symbols: isolated single spikes and short-interval spike doublets. Given the ubiquitous nature of similar neural activity reported in other systems, we suspect that the implementation of such temporal encoding schemes may be widespread across animal phyla. Knowledge of the basic coding units used by single cells will help in building the large-scale neural network models necessary for understanding how nervous systems function.

Regulation of conduction velocity in axons from near-field receptors of the crayfish antennule

The antennular flagella of the crayfish Procambarus clarkii each possess a linear array of near-field receptors, termed standing feathered sensilla, that are extremely sensitive to movement of the surrounding water. Previously it had been shown that, within each flagellum, the axonal conduction velocity of the sensory neuron pair associated with each feathered sensillum was linearly related to the position of the sensillum along the flagellar axis. In the current studies I show that the conduction velocity of axons within the proximal three segments of the antennules, between the flagellum and the brain, is somewhat higher than the corresponding conduction velocity of the same axons in the flagellum, especially for those whose flagellar conduction velocity is between 1 and 3 m s–1, even though there is no net change in axonal diameter within this part of the afferent pathway. One consequence of this change in axonal conduction properties is an effective compression of the temporal spread – potentially by as much as tenfold – which otherwise would occur in arrival times of initial spikes from each sensillum following a mechanical stimulus to the antennule. Furthermore, the pattern signature of initial spike volleys at the brain following a global hydrodynamic stimulus to the flagellum is remarkably consistent and conceivably could be recognized as such by central processing centers. I conclude that conduction velocity adjustments improve temporal summation and resolution from input volleys that originate in the highly sensitive and, hence, inherently noisy near-field receptors, thereby more effectively triggering startle response circuitry at the approach of potential predators.

Why do insects have such a high density of flow-sensing hairs? Insights from the hydromechanics of biomimetic MEMS sensors

Insects and arachnids are often quite hairy. The reasons for this high density of sensory hairs are unknown. Previous studies have predicted strong hydrodynamic coupling between densely packed airflow-sensitive hairs. Flow perturbation owing to single hairs and between tandem hairs, however, has never been experimentally measured. This paper aims to quantify the extent of flow perturbation by single and tandem hairs directly, using biomimetic microelectromechanical system (MEMS) hairs as physical models and particle image velocimetry (PIV) for flow visualization. Single and tandem MEMS hairs of varying interhair distances were subjected to oscillatory flows of varying frequency. Decreasing hair-to-hair distance markedly reduced flow velocity amplitude and increased the phase shift between the far-field flow and the flow between hairs. These effects were stronger for lower flow frequencies. We predict strong hydrodynamic coupling within whole natural hair canopies exposed to natural stimuli, depending on arthropod and hair sizes, and hair density. Thus, rather than asking why arthropods have so many hairs, it may be useful to address why hairs are packed together at such high densities, particularly given the exquisite sensitivity of a single hair.