Interaction between arthropod filiform hairs in a fluid environment

Many arthropods use filiform hairs as mechanoreceptors to detect air motion. In common house crickets (Acheta domestica) the hairs cover two antenna-like appendages called cerci at the rear of the abdomen. The biomechanical stimulus-response properties of individual filiform hairs have been investigated and modeled extensively in several earlier studies. However, only a few previous studies have considered viscosity-mediated coupling between pairs of hairs, and only in particular configurations. Here, we present a model capable of calculating hair-to-hair coupling in arbitrary configurations. We simulate the coupled motion of a small group of mechanosensory hairs on a cylindrical section of cercus. We have found that the coupling effects are non-negligible, and likely constrain the operational characteristics of the cercal sensory array.     

Mobilities of the cercal wind-receptor hairs of the cricket, Gryllus bimaculatus

The deflection sensitivities of cercal filiform hairs of the cricket, Gryllus bimaculatus, were determined by direct measurement. The tangential velocity of deflecting hair shafts in response to stimulus air motion was measured in situ by a laser-Doppler velocimeter with surface scattering of the shaft. The velocity of the stimulus air motion in a small wind tunnel was calibrated by the same velocimeter with smoke from a joss-stick. The mobility of the hair was obtained from former measurements with reference to the latter calibration of the single apparatus. A Gaussian white noise signal was employed as a stimulus waveform, and the stimulus-response transfer function was calculated through a cross-correlation method, which provides greater precision and wider frequency for a longer period of measurement. The mobility of hair was expressed in deflection amplitudes and phase shifts in reference to the velocity sinusoid of a stimulus at various frequencies. The measurements established the following conclusions. The wind receptor hairs comprise an array of mechanical band-pass filters whose best frequencies are inversely proportional to the length. The motion dynamics of the wind-receptor hairs have strong damping. Accepted: 24 February 1998     

A computational fluid dynamics model of viscous coupling of hairs

Arrays of arthropod filiform hairs form highly sensitive mechanoreceptor systems capable of detecting minute air disturbances, and it is unclear to what extent individual hairs interact with one another within sensor arrays. We present a computational fluid dynamics model for one or more hairs, coupled to a rigid-body dynamics model, for simulating both biological (e.g., a cricket cercal hair) and artificial MEMS-based systems. The model is used to investigate hair–hair interaction between pairs of hairs and quantify the extent of so-called viscous coupling. The results show that the extent to which hairs are coupled depends on the mounting properties of the hairs and the frequency at which they are driven. In particular, it is shown that for equal length hairs, viscous coupling is suppressed when they are driven near the natural frequency of the undamped system and the damping coefficient at the base is small. Further, for certain configurations, the motion of a hair can be enhanced by the presence of nearby hairs. The usefulness of the model in designing artificial systems is discussed.     

Mechanics of trichobothria in orb-weaving spiders (Agelenidae,Araneae)

Summary When a fly is humming at a distance of about one centimetre from an orb-weaving spider (Agelenidae) the trichobothria on the spider's extremities are deflected by air streams and air vibrations. Frequency analysis of the hum of the two prey animals,Drosophila andMusca, shows that the effective sound velocities of the harmonics with frequencies inferior to some five hundred Hz exceeds that of higher frequencies by a factor of at least 5. Biologically relevant resonances would, therefore, have to be looked for in the range of a few hundred Hz. Frequency response diagrams show that single hairs have no resonance between a few Hz and approximately 2 kHz. The maximal relative amplitude of hairs of different lengths shifts from the longer to the shorter hairs with increasing frequency. As this is only a minor effect, however, it appears that there is no frequency discrimination by the mechanical apparatus. Constant air streams with a velocity of 40 mm/s cause hair deflection of about 10 degrees (the hair's bend is neglibible). Similarly, near-field particle velocity of sound fields up to a few hundred Hz is well transmitted. The mechanical directional sensitivity does not depend on the azimuthal angle of deflection. Thus, information about direction and velocity of stationary and near-field air movements is transmitted without deformation by the mechanical apparatus. This is well matched with the fact that the hair is multiply innervated.Supported by grants of the Deutsche Forschungsmeinschaft in the field of the Schwerpunktprogramm Rezeptorphysiologie     

Mechanosensory afferents innervating the swimmerets of the lobster

Feathered hair sensilla fringe both rami of the lobster (Homarus americanus) swimmeret. The sensory response to hair displacement was characterized by recording afferent impulses extracellularly from the swimmeret sensory nerve while deflecting sensilla with a rigidly-coupled probe or controlled water movements. Two populations of hairs were observed: "distal" hairs localized to the distal 1/3 of each ramus and "proximal" hairs near its base. Distal hairs are not innervated by a mechanosensory neuron but instead act as levers producing strain within adjacent cuticle capable of activating a nearby hypodermal mechanoreceptor. Hair deflections of 25 degrees or more are required to evoke an afferent response and this response is dependent on hair deflection direction. The frequency and duration of the afferent discharge evoked are determined by the velocity of hair displacement. Each proximal hair is innervated by a single mechanosensory neuron responding phasically to hair deflections as small as 0.2 degrees in amplitude. Deflection at frequencies up to 5 Hz elicits a single action potential for each hair movement; at higher frequencies many deflections fail to evoke an afferent response. These sensilla, which are mechanically coupled, may be activated by the turbulent flow of water produced by the swimmerets during their characteristic beating movements.     

Modeling arthropod filiform hair motion using the penalty immersed boundary method

Crickets are able to sense their surrounding environment through about 2000 filiform hairs located on a pair of abdominal cerci. The mechanism by which the cricket is able to sense a wide range of input signals using these filiform hairs of different length and orientation is of great interest. Most of the previous filiform hair models have focused on a single, rigid hair in an idealized air field. Here, we present a model of the cercus and filiform hairs that are mechanically coupled to the surrounding air, and the model equations are based on the penalty immersed boundary method. The key difference between the penalty immersed boundary method and the traditional immersed boundary method is the addition of forces to account for density differences between the immersed solid (the filiform hairs) and the surrounding fluid (air). The model is validated by comparing the model predictions to experimental results, and then the model is used to examine the interactions between multiple hairs. With multiple hairs, there is little interaction when the hairs are separated by more than 1mm, and, as they move closer, they interact through viscous coupling, which reduces the deflection of the hairs due to the air movement. We also examine the computational scalability of the algorithm and show that the computational costs grow linearly with the number of hairs being modeled.     

Hair density in the Eurasian otter <Emphasis Type="Italic">Lutra lutra</Emphasis> and the Sea otter <Emphasis Type="Italic">Enhydra lutris</Emphasis>

The hair density of adult Eurasian otters Lutra lutra (Linnaeus, 1758) and sea otters Enhydra lutris (Linnaeus, 1758) was analysed using skin samples taken from frozen carcasses. Lutra lutra exhibited a mean hair density of about 70 000 hairs/cm² (whole body, appendages excepted), the mean individual density ranging from about 60 000 to 80 000 hairs/cm². The dominant hair type were secondary hairs (wool hairs), the hair coat comprising only 1.26% of primary hairs (PH). Secondary hair (SH) density remained constant over the body (appendages excepted), whereas a few variations in PH density were observed. Neither an influence of the sex, nor a seasonal variation of the hair coat was found, moulting seems to be continuous. Enhydra lutris had a hair density between 120 000 and 140 000 hairs/cm², the primary hairs representing less than 1% within the hair coat. Hair density remained quite constant over the regions of the trunk but was lower at the head (about 60 000 hairs/cm² on the cheek). The hair follicles were arranged in specific groups with different bundles of varying size, normally comprising dominant numbers of wool hair (SH) follicles. Invariably there was always a large central primary hair follicle and numerous sebaceous glands between the bundles and principally around the PH follicles. The results are discussed related to possible ecological influences on hair coat density.     

Hair canopy of cricket sensory system tuned to predator signals

Filiform hairs located on the cerci of crickets are among the most sensitive sensors in the animal world and enable crickets to sense the faintest air movements generated by approaching predators. While the neurophysiological and biomechanical aspects of this sensory system have been studied independently for several decades, their integration into a coherent framework was wanting. In order to evaluate the hair canopy tuning to predator signals, we built a model of cercal population coding of oscillating air flows by the hundreds of hairs on the cerci of the sand cricket Gryllus bimaculatus (Insecta: Orthoptera). A complete survey of all hairs covering the cerci was done on intact cerci using scanning electronic microscopy. An additive population coding of sinusoid signals of varying frequencies and velocities taking into account hair directionality delivered the cercal canopy tuning curve. We show that the range of frequencies and velocities at which the cricket sensory system is best tuned corresponds to the values of signals produced by approaching predators. The relative frequencies of short (< 0.5 x 10(-3) m) and long hairs and their differing responses to oscillating air flows therefore enable crickets to detect predators in a time-frequency-intensity space both as far as possible and at close range.     

Evolution of the cercal sensory system in a tropical cricket clade (Orthoptera: Grylloidea: Eneopterinae): a phylogenetic approach

The diversity of sensory systems in animals has poorly been explored on a phylogenetic basis at the species level. We addressed this issue using cricket cerci, comprising abdominal appendages covered with touch‐ and air‐sensitive hairs. Scanning electron microscopy measurements and spatial analyses of hair positioning were used to quantify the structural diversity of cercal structures. Eighteen Eneopterinae and two Gryllidae (outgroups) were studied from a phylogenetic perspective. Cerci were revealed to be complex, diverse, and variable between cricket species. Based on maximum likelihood estimations, the ancestral Eneopterinae cercus had a small size, and its hair equipment allowed the use of both air and touch mechanoreception. The evolution of Eneopterinae cerci was mainly unconstrained by the phylogeny; it was rather a punctuated process, involving apical transformations, and was mostly unrelated to environmental patterns. All studied species have enhanced their overall perceptive capacities compared to the ancestor. Most have longer cerci with more and/or longer hairs. Sensory abilities have improved either in the direction of touch or air movement detection, or both, without discarding the potential for any sensory capacity that was already present ancestrally. This pattern is consistent with the hypothesis of an evolutionary trade‐off for sensory performances. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 614–631.     

A Model of Filiform Hair Distribution on the Cricket Cercus

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 covered with between 500 to 750 filiform mechanosensory hairs. The distribution of the hairs on the cerci, as well as the global patterns of their movement axes, are very stereotypical across different animals in this species, and the development of this system has been studied extensively. Although hypotheses regarding the mechanisms underlying pattern development of the hair array have been proposed in previous studies, no quantitative modeling studies have been published that test these hypotheses. We demonstrate that several aspects of the global pattern of mechanosensory hairs can be predicted with considerable accuracy using a simple model based on two independent morphogen systems. One system constrains inter-hair spacing, and the second system determines the directional movement axes of the hairs.     

The shape of wind-receptor hairs of cricket and cockroach

We examined the exact shapes of the thread-like wind-receptor hairs in the cricket and cockroach. The diameters of hairs at various distances from the hair tip as measured by scanning electron microscopy revealed unexpected hair shapes. We had expected, a priori, that the shape of the hair would be a slender linearly tapered cone, but the measurements revealed hairs in the form of extremely elongated paraboloids. The diameter of the wind-receptor hairs varies with the square root of the distance from the hair tip, i.e., the diameter rapidly increases with the distance from the tip and is asymptotic to the base diameter. Both the cricket, Gryllus bimaculatus, and the cockroach, Periplaneta americana, showed the same hair shape. In both insects, the formation of the wind-receptor hair during metamorphosis seems to be controlled by a common cytological program. The shape of the hair constrains the mobility of the wind-receptor hair, because both the drag force caused by moving air and the moment of inertia of motion dynamics are functions of shaft diameter. The shape of the hair is a biological trait which affects the sensory information transmitted to the central nervous system. Accepted: 24 February 1998     

Common species affects the utility of non‐invasive genetic monitoring of a cryptic endangered mammal: The bridled nailtail wallaby

Non‐invasive methods of monitoring wild populations (such as genotyping faeces or hair) are now widely used and advocated. The potential advantages of such methods over traditional direct monitoring (such as live capture) are that accuracy improves because sampling of non‐trappable individuals may be possible, species in difficult and remote terrain can be surveyed more efficiently, and disturbance to animals is minimal. Few studies have assessed the effects of interactions between species on remote sampling success. We test the use of non‐invasive monitoring for the cryptic, forest‐dwelling, solitary and endangered bridled nailtail wallaby (Onychogalea fraenata) that is sympatric with the ecologically similar and more common black‐striped wallaby (Macropus dorsalis). Six types of hair traps were tested for 3668 trap days, and hairs were caught with about a 10% success rate. Camera traps showed that baited hair traps targeted both wallaby species. We microscopically identified hair as bridled nailtail wallaby or black‐striped wallaby. We compared these hairs and their genotypes (using seven microsatellite loci) with known bridled nailtail wallaby hairs and genotypes derived from animal trapping. Trapped bridled nailtail wallaby hairs had characteristics that could be mistaken for black‐stripe wallaby hairs; characteristics were not diagnostic. Genetic assignment tests consistently differentiated the known bridled nailtail wallaby samples from identified black‐striped wallaby samples, however genetic overlap between most of the microsatellite markers means that they are not suitable for species identification of single samples, with the possible exception of the microsatellite locus B151. With similar trapping effort and within the same area, live‐capture mark‐recapture techniques estimated 40–60 individuals and non‐invasive methods only detected 14 genotypes. A species‐specific genetic marker would allow more efficient targeting of bridled nailtail wallaby samples and increase capture rates.     

Daily cycles for emission of urticating hairs from the pine processionary caterpillar (Thaumetopoea pityocampa S.) and the brown tail moth (Euproctis chrysorrhoea L) (Lepidoptera) in laboratory conditions

Summary In laboratory conditions, urticating hairs from the pine processionary caterpillar (Thaumetopoea pityocampa S.) and from the brown tail moth (Euproctis chrysorrhoea L.) are detectable in the air using an apparatus designed for the capture of airborne microorganisms and pollen research studies. The hairs produced by the caterpillars of these two species are distributed either via air currents or moths (only forEuproctis). Daily cycles of hair emission were observed and were in relation with locomotion and feeding activities of the caterpillars and with flying and reproductive activities of moths.     

Development of cricket sensory hairs: changes of dynamic mechanical properties

Summary Mechanical oscillation properties of cricket (Acheta domesticus) filiform hair sensilla were measured at different larval stages, as an indication of larval sensory capacities and for comparison with data in the literature on central nervous changes during development. The hairs were stimulated by airborne vibration over a frequency range of 10 to 220 Hz. Best frequency, angular displacement at best frequency, slope of angular-displacement tuning curve and phase of hair deflection relative to air particle velocity were tested for correlation with hair length, which is proportional to the age of a sensillum. The ranges found for the various oscillation parameters in early larval stages were similar to or larger than those in adults. Oscillation properties changed with both the developmental stage of the hair sensilla and that of the whole animal. Four individually identifiable hair sensilla were analysed separately; the sensory neurons of two of them are known to change synaptic properties during maturation. Angular displacement at a given stimulus intensity was maximal for all hairs after differentiation, and decreased during further development. The hairs did not show clear common changes for any of the other oscillation parameters. Yet particular changes were found for individual hairs.     

A note on the specific cuticle structure of wool hairs in otters (Lutrinae)

The cuticle structure of the wool hairs (secondary hairs) of six otter species was examined by scanning electron microscopy to clarify the specific function of this hair type in the Lutrinae. The species studied were chosen according to the different genera, climatic regions, and degrees of association to water of the Lutrinae. Independent of their preferred habitats, the cuticle of every wool hair examined exhibited in all animals a rather similar shape and arrangement of the scales. This specific adaptive feature allows a flexible interlocking of adjacent wool hairs, which also helps to form thin wool hair bundles that surround small oval shaped spaces. Thus, the trapping of an effective insulating air layer is facilitated and heat loss from the body is reduced.     

Hair morphogenesis inAcetabularia mediterranea: Temperature-dependent spacing and models of morphogen waves

Summary Whorls of sterile hairs inA. mediterranea show, at the moment of first appearance of hair initials, a spacing independent of number of hairs in the whorl but dependent on temperature. By changing the temperature at various times before appearance of hair initials, the pattern-forming event can be located at about 3–4 hours before initials become visible.The temperature dependence of spacing is like that of a chemical rate parameter: In (spacing)versus 1/T is linear. This suggests that the spacing is controlled by kinetic rather than structural factors, and correlates well with reaction-diffusion theory.Mathematical analysis and computer simulation have been used to show that the observed sequence of tip-flattening followed by whorl initiation can be interpreted in terms of published models for generation of dissipative structures by reaction and diffusion, and that at least two sequential processes must occur, the first of which shifts growth activity from extremity to circumference of the growing tip, permitting the second to operate around the circumference.Submitted to workshop on Morphogenesis inAcetabularia, Berlin (West), September 1980.     

A root hairless barley mutant for elucidating genetic of root hairs and phosphorus uptake

This paper reports a new barley mutant missing root hairs. The mutant was spontaneously discovered among the population of wild type (Pallas, a spring barley cultivar), producing normal, 0.8 mm long root hairs. We have called the mutant bald root barley (brb). Root anatomical studies confirmed the lack of root hairs on mutant roots. Amplified Fragment Length Polymorphism (AFLP) analyses of the genomes of the mutant and Pallas supported that the brb mutant has its genetic background in Pallas. The segregation ratio of selfed F₂ plants, resulting from mutant and Pallas outcross, was 1:3 (–root hairs:+root hairs), suggesting a monogenic recessive mode of inheritance.In rhizosphere studies, Pallas absorbed nearly two times more phosphorus (P) than the mutant. Most of available inorganic P in the root hair zone (0.8 mm) of Pallas was depleted, as indicated by the uniform P depletion profile near its roots. The acid phosphatase (Apase) activity near the roots of Pallas was higher and Pallas mobilised more organic P in the rhizosphere than the mutant. The higher Apase activity near Pallas roots also suggests a link between root hair formation and rhizosphere Apase activity. Hence, root hairs are important for increasing plant P uptake of inorganic as well as mobilisation of organic P in soils.Laboratory, pot and field studies showed that barley cultivars with longer root hairs (1.10 mm), extracted more P from rhizosphere soil, absorbed more P in low-P field (Olsen P=14 mg P kg^–1 soil), and produced more shoot biomass than shorter root hair cultivars (0.63 mm). Especially in low-P soil, the differences in root hair length and P uptake among the cultivars were significantly larger. Based on the results, the perspectives of genetic analysis of root hairs and their importance in P uptake and field performance of cereals are discussed.     

Dry under water: Comparative morphology and functional aspects of air‐retaining insect surfaces

Superhydrophobic surfaces prevent certain body parts of semiaquatic and aquatic insects from getting wet while submerged in water. The air layer on these surfaces can serve the insects as a physical gill. Using scanning electron microscopy, we investigated the morphology of air‐retaining surfaces in five insect species with different levels of adaptation to aquatic habitats. We found surfaces with either large and sparse hairs (setae), small and dense hairs (microtrichia), or hierarchically structured surfaces with both types of hairs. The structural parameters and air‐film persistence of these surfaces were compared. Air‐film persistence varied between 2 days in the beetle Galerucella nymphaea possessing only sparse setae and more than 120 days in the bugs Notonecta glauca and Ilyocoris cimicoides possessing dense microtrichia (up to 6.6 × 10⁶ microtrichia per millimeter square). From our results, we conclude that the density of the surface structures is the most important factor that affects the persistence of air films. Combinations of setae and microtrichia are not decisive for the overall persistence of the air film but might provide a thick air store for a short time and a thin but mechanically more stable air film for a long time. Thus, we assume that a dense cover of microtrichia acts as a “backup system” preventing wetting of the body surface in case the air–water interface is pressed toward the surface. Our findings might be beneficial for the development of biomimetic surfaces for long‐term air retention and drag reduction under water. In addition, the biological functions of the different air retention capabilities are discussed. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.