Hair canopy of cricket sensory system tuned to predator signals

Abstract

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 (o0.5 Â 10 À3 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. r

Interaction between arthropod filiform hairs in a fluid environment

“…As in previous studies [3,12,25,33], we assume that the i-th cricket hair resists the motion of the air with a torque of (15) where I (i) is the moment of inertia, R (i) is the torsional resistance coefficient, S (i) is the spring constant, and θ (i) (t) is the angular position of the i-th hair. In this section, we consider only a single hair, so we will drop the superscript indicating the identity of the hair.…”

“…At every position the velocity v has four components, three of which make up the perturbation velocity: (25) where u IRS is the velocity induced by all dynamic resistive forces, , on all hairs; u con is the velocity induced by all forces that ensure a no-slip condition about a rigid hair, ; u bc is the velocity caused by all forces that ensure a no-slip boundary condition on the cercus, ; and u b is the boundary layer or driving velocity. Our goal is to explicitly compute θ̈ (j) for j = 1, …, N in the expression of the velocity v on the right hand side of (24).…”

“…Furthermore, through this dependence on v, the computation of u bc depends on u con and vice-versa. In order to be able to solve for θ̈ (j) explicitly, we make an assumption that, in a given time step, F bc and F con are computed only from u IRS + u b , rather than from v given by equation (25). With this assumption the fifth and sixth equations above (equations (20) and (9)) take the form where and are to be taken as concatenations of velocity vectors along hair i and is dropped from the second equation since u b = 0 at all points in .…”

“…Computational efficiency is crucial, since each cercus contains about 1000 hairs of different lengths whose movements affect the air flow and, hence, the movement of other hairs. Previous work in this area was concerned with modeling the response of a single hair to the motion of the air and only recently has the focus shifted to the analysis of the viscosity-mediated coupling between hairs [3] and characterization of a total canopy response [25].…”

“…Their experimental results indicate limited interaction between these hairs, but, as the authors note, their results do not preclude significant hair interactions in other arthropods. In another recent article Magal et al [25] attempts to characterize the response of the entire collection of hairs on the cerci of the sand cricket Gryllus bimaculatus. The response characteristic and position of each cercal hair is combined to form the canopy response.…”