Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Morphology and mechanical response of cricket airflow sensors

As in most biological sensors, the cricket hairs sensors investigated in the IST-2001-34718 CICADA project show a high level of integration between the "hardware" and the "software" sides. Hair-based mechano-sensors in insects and arthropods achieve very high sensitivities, being capable of detecting acoustic signals at thermal noise levels. Typical threshold levels for neural response are associated with angular displacements of the hairs of the order of 0.001 degrees response to stimuli with air velocities of the order of mm/s or less. To a significant extent this high sensitivity is due to the detailed morphology of the socket-hair subsystem and to the mechanical properties of the tissues involved. The organization of the tissues making up the sensing unit (hair + socket + hair-base supporting membrane) and their mechanical properties can provide local amplification and filtering of the stimulus. The hair and the socket are stiff composites made of chitin fibres bonded together by protein and phenolic compounds. The orientation of the fibres in the hairs is longitudinal but the orientation of the fibres in the socket is yet to be determined. In the regions of the exoskeleton between hairs the chitin fibres are organized plywood fashion. The membrane, which anchors the base of the hair to the socket, is likely to by made of resilin, a rubber-like protein found also in the flight organs of insects. The elastic modulus of this membrane determines the stiffness of the rotational spring supporting the hair and, in turn, this is critical for the mechanical response of individual hairs to steady state or oscillatory airflow stimuli.

The objective of WP2 was to characterize as fully as possible the relevant morphological aspects and mechanical properties of tissues, which make up the mechanosensors and to investigate the stimulus-response characteristics of single biological sensors. A data set of correlations between various important morphological and geometrical parameters of the cricket's hair sensing organs has been established using a variety of microscopy techniques. The length of hairs, the easiest geometrical feature to measure using these methods, has been cross-correlated to the diameter at the base, the degree of taper along the length, the diameter of the socket and the ellipticity of the socket base.

This last parameter is relevant to the polarity of the hair response, i.e. to the fact the sensing hairs deflect preferentially in one particular plane, with respect to axis of the cerci, determined by the geometry of the elliptical insertion of the hair at the base of the socket. Also, the orientation of this preferential plane with respect to the position of two (occasionally three) campaniform organs associated with at the base of the socket has been verified. The characterization of the elastic properties of the hair-supporting membrane has been obtained using vibration methods and high-speed camera filming. As far as the response of single biological sensors to stimuli is concerned, the combined use of Laser Doppler Vibrometry and high speed camera has shown that over a wide range of input frequencies of the oscillatory stimulus (from a few Hz to several kHz), the hairs 'follow' the stimulus and appear to be in phase with it.

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