so
Henry C. Bennet-Clark
4.2 The Primary Vibrating Structure
The simplest type of sound detector is a vane extended in the sound field which is
driven by the particle velocity or fluid movement due to the sound wave and
actuates sensilla at the base of the vane: examples are the antennae of mosquitoes
(Gopfert et al. 1999) or the sensory hairs of many insects and arachnids (e.g.,
Michelsen 1983; Fletcher 1978, 1992; Barth et al. 1993).
In pressure-driven ears, a thin diaphragm extends over an air-filled cavity; the
pressure in the sound wave causes vibration of the diaphragm which drives the
sensory cells. Because acoustic pressures are very small relative to, e.g.,
atmospheric or ventilatory pressure changes, the cavity is usually vented by a
narrow pipe, such as the tracheal system in insects or the eustachian tube in
tetrapods. Such ears are found in insects such as locusts, cicadas or moths and
throughout the tetrapods (see, e.g. Autrum 1963, Michelsen 1983).
In both types of sound detector, the primary vibrating structuretends to be light
in weight but to have a large effective surface area. In antennae, lightness with
large surface area may be achieved, as with a feather, by a central rod bearing
lateral hairs. At the low velocities at which such structures vibrate, the relatively
thick air boundary layer increases the effective area of the antenna. If the vibrating
structure is too thin, it will bend along its length, and if it is too thick, it will be too
heavy to bend in the sound wave.
In pressure-driven ears, the diaphragm may be thin: thicknesses of 0.5 to 2 J.Lm
have been reported for parts of the tympana of locusts (Stephen and Bennet-Clark
1982) and cicadas (Young 1990). However, ifthe diaphragm is thin, it will readily
transmit rather than absorb sound and it may be difficult to extract energy from its
vibration; a thick heavy diaphragm may have a small amplitude of vibration but it
will be able to drive a larger group of sensory cells.
Many sound sensors have resonant properties. In antennae, the mass or
rotational moment of inertia of the vane will interact with the stiffuess of the basal
articulation (Humphrey et al. 1993). In a pressure-driven ear, the mass of the
diaphragm and its stiffness will interact (Fletcher 1992).
In general, smaller ears of either type tend to respond at higher frequencies.
Simple resonant properties give rise to a V -shaped threshold response curve, with
maximal sensitivity at the resonant frequency (e.g. Gopfert et al. 1999). At lower
frequencies, the response will be limited by the stiffness, and at higher frequencies
by the mass or inertia of the system. Ears tend to have arrays of sensory cells that
allow analysis over a band of frequencies: these have been described in as diverse
animals as bush crickets (Oldfield 1982), locusts (Michelsen 1971) and mammals
(von Bekesy 1960). In many cases, the mechanism of tuning is incompletely
understood.
Henry C. Bennet-Clark
4.2 The Primary Vibrating Structure
The simplest type of sound detector is a vane extended in the sound field which is
driven by the particle velocity or fluid movement due to the sound wave and
actuates sensilla at the base of the vane: examples are the antennae of mosquitoes
(Gopfert et al. 1999) or the sensory hairs of many insects and arachnids (e.g.,
Michelsen 1983; Fletcher 1978, 1992; Barth et al. 1993).
In pressure-driven ears, a thin diaphragm extends over an air-filled cavity; the
pressure in the sound wave causes vibration of the diaphragm which drives the
sensory cells. Because acoustic pressures are very small relative to, e.g.,
atmospheric or ventilatory pressure changes, the cavity is usually vented by a
narrow pipe, such as the tracheal system in insects or the eustachian tube in
tetrapods. Such ears are found in insects such as locusts, cicadas or moths and
throughout the tetrapods (see, e.g. Autrum 1963, Michelsen 1983).
In both types of sound detector, the primary vibrating structuretends to be light
in weight but to have a large effective surface area. In antennae, lightness with
large surface area may be achieved, as with a feather, by a central rod bearing
lateral hairs. At the low velocities at which such structures vibrate, the relatively
thick air boundary layer increases the effective area of the antenna. If the vibrating
structure is too thin, it will bend along its length, and if it is too thick, it will be too
heavy to bend in the sound wave.
In pressure-driven ears, the diaphragm may be thin: thicknesses of 0.5 to 2 J.Lm
have been reported for parts of the tympana of locusts (Stephen and Bennet-Clark
1982) and cicadas (Young 1990). However, ifthe diaphragm is thin, it will readily
transmit rather than absorb sound and it may be difficult to extract energy from its
vibration; a thick heavy diaphragm may have a small amplitude of vibration but it
will be able to drive a larger group of sensory cells.
Many sound sensors have resonant properties. In antennae, the mass or
rotational moment of inertia of the vane will interact with the stiffuess of the basal
articulation (Humphrey et al. 1993). In a pressure-driven ear, the mass of the
diaphragm and its stiffness will interact (Fletcher 1992).
In general, smaller ears of either type tend to respond at higher frequencies.
Simple resonant properties give rise to a V -shaped threshold response curve, with
maximal sensitivity at the resonant frequency (e.g. Gopfert et al. 1999). At lower
frequencies, the response will be limited by the stiffness, and at higher frequencies
by the mass or inertia of the system. Ears tend to have arrays of sensory cells that
allow analysis over a band of frequencies: these have been described in as diverse
animals as bush crickets (Oldfield 1982), locusts (Michelsen 1971) and mammals
(von Bekesy 1960). In many cases, the mechanism of tuning is incompletely
understood.