Chordotonal Organs of Insects

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L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 86contribution of electrical membrane components to chordotonal neuron responses.Discrete subthreshold depolarisation events such as those observed in tympanal organs havealso been found in tonically-spiking neurons of the femoral chordotonal organ (FeCO), wherethey summate and give rise to overshooting spikes (Field and Matheson, unpublishedobservations). In the subgenual organ (SGO) of the cockroach, a slow receptor potential givesrise to non-overshooting spikes, but shows no sign of discrete subthreshold events. Therecording site was in an electrotonically coupled accessory cell, which may explain the absenceof discrete depolarisations (Shaw, 1994b). Therefore it is not clear whether Hill’s (1983a)results for tympanal scolopidia can be generalised to other scolopidial receptor potentials.The transduction region of chordotonal sensilla is much more susceptible to blockage bydimethyl sulfoxide (DMSO) than the axonal region (Theophilidis and Kravari, 1994). DMSOhas an analgesic effect on vertebrate sensory neurons but the mechanism of action is unknown.In the locust FeCO, evidence suggests that potassium transduction channels in the dendrite areblocked at low concentrations (0.85%) and axonal potassium conduction channels are blockedat higher concentrations (4.8% reduces the firing rate by half; Theophilidis and Kravari, 1994).7. Physiological responses of chordotonal organs7.1 MECHANOSENSITIVE ROLES OF CHORDOTONAL ORGANSIn an ecological approach to insect mechanoreception, Schwartzkopf (1974) summarised thesenses that insects use to detect mechanical signals in the environment. When focused onchordotonal organs specifically, the scheme gives an appreciation of their versatility.A gravitational sense is achieved by evaluation of displacement of body parts as an insect restsin, or alters its position in, the gravitational field (a constant mechanical force). Leg jointchordotonal organs detect errors caused by gravitational pull, in a feedback loop set to maintainposture (Bässler, 1993; Field and Coles, 1994), whereas antennae in resting dipterans signal thedirection of the gravitational field (Schneider, 1953).A sense of air or water current is given by Johnston’s organ, and by hair sensilla, in detectingdisplacement of the surrounding medium relative to the insect’s body. If the insect is resting onthe substrate, deflection of the antennae by moving air or water causes orientation into thecurrent. In flying insects, the Johnston’s organ, and in some cases campaniform and hairsensilla, detect antennal deflection in a feedback system that controls wing beat rate, andtherefore flight speed, to achieve a stable airflow against the antennae (reviewed inSchwartzkopf, 1974).
L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 87A vibration sense (oscillations of a solid substrate, as opposed to air or water) is highlydeveloped in insects, owing to the SGO in each leg. Usually the SGO scolopidia respond best tolow frequencies up to several kHz and over comparatively short distances (because of dampingby the substrate).A sense of hearing, on the other hand, involves detection of oscillations in a fluid (air or water)over long distances. Both vibration detection and hearing are important in communicationamongst insects. As seen in Section 3, insects have a great diversity of chordotonal organs andassociated structures adapted for hearing, allowing acoustic sensitivity peaks ranging from 600Hz to 200 kHz (see Table II compiled by Schwartzkopf, 1974). The range includes ‘primitive’hearing organs, such as tarsal chordotonal organs of backswimmers (Notonectidae) whichdetect water surface oscillations as weak as 0.5 µm at 150 Hz, and ‘multifunctional’chordotonal organs, such as Johnston’s organ, which detects not only wind current but also thespecific sound frequency of female wing beats in culicid dipterans (380 Hz for the mosquitoAnopheles subpictus). Hearing organs have developed in mouthparts (sphingid moths) andwings (lacewings, Neuroptera) for specific detection of high frequency airborne sound, albeitwithout elaborate tympana. The most specialised hearing organs are the tympanal organs, withelaborate modifications for high sensitivity, and frequency tuning of individual scolopidia(Schwartzkopf,1974; Michelsen and Larsen, 1985).There are two senses in which insect chordotonal organs do not appear to participate. Thesense of touch is by definition the domain of cuticular sensilla and campaniform sensilla. Thesense of isometric tension within and between internal body parts, especially associated withmuscles, appears to be restricted to Type II multipolar receptors (e.g. the tibial flexor muscletension receptors in locusts: Theophilidis and Burns, 1979; Matheson and Field, 1995),although in crustaceans, chordotonal organs can detect muscle tension as well as displacementof joints (Macmillan et al. 1982).7.2 THE NATURE OF THE STIMULUSThe ultrastructure of scolopidia indicates that these sensilla are designed to be exquisitelysensitive bio-mechanical transducers, but it does not suggest the kind mechanical signals towhich scolopidia respond. Mechanical energy occurs in a variety of forms, including staticpotential energy involved in position and force (shear, torsion, compression, stretch) and kineticenergy involved in displacement (direction, velocity and acceleration). When this energyimpinges on the rigid arthropod exoskeleton, an excellent physical frame of reference isprovided for the generation of highly reproducible distortions. Not all forms of mechanicalenergy are detectable; static pressure, for example, is ineffective on the virtually incompressiblecuticle (Schwartzkopf, 1974). The mechanical energy to which chordotonal organs are
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Chordotonal Organs of Insects

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