ABSTRACT
Chordotonal organs are internal proprioceptors that have evolved only in insects and crustaceans. Although they serve as versatile stretch receptor organs in these classes, their sensory role is greater, owing to their extraordinary sensitivity to micromechanical displacement (Figure 3.1). These sense organs still contain unsolved mysteries of operation: unknown mechanisms of transduction, unusual and little-known spike potentials and unitary receptor “quantum bumps,” extraordinary partitioning of stimulus specificity within single organs, and the possibility of several unusual molecular mechanical events involved in transduction and sensitivity (Field and Matheson, 1998). Such features contribute to their scientific appeal. Earlier reviews provided limited coverage of chordotonal organs under the topics of insect or arthropod mechanoreception (Finlayson, 1968; Rice, 1975; Moulins, 1976; Wales, 1976; Wright, 1976; McIvor, 1985). A number of these reviews formed chapters in the excellent book by Mill (1976), which gives a comprehensive illustrative coverage of chordotonal organ anatomy, histology, ultrastructure, and physiology. This mate-
FIGURE 3.1 Connective and nonconnective chordotonal organ gross morphology. a-c: Connective chordotonal organs; d: Nonconnective chordotonal organ. a: Femoral chordotonal organ (FeCO) in distal femur (see inset) of locust hind leg, showing attachment across joint via ligament connected to FeCO apodeme, and additional flexor strand containing a single strand organ. b: Variations in fusion of scoloparia (sensory neuronal subgroups) and connective ligaments of joint chordotonal organs (FeCO) from different legs and orthopteran species: upper, locust; middle, New Zealand tree weta (Anostostomatidae); lower, locust. c: Tibio-tarsal chordotonal organ of cockroach showing subgroups of scolopidial neurons embedded in connective tissue strands that span the tibio-tarsal joint. d: Nonconnective chordotonal organs in proximal region of hind tibia of New Zealand tree weta. These scoloparia, composing the complex tibial organ, include the subgenual organ (SGO), the intermediate organ (IO), and the crista acustica (CA). They lack long connective tissue ligaments and instead are intimately associated with tracheae (CA, IO) or a septate mass occluding the hemocoel (SGO). (a, b from Field, L.H. and Matheson, T., Adv. Insect Physiol. 27:1-228, 1998. With permission. c from Young, D., Phil. Trans. Roy. Soc. Lond. B 256:401-426, 1970. With permission. d from unpublished data of author.)
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rial was brought up to date in a recent review that focused on insect chordotonal organs (Field and Matheson, 1998). Extensive coverage was provided for many aspects of these sense organs, including newly developed genetic approaches to studying chordotonal organ function and structure. Here, a descriptive overview of anatomy and terminology, diversity, and function will be presented to lay the groundwork for discussing techniques used to study the structure and sensory physiology of chordotonal organs.
