Chordotonal Organs of Insects

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L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 192tonically active neuron unresponsive to sound, and 1-2 smaller neurons phasically responsive tosound. The case for homology of the chordotonal organs is thus built on morphological andelectrophysiological evidence (Yack and Fullard, 1990), while differences between the twoorgans are mechanical and lie in non-neural structures such as the enlarged tympanal chamber,thinned tympanum and isolation of the tympanum from body movements (Fullard and Yack,1993).Ultrastructural examination of scolopidia from tympanal and atympanal chordotonal organsallowed Yack and Roots (1992) to suggest hypothetical changes, in addition to the mechanicalones noted above, involved in the transition from proprioceptive to hearing function.Mononematic, monodynal scolopidia are apparently the optimal type for hearing. In addition,characteristics of the tympanate condition include a decrease in length of the distal attachmentstrand, a decrease in microtubule density within the scolopidial attachment cells, and a loss ofthe sheath surrounding the strand. The resulting structure would result in a lower scolopidiumthreshold by stiffening the system in the tympanal strand. The mononematic, monodynalcondition could provide enhanced mechanical coupling between structures in the evolutionarytransition toward detection of minute and rapid tympanal membrane displacement (Yack andRoots, 1992; Ghiradella, 1971).Other homologues for tympanal ears have been proposed in closely related atympanate insects.The prosternal hearing organ found in parasitic tachinid flies is thought to be derived from theprosternal chordotonal organ in the neck of atympanate flies such as the drosophilids andmuscovids. The prosternal chordotonal organ of atympanate insects is thought to monitor neckmovements (Hengstenberg, 1991), whereas the tympanal organ has many more scolopidia andmonitors vibration of the anterior cuticular membranes of the ear, as a sound pressure receiversystem. Both organs enter the frontal nerve of the thoraco-abdominal ganglion and project to allthree thoracic neuromeres (Robert et al. 1994; Lakes-Harlan and Heller, 1992; see Section8.4.3). Recently, one of the two paired prosternal chordotonal organs in the locust (Locusta)was shown to connect medially to the ventral cervical membrane, and to be sensitive to soundand gross neck movements; this system may represent a transition toward a tympanal organ(H.-J. Pflüger and L.H. Field, pers. comm.). In grasshoppers, comparative developmentalstudies show that the tympanal organ in the first abdominal segment of Schistocerca appearshomologous to the pleural chordotonal organ found in the first abdominal segment of the moreprimitive atympanate species Heide amiculi. This interspecific homology supports Meier andReichert’s hypothesis of segmental specialisation of serially homologous pleural chordotonalorgans, which are essentially considered to be plesiomorphic (ancestral) (Meier and Reichert,1990). The cyclopean tympanal organ of mantids is an exception to the above examples ofatympanate prototype homologues. The organ projects to metathoracic nerve 7, which incockroaches (in the same order, Dictyoptera)
L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 193supplies cuticular exteroreceptor sensilla of the basisternum and furcasternum, and is notknown to supply any chordotonal organs. Furthermore the ganglionic projections of the sensoryaxons differ from those of all other tympanate insects (Yager and Hoy, 1987). Although aprototype homology is lacking for the mantid ear, an understanding of the evolution of this earin the Mantodea is not. Within 330 mantid genera, five anatomical types of external cuticularmodifications are associated with the ear. One of the types is associated with sensitiveultrasonic hearing, whereas another (Chaeteesidae) appears to be the plesiomorphic conditionand is reflected in newly-hatched nymphs of other families showing more derived conditions inthe adults (Yager, 1989).12.4 LEG CHORDOTONAL ORGANS12.4.1 Femoral chordotonal organIn Orthoptera the femoral chordotonal organ consists of a proximal and a distal scoloparium(Fig. 8), both of which insert onto a common apodeme pulled by the tibia, but which are usuallyseparated within the femur (Slifer, 1935; Burns, 1974; Field and Pflüger, 1989). The proximalscoloparium contains many very small sensory neurons, a form that is reminiscent of cicadaabdominal chordotonal organs (Fig. 7b), whereas the distal scoloparium contains larger neuronscharacteristic of joint chordotonal organs. In the hind legs, the FeCO may differ from this plan,as there appears to be a variable degree of fusion of the two scoloparia throughout the ensiferanorthopterans (Fig. 8c-e). In the caeliferan orthopterans, the proximal scoloparium was thoughtto be lost in the hind leg FeCO, since only one cluster of neurons had been described(Usherwood et al. 1968; Zill, 1985a). However, a distal group of tiny sensory neurons in themetathoracic FeCO of the locust are supplied by a separate nerve which overlays the FeCO andjoins its sensory nerve proximally (Matheson and Field, 1990). Although the group appears tobe an integral part of the main FeCO, its innervation by a separate nerve, and composition oftiny neurons were taken as evidence for serial homology to the proximal scoloparium (whichcontains tiny neurons) in the fore- and middle legs. Further support for homology comes fromdevelopmental evidence in the grasshopper Melanoplus. The FeCO of all three leg pairs arisesas a single invagination and subsequent proliferation of an undifferentiated cell mass in thefemur. At day 23, the mushroom-shaped cell mass of the fore- and middle legs splits off a distalportion and thereby forms the two separate scoloparia. In the hind leg, the cell mass does notsplit off a separate portion, but it differentiates into the large and small neurons of the adultFeCO (Slifer, 1935). Thus the tiny neurons in the FeCOs of all three legs have a commonorigin.The advantages underlying the evolution of two scoloparia (three in dipteran FeCO: Lakes-Harlan and Pollack, 1993; see Section 3.5.3) and the
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Chordotonal Organs of Insects

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