L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 1 Chordotonal Organs of Insects Laurence H. Fielda and Thomas Mathesonb a Department of Zoology, University of Canterbury, PB 4800, Christchurch, New Zealand b Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK Please note that this PDF may differ very slightly from the published version as it was created from the original text and figures at a later date. All the page numbering is identical, except for the Plates (p 229, 230), which did not have page numbers and were inserted between pages 56 and 57 in the published version. 1. Introduction 2 2. Histological methods for chordotonal organs in insects 7 2.1 Histochemical staining of fixed tissue 7 2.2 Intravital perfusion techniques 8 2.3 Uptake of dye by cut axons and nerves; intracellular dye injection 8 2.4 Immunochemical techniques 9 3. Diversity in distribution, structure and function 11 3.1 Overview of diversity 11 3.2 Head 12 3.3 Thorax 14 3.4 Abdomen 22 3.5 Legs 25 4. Ultrastructure 40 4.1 General scolopidial structure 40 4.2 Method of fixation affects ultrastructure 41 4.3 The bipolar sensory neuron 42 4.4 The scolopale cell 61 4.5 The attachment cell 67 5. Mechanics of the scolopidium 69 5.1 Compliance of the scolopidium 70 5.2 Hypotheses for role of cilia in mechanical coupling 75 6. Transduction mechanisms 80 6.1 Mechanically activated channels (MACs) 80 6.2 Receptor currents and potentials 81 6.3 Transducer coupling to spike generator 83 7. Physiological responses of chordotonal organs 86 7.1 Mechanosensitive roles of chordotonal organs 86 7.2 The nature of the stimulus 87 7.3 Methods of analysis 88 7.4 Response properties 89 8. Central projections 108 8.1 Overview 108 8.2 Neuropilar areas containing chordotonal organ projections 109 8.3 Comparisons between species 109 L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 2 8.4 Central organisation of auditory and vibratory chordotonal afferents 110 8.5 Central organisation of other chordotonal organ afferent neurons 119 9. Processing of information from chordotonal organs 126 9.1 Femoro-tibial chordotonal organ 126 9.2 Processing of information from other chordotonal organs 148 9.3 Presynaptic inhibition 152 9.4 Neuromodulation 154 10. Development of chordotonal organs 156 10.1 Taxonomic overview 157 10.2 Scolopidium cell lineage 158 10.3 Scolopidium cell differentiation 160 10.4 Morphogenesis of chordotonal organs 161 10.5 Morphogenesis of nerve pathways 168 10.6 Physiological consequences of delayed development in Orthoptera 171 11. Genetics and molecular biology 172 11.1 Model for genetic determination of sensilla 173 11.2 Gene structure and expression control 178 11.3 Molecular biology of gene products 180 11.4 Future research 185 12. Evolution and homology 185 12.1 Concepts of homology 186 12.2 Antennal chordotonal organs 187 12.3 Thoracic and abdominal chordotonal organs 188 12.4 Leg chordotonal organs 193 12.5 Homology of scolopidia and cuticular sensilla 196 12.6 Origin and evolution of scolopidia 198 13. Conclusions 200 14. Acknowledgements 202 15. Abbreviations 202 15.1 Sense organs 202 15.2 Ganglionic regions and cell types 203 15.3 Other 204 16. References 204 1. Introduction Research on chordotonal organs has extended from the domain of sensory neurophysiologists to now include a diverse group of investigators in the areas of genetics, development, biochemistry, immunology, cytology, and evolution. Even within neurobiology, the demands for highly-specialised training and focus (e.g. motor control, transduction mechanisms, development or architecture of central circuitry) mean that many arthropod neurobiologists are not familiar with chordotonal organs. Comprehensive reviews of modern knowledge about chordotonal organs are lacking. Therefore it is our intention to provide not only a review of recent literature, but also to generate a practical guide for the diverse range of researchers mentioned above. We hope that by offering comments on fruitful and critical areas for research, we will encourage a cross-disciplinary approach in future research on chordotonal organs. Chordotonal organs are found in the Insecta and Crustacea, but do not appear in any other arthropod classes. In insects, chordotonal organs occur in great morphological diversity (see L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 3 Section 3), and are found at nearly every exoskeletal joint, and between joints within limb and body segments. All chordotonal organs are internal mechanoreceptors and thus may serve as proprioceptors, or as highly specific mechanoreceptor organs (e.g. hearing organs). Morphologically, a chordotonal organ is a cluster of sensilla (rarely a single sensillum) connected to moveable parts of skeletal cuticle or to the tracheal system, or sometimes inserting into a connective tissue strand (Fig. 1a). Often, additional connective tissue elements anchor or support the organ. Within some chordotonal organs, neurons are clustered into one or two groups, termed scoloparia, which may be morphologically separated from each other (e.g. the mesothoracic femoral chordotonal organ of the orthopteran leg; Fig. 8d,e). Chordotonal organs were first recognised in insects, and described by Graber (1882) based upon the presence of a terminal element (Stifte) in each sensory neurone, as viewed in the light microscope. This is a component of the scolopidium, the sensillum which characterizes chordotonal organs, and which is an elaborate micromechanical transducer. A scolopidium consists of one or more bipolar sensory neurons, each with a ciliated dendrite enveloped by two specialised cells, the scolopale cell and the attachment (cap) cell, plus a glial cell. The modified cilium forms a distal extension of each neuronal dendrite. The scolopale cell produces a barrel- shaped sleeve of fibrous scolopale rods (collectively termed the scolopale, or Stifte seen by Graber) which surround the cilium (or cilia) and insert distally into a dense cap produced by the attachment cell (Fig. 1b). The above terminology (reviewed by Howse, 1968) specifically applies to such internal sense organs and should not be applied to cuticular sensilla, as sometimes encountered in the literature (e.g. Dethier, 1963; Davies, 1988; Gillott, 1991; Wigglesworth, 1972). Chordotonal organs are not normally associated with external cuticular structures, such as hairs, bristles or campaniform sensilla, although evolutionary homologies to these are discussed in Section 12. Other sense organs in insects (e.g. hair sensilla and campaniform sensilla) also contain bipolar sensory neurons with centriolar derivatives (“tubular bodies”) in the dendrites (Wright, 1976), but only chordotonal organ scolopidia contain the solid scolopale structure (scolopale rods). All mechanosensory sensilla with ciliated bipolar sensory neuron dendrites are together classified as Type I arthropod mechanoreceptor organs, whereas those mechanosensilla with multipolar, non-ciliated sensory neurons (e.g. joint and arthrodial membrane stretch receptors, and muscle receptor organs) are classified as Type II (Zawarzin, 1911). The following terminology will assist in understanding the morphological variation found in insect chordotonal organs. Moulins (1976) described the various means by which the apical end of a chordotonal organ may be associated with the tegument. One of two basic types of structure are usually L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 4 Fig. 1. Chordotonal organ and scolopidium morphology, illustrated for the cockroach tarsal chordotonal organ. a. Diagrammatic illustration showing bipolar neurons with dendrites terminating in scolopidia embedded in connective tissue strands. The strands attach distally to the internal surface of the cuticle (not illustrated), and are stretched or relaxed during mechanical stimulation. b. Essential components of two scolopidia. In this case, each neuron gives rise to a ciliated dendritic outer segment which inserts along with the other from the paired neuron into the cap, and are surrounded by a scolopale produced by the scolopale cell. Attachment cells are embedded in strands. Neurons are surrounded by thick (left) or thin (right) glial cells (sheath and Schwann cells) with small nuclei. a and b, modified from Young (1970), with permission. L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 5 observed. To avoid confusion, we refer to either (a) non-connective chordotonal organs, or (b) connective chordotonal organs. In non-connective organs the scolopidia are connected distally to the hypodermis by the attachment cell (sometimes also an intermediate accessory cell); in connective chordotonal organs the scolopidia are inserted into a connective tissue strand via the attachment cell (the “connective” chordotonal organ of Howse, 1968). In the latter, the connective tissue strand often bridges a joint, as in some leg or antennal chordotonal organs. Although Moulins referred to this type as a “strand chordotonal organ”, we propose to use the terminology of Howse because a different kind of innervated connective tissue strand receptor (a multipolar Type II neurone) with a central cell body has subsequently been described in insect legs (“strand receptor” [SR] of Bräunig et al. 1981; Bräunig, 1985). Scolopidia that contain a single cilium and neuron are referred to as monodynal, whereas those with two or three cilia and neurons inserting into a common cap are