Chordotonal Organs of Insects

L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 7with increasingly narrower aspects of mechanoreceptor roles in insect behaviour (e.g. Bässler,1993; Burrows, 1994). Others have reviewed chordotonal organ ultrastructure (Moulins, 1976),transduction mechanisms (French, 1988), homologies (Boyan, 1993) and the roles of tympanalorgans in sound reception and behaviour (Michelsen and Larsen, 1985). In recent years newresearch areas, such as the genetic control of chordotonal organ development, have greatlyincreased our knowledge of chordotonal organs and their relationships to other sense organs(reviewed for Drosophila by Jan and Jan, 1993). It is timely that this diversity of information begathered together and placed into an overall perspective through the present review. We buildon the earlier general reviews of chordotonal organs rather than repeat the informationcontained therein.At the time of the last major review on chordotonal organs (McIver, 1985) a number of aspectsof the physiology of chordotonal organs remained unknown, although they had been posed asmajor research questions since the reviews by Wright (1976) and Moulins (1976). The foremostof these problems was the vast gap in knowledge about how a stimulus is applied to thedendrite. The question involves details about (a) the roles of scolopidial components intransferring mechanical distortion to the dendrite, (b) the functions of differentiated regions inthe cilium, and (c) the differences in mode of sensory activation of the different scolopale typesfound in chordotonal organs. Initial progress was made on the role of the ciliary component(Moran et al. 1977). Another major question asked how different sensory responses are realisedby adjacent chordotonal neurons, which often are clustered in a group, all receiving the samemechanical stimulus from a common cuticular termination. Differences must be sought in theelectrical properties of the excitable membrane and associated transduction mechanisms,mechanical properties of the multicellular link between dendrite and stimulus, andultrastructural or molecular differences between scolopidia. Two further questions remained:(1) what is the embryonic derivation and genetic control of chordotonal sensilla duringdevelopment, and (2) what are the roles of chordotonal organs in the control of behaviour?McIver (1985) reviewed the scant literature on these subjects, but only in the past decade has aremarkable boom of research taken place to address these questions, and this forms animportant part of the present review.2. Histological methods for chordotonal organs in insects2.1 HISTOCHEMICAL STAINING OF FIXED TISSUE2.1.1 Baker-Masson's triple stainYoung (1970) fixed chordotonal organs in alcoholic Bouin's fluid, embedded them in paraffinand stained the 15 µm sections in Baker's modification of