Chordotonal organs associated with the sound producing apparatus of cicadas (Insecta, Homoptera)

Abstract

1. There are three connective chordotonal organs associated with the sound producing apparatus of cicadas: a) the tymbal chordotonal organ, connected to the lower anterior margin of the tymbal; b) the tensor chordotonal organ, connected to the special protuberance bearing the origin of the tensor muscle; c) the detensor chordotonal organ, connected to the cuticle near the inner margin of the tympanum, beneath the detensor tympani muscle. 2. A newly described muscle, the accessory tensor muscle, originates next to the proximal attachment of the tymbal organ and inserts alongside the insertion of the tensor muscle. 3. These chordotonal organs contain an exceptionally large number of sensillae: tymbal organ, 650–1300 scolopidia; tensor organ, 200–350 scolopidia; detensor organ, 325–400 scolopidia. 4. Each scolopidium consists of the following cells: a bipolar neuron with dendrite bearing a sensory cilium, a scolopale cell, a proximal attachment cell, a distal attachment cell. Scattered Schwann cells surround the sensory axons. 5. The scolopale has the usual fine structure but the following features are distinctive: a) the scolopale is composed of a continuous tube of scolopale material, not separate scolopale rods; b) the scolopale material varies markedly in thickness in different regions of the scolopale; c) the extracellular cap, which anchors the cilium distally, is very small and is entirely surrounded by the scolopale. 6. From the morphological evidence, it is suggested that these chordotonal organs provide a very sensitive monitoring system for the activity of the sound producing apparatus.