84 THE PHYSIOLOGY OF INSECT SENSES 



The tympanic membrane faces only part of the tracheal air sac. 

 The rest of it is faced by another membrane whose upper surface is 

 exposed to the air by a narrow slit which is the orifice of a large 

 counter-tympanic cavity. This cavity and membrane are regarded as 

 accessory resonating structures. 



The paired tympanic organs of cicadas are located ventrally in the 

 region of the first and second abdominal segments. They differ from 

 others described chiefly with reference to the point of attachment of 

 the chordotonal sensilla. These, approximately 1,500 in number, occur 

 in a single large bundle, their proximal ends attached to one corner of 



Fig. 59. Frontal section through the base of the abdomen of Cicadetta 

 coriaria showing the left auditory capsule. S, sense cells ; E, enveloping 

 cells; N, nerve; B, blood; Sp, spiracle; O, outer air; I, inner air space; 

 T, tympanic membrane. (Redrawn from Vogel, 1923.) 



the tympanic membrane and their distal ends to the cuticle of the body 

 wall (Figs. 58 and 59). 



The tympanic organs of Orthoptera and Lepidoptera have two so- 

 called tympanic muscles associated with them. The function of these 

 muscles is unknown, although it has been suggested from time to time 

 that they might be concerned with altering stresses on the tympanic 

 organ. In moths it is certainly true that distortion of the skeletal 

 elements of the tympanic organ reversibly changes the excitability of 

 the sensilla (Roeder and Treat, 1957). 



The Sensilla 



The chordotonal sensilla, as viewed with the light microscope, are all 

 basically the same. So far only those of the tympanic organ of the 



