602 



THE SENSES AND SENSORY ORGANS. 



abdomen. The mirrors in many respects resemble the tym- 

 panic organs of the Acrididse, but they have been insufficiently 

 investigated. The tymbal is a large plicated membrane 

 moved by a strong muscle, which has been observed to 

 exhibit pulsating movements during the emission of sound. 

 As the note of the Cicadae is very high, no visible vibrations or 



changes of form can even be the cause 

 of its deepest ground tone. As there is 

 undoubtedly a narrow spiracle opening 

 into the special cavity in which the 

 mirrors are situated, it appears to me 

 probable that the so-called tymbals are 

 functionally bellows which produce the 

 sound by expelling air through this 

 spiracle. It further appears probable 

 that the air so expelled impinges upon 

 the mirrors and sets up a vibratory 

 movement of the air in the drum-like 

 cavity in front of them. Landois [281] 

 refers the sound to an expiratory 

 current from the posterior thoracic 

 spiracles, which he erroneously asserts 

 open into the cavities in which the 

 tymbals lie ; the spiracles are separated 

 from these cavities by the opercula, 

 and are quite distinct from the spiracles 

 which open into the mirror chamber. A structure somewhat 

 intermediate between the sound-producing organs of the Cicada} 

 and the tympanic apparatus of the Orthoptera occupies the 

 tympanic fissure in the Blow-fly. It receives small nerves 

 which are branches of the great nerves to the halteres 

 (see p. 623). 



FIG. 78. The supratym- 

 panic (Graber's) organ of 

 Gryllus campestris, after 

 Graber [283]: X, ganglion; 

 //r, haemal cavity ; pt, pos- 

 terior tympanum ; st, supra- 

 tympanal organ; trat,trpt, 

 anterior and posterior tym- 

 panic sacs. 



