A UDITOR Y AND SO UND-PROD UCING MECHANISMS. 599 



b. Tympanic Organs. 



The terms tympana and mirrors have been applied to 

 certain more or less transparent tense membranes, protected 

 by over-lapping plates, or situated at the inner extremity of a 

 short tubular depression of the integument, or upon the ex- 

 posed surface of one of the elytra. Such are (i) the tympanic 

 ear-like organs of certain Orthoptera (Acrididce), which were 

 fully described by Siebold [278] ; (2) the sound-intensifying 

 mirrors of the Locustidae ; and (3) the so-called tymbals and 

 mirrors of the Cicadas. 



The Tympana of the Orthoptera are thin, round or oval 

 membranes. One half of the tympanum is usually thicker 

 and darker than the other, and a small sclerite projects 



FIG. 75. Diagrammatic representations of the ear-like organs of the Orthoptera, 

 copied from Graber's paper [286]- 



A. The ear-like organ from the first abdominal segment of an Acridium : 

 in, Mtillerian organ, on the inner surface of the tympanic membrane ; , nerve 

 fibre ; s, spiracular opening ; /;-, subtympanic tracheal sac. 



B. A section of the anterior tibia of a Locusta ; a tr, anterior, and p tr, pos- 

 terior, subtympanic tracheal sac ; cr, crista of the organ of Siebold ; st, supra- 

 tympanic oigan, or organ of Graber. 



inwards from its border, and has a resemblance at least to 

 the auditory ossicles of a Vertebrate. This sclerite is some- 

 times connected with a tensor muscle. Beneath the tympanic 

 membrane there is always a large tracheal sac connected with 

 the exterior by a spiracle of peculiar construction. 



There is always a special nerve end organ, and sometimes 



