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AUDITORY AND SOUND-PRODUCING 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 (1) the tympanic 
ear-like organs of certain Orthoptera (Acridide), which were 
fully described by Siebold [278]; (2) the sound-intensifying 
mirrors of the Locustide ; and (3) the so-called tymbals and 
mirrors of the Cicade. 
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 
Fic. 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: 
m, Miillerian 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 ¢”, anterior, and / ¢”, pos- 
terior, subtympanic tracheal sac ; c, crista of the organ of Siebold; st, supra- 
tympanic organ, 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 
