602 THE SENSES AND SENSORY ORGANS. 
abdomen. The mirrors in many respects resemble the tym- 
panic organs of the Acrididz, 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 Cicadz 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] 
Gh a are refers the sound to an_ expiratory 
Gryllus campestris, after current from the posterior thoracic 
gas [283]: g; ganglion; spiracles, which he erroneously asserts 
hc, heemal cavity ; pf, pos- z on ‘ . 
terior tympanum ;s¢, supra- Open into the cavities in which the 
tympanal organ; tr at,tr pt, tymbals lie; the spiracles are separated 
anterior and posterior tym-  fF475) these cavities by the opercula, 
panic sacs. 
and are quite distinct from the spiracles 
which open into the mirror chamber. A structure somewhat 
intermediate between the sound-producing organs of the Cicadz 
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). 
