626 THE SENSES AND SENSORY ORGANS. 
that much more work is needed before it can be said that these 
remarkable structures have been satisfactorily worked out. 
b. On the Probable Function of the Tympanic Organs, and their 
Relation to the Halteres. 
Edison’s phonograph shows that the same membrane may 
alternately act as a sound-producing and sound-receiving organ ; 
and there is no a priori reason for denying such a double func- 
tion to the tympanic membranes and mirrors in insects ; but 
I shall endeavour to show that there is much evidence in favour 
of the view that these membranes are concerned in the per- 
ception rather than in the production of sounds. 
The existence of sound-receiving tympanic membranes has 
hitherto been only admitted in the case of certain Orthoptera, 
the Acridide and Locustide. The latter at least possess a 
complex auditory mechanism. Siebold’s and Graber’s organs 
are found highly developed in relation with the tracheal sacs 
connected with the subgenual or tibial tympana, which cannot 
be regarded as sound-producing organs, since the sounds are 
clearly produced by the elytra. 
On the other hand, in the Acridide (Field Crickets), the 
tympanic organ is much larger, and only exhibits, so far as is 
known, Miiller’s organ, a group of chordotonal organs situated 
in a terminal ganglion, which is directly applied to the tympanic 
membrane. It is further probable that such ganglia exist in 
relation with the mirrors of the Cicadz, as Swinton affirms [287]. 
Hence it appears to me far from improbable that the mirrors of 
the Acrididz, and those of the Cicade, are analogous structures. 
Further, the tympanic membranes in the Locustide, in the 
Gryllide, and the mirrors in the Cicadze, exhibit a chitinised 
plate which occupies a moiety of the membrane, and I have 
shown that a similar sclerite exists in my newly discovered 
tympanic membrane in the Blow-fly. Whether these mem- 
branes are subgenual, thoracic or abdominal, they exhibit 
precisely similar peculiarities. There is therefore an a priort 
argument in favour ofa similarity of function. Although the 
mirrors of the Cicade are usually regarded as sound-producing 
