288 Transactions. —Zoology. 
another click. That this is really the mode in which the sounds are caused 
may be proved by exposing the parts immediately after killing the insect; on 
then allowing the muscles to harden a little by exposure, and on pulling 
them with the point of a pin, the membranes will be seen to straighten and 
fly back again, accompanied by the production of the usual sound. 
Now what part do the large transparent drum-like membranes take in the 
production of the sound? All writers on the subject have attributed to them 
reverberating qualities for the intensifying of the sound, but a simple 
experiment appears to disprove this, for if an insect be taken while stridu- 
lating and all four of the membranes be destroyed with a pin the sounds are 
not materially affected, but if one of the stridulating membranes be destroyed 
the sounds suffer great diminution, and on destroying the other they cease 
entirely. I was much surprised the first time I tried the experiment to find 
that the large drums seemed to take no part in the production of the sound, 
and the idea occurred to me that they might be hearing organs, but on 
examining the females, which, most remarkable to relate, are dumb and do 
not possess the stridulating organs, I found that the drums exist indeed, but 
are quite rudimentary instead of being large as we should expect to find them 
were they subservient to the sense of hearing. The question remains then of 
what use are they? That such highly developed structures must be of some 
use is clear. The three cicade found commonly in Canterbury differ in the 
sounds produced. The small green cicada utters a sound which may be 
represented by the repetition of the letter “r” thus “r-r-r-r-r-r,” the voice of 
the larger green species would be expressed by “ crrrk—crrrk—crrrk,” while the 
small black ones found in the hills say “ crrrk-r-r-r-r-r.” The voice of this 
species is remarkably loud and piercing. 
In connection with the voice of the cicada I may allude to a circumstance 
which has been frequently observed, viz, inability of some individuals to 
perceive very acute sounds. This is very noticeable with the song of the 
small green cicada. I have found many persons who are totally unable to 
hear any sound when my ears are being pierced with their shrill voices so 
as almost to give rise to a feeling of pain. There would seem to be in some 
cases less a deficiency in the organisation of the ear than in the faculty of 
perception, which is akin to the difficulty experienced by a landsman in per- 
ceiying very distant objects at sea. In some individuals, however, there is an 
absolute inability to hear very acute sounds, and inasmuch as the entire range 
of the human ear is, according to Helmholtz, eleven octaves, it has been 
justly remarked that the air may be filled with shrill insect sounds, which 
may be perfectly audible to the insects themselves but absolutely inaudible 
to our grosser sense. 
