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 suifer great diminution, and on destroying the other they cease 

 entirely. T 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 cicadse 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, v^iz., 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- 

 ceiving 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. 



