BUTTERFLIES, MOTHS AND CICADAS 213 



and that all the essential organs are compressed into a 

 narrow band above. This air cavity may be called the 

 abdominal sac. We look forward into this vacant sac. 

 In front we see a pair of stout white rectangular muscles 

 that meet below in the middle line and diverge as 

 they ascend to be finally attached to the posterior 

 margin of the drum. 



These are the five organs to which I wish to direct 

 attention : (i) the opercula, (2) the drum, (3) the 

 shields, (4) the air-sacs, and (5) the muscles. 



In order to understand the mechanism of these 

 different organs it is necessary to examine the cicada 

 while in the act of emitting its sound. I find the 

 insect at sunset singing on the trees. When captured, 

 it immediately ceases, but I squeeze its thorax and it 

 again gives forth its note. With a pair of sharp scissors 

 I snip off the opercula ; the sound continues ; the oper- 

 cula therefore are not essential to the production of 

 the noise. I amputate the shields. The music still 

 continues ; so we reach a similar conclusion here. I 

 decapitate the insect. Even then the sound may re- 

 appear ; the mechanism is therefore reflex ; it is not 

 in necessary subjection to the brain. I seek another 

 specimen. It is playing loudly on a tree. Its abdomen 

 is seen to distend and collapse in accordance with the 

 increase and diminution of the pitch. While the note 

 is low the body is more flaccid, but with a rising pitch 

 the abdomen swells into a tense transparent globe. 

 It would seem as though the abdominal sac was there- 

 fore an essential organ for the production of the sound. 

 Let us see. I catch the insect and make a little 

 window into its abdomen so as to open freely into the 

 sac, There can be no air pressure now, The air will 



