3&J GOUREAU ON THE 



However, I procured some of these insects in the summer of 

 1836, on which I made the following observations. When 

 this insect sings in a box, no motion is observable in the wings, 

 nor in any other part of the body ; when I held it between the 

 fingers in such a way as left the abdomen free, its voice was as 

 strong as usual ; but if 1 held its body and pressed the opercula 

 against the abdomen, it was dull, faint, and stifled: if, on the 

 contrary, I raised the abdomen, so as to leave the cavities, 

 which are generally covered by the opercula, open, the voice 

 became unusually loud and strong : this led me to conclude, 

 that the opercula perform the office of keys, and serve to 

 modify sounds ; but these keys, instead of being movable, as 

 in wind instruments, are fixed : the abdomen is movable. 

 This will explain why the Cicada, when at liberty, is constantly 

 moving its abdomen during the time it is singing, alternately 

 elevating and depressing it; by this means it obtains a variety 

 of sounds from the musical instrument, and produces modula- 

 tions, which, though we are not sensible of them, are assuredly 

 perceptible to the insect himself, and to the female whom it 

 is his object to please. The trochanter acts as a check, as 

 Reaumur thought ; but instead of preventing the operculum 

 from being raised too high, it merely prevents its yielding to 

 the pressure of the abdomen during depression ; for it is of 

 itself incapable of motion. 



Being desirous of seeing what took place in the timbals 

 (timbalen) during the time the animal was singing, I raised with 

 a penknife that portion of the upper part of the ring which 

 covers one of them, and thus exposed it to sight ; immediately 

 the sound became much louder, and I saw a movement in this 

 organ similar to that which Reaumur has indicated, without 

 having himself observed it. The tirabal (timbale) vibrates and 

 alters in form, passing successively from a convex to a concave 

 form ; it is this movement which produces the sound and the 

 song. These vibrations are very apparent when the insect 

 sings loudly ; and less so when only a faint sound is produced ; 

 and when the song is scarcely audible by us they are imper- 

 ceptible. It appears to me that Reaumur was well acquainted 

 with the sonorous organ of these insects, and that his Memoir 

 left nothing further to wish for on the subject — nothing at least 

 of importance. 



The Cicadce of the genus Tibicen have the least sonorous 



