118 LETTERS ON ENTOMOLOGY. 
nervures against each other, produces a vibration 
in the membrane, augmenting the sound. The 
males in question sing continually in the hedges 
and trees during the months of July and August, 
especially towards sunset and part of the night ; 
but when any one approaches., they immediately 
stop. Some of the tribe of cicadae are far more 
noisy than any of the preceding, and the Brazilian 
cicada is said to be heard at the distance of a 
mile, which is as if a man of ordinary stature 
could be heard all over the country. To produce 
this amazing sound they have a very complex 
and wonderful organ. Under the body of the 
male is a pair of large plates of an irregular form 
— in some semi-oval, in others triangular, in 
others again a segment of a circle of greater or 
less diameter — covering the anterior part of the 
abdomen, and fixed to the trunk between that 
and the hind legs : these are the drum-covers or 
opercula, from beneath which the sound issues. 
At the base of the posterior legs, just above each 
operculum, there is a small pointed triangular 
process, the object of which, as Reaumur sup- 
poses, is to prevent them from being too much 
elevated. When an operculum is removed, be- 
neath it is perceived, on the exterior side, a hollow 
cavity, with a mouth somewhat linear, which 
