60 INSECTS AND MAN 



The organ of song (fig. 8) is found in the male insect only, 

 " and the true sound apparatus consists of two small ear- 

 like or shell-like inflated drums situated on the sides of the 

 basal segment of the abdomen. These drums are caused to 

 vibrate by the action of powerful muscles, and the sound 

 is variously modified by adjacent smaller discs the so- 

 called 'mirrors' or sounding boards and issues as the 

 peculiar note of the species, which, once heard, is never 

 likely to be forgotten, or, if heard again, mistaken for that 

 of some other insect. The true sound organs are entirely 

 exposed in the seventeen-year-old cicada, except for the 

 covering afforded by the closed wings of the resting insect. 

 In other cicadas these drums are usually protected by over- 

 lapping valves or expansions of the body wall. The sound- 

 ing drum, or ' timbal,' as Reaumur termed it, of the peri- 

 odical cicada is a tense, dry, crisp membrane numerously 

 ribbed or pleated, with the convex surface turned outward. 

 The ribs are chitinous thickenings or folds in the surface 

 of the parchment-like drum, and strengthen the drum, while 

 perhaps rendering it at the same time more elastic. The 

 sound is produced by the rapid vibration, or undulation, 

 caused by the springing or snapping in and out of these 

 corrugated drums. Two powerful muscles of very peculiar 

 structure situated within the base of the abdomen set these 

 drums in motion, producing the rattling, so-called song of 

 the cicada, very much, as has been suggested, as sound is 

 produced by pressing up and down the bottom of a tin pan 

 which is somewhat bulged. Beneath each ' timbal ' in the 

 base of the abdomen of the insect is a large sound or air 

 chamber, and a third occurs in the thorax joining the first 

 two. These are closed by the body walls and membranes, 

 and the two abdominal ones beneath by the very peculiar 

 'mirrors' or 'spectacles' the tense, mica-like membranes 

 situated at the base of the abdomen and protected and 

 covered by the semicircular rigid discs projecting from the 

 thorax. These transparent membranes are often mistaken 



