90 



AN INTRODUCTION TO ENTOMOLOGY 



Fig. 102. — -The musical apparatus of a cicada; fm, 

 folded membrane; /, base of leg; Ic, lateral cavity; 

 m, mirror; o, operculum, that of the opposite 

 side removed; sp, spiracle; t, timbal; vc, ventral 

 cavity (After Carlet). 



from the hind border of the thorax and overlap the basal part of the 

 abdomen; these are the opercula (Fig. 102, o). The opercula are 

 expansions of the ster- 

 nellum of the meta- / 



thorax, and each 1 



serves as a lid covering V^!^^v%?:'nV%:/^^ y'l^^-^'-r'^i-^^ ^P 



a pair of cavities, con- 

 taining the external 

 parts of the musical 

 apparatus of one side 

 of the body. 



The two cavities 

 covered by a single 

 operculum may be de- 

 signated as the ventral 

 cavity (Fig. 102, v. c.) 

 and the lateral cavity 

 (Fig. 102, /. c.) respec- 

 tively. Each cavity is formed by an infolding of the body-wall. 



In the walls of these cavities are three membranous areas; these 

 are known as the timbal, the folded membrane, and the mirror. 



The timbal is in the lateral cavity on the lateral wall of the parti- 

 tion separating the two cavities (Fig. 102, i); the other two mem- 

 branes are in the ventral cavity. The folded membrane is in the 

 anterior wall of the ventral cavity (Fig. 102, /. m.); and the mirror 

 is in the posterior wall of the same cavity (Fig. 102, m). Within the 

 body, there is in the region of the musical apparatus a large thoraco- 

 abdominal air chamber, which comnliinicates with the exterior 

 through a pair of spiracles (Fig. 102 5^); and a large muscle. Which 

 extends from the furca of the second abdominal segment to the inner 

 face of the timbal. 



By the contraction of this muscle the timbal is pulled towards the 

 center of the body; and when the muscle is relaxed, the elasticity of 

 the chitinous ring supporting the timbal causes it to regain its former 

 position. By a very rapid repetition of these movements of the timbal 

 the sound is produced. 



It is probable that the vibrations of the timbal are transmitted to 

 the folded membrane and to the mirror by the air contained in the 

 large air chamber mentioned above; as the strings of a piano are 

 made to vibrate by the notes of a near-by violin. The sound, how- 

 ever, is produced primarily by the timbal, the destruction of which 



