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ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1923 



far back into the abdomen (fig. 31, A, Air Sc). Its walls are very 

 thin and are closely applied to the inner faces of the tympana. The 

 whole abdomen of the cicada is, therefore, virtually a drum. Besides 

 the drum heads themselves there are two other pairs of large mem- 

 branes in the body wall below them, likewise covered internally by 

 the walls of the air sac. One pair of these membranes is particularly 

 thin and tense and must act as secondary resounding boards to give 

 added resonance to the sound produced by the drums. These lower 

 membranes are concealed above valvelike flaps of the body wall 

 projecting backward below them, but when the cicada begins to play 

 his drums he elevates the abdomen a little and thus opens the space 



/TmMcl 



-VMcl 



IIS ms 



Pig. 31. — The abdomen and sound-making organs of the 17-year cicada. A, the abdo- 

 men cut open from above to show the large air sac (AirSt!) it contains, and the great 

 tympanal muscles (TmMcl) that vibrate the drums or tympana (Tin). N 2 , the back of 

 the third thoracic segment carrying the hind wings (W 3 ) cut off near their bases. 

 The arrows indicate the position of the air holes or spiracles on the sides that open 

 into the air sac. B, inner view of right side of first two segments of abdomen, show- 

 ing the right tympanal muscle (TmMcl) attached to the tympanum (Tm) and the 

 spiracle (ISp) of the first segment ; IT — HIT, IS — HIS, dorsal and ventral plates of 

 first three abdominal segments ; DMcl, YMcl, dorsal and ventral longitudinal muscles ; 

 n, hinge between abdomen and thorax ; u, supporting plate of tympanal muscles 



between the membranes and their coverings. At the end of the song 

 the abdomen drops down again. 



It has generally been supposed that the air chamber of the 

 cicada's body is a part of the tracheal or respiratory system, cor- 

 responding with the smaller air sacs of other insects; but a recent 

 investigator, L. M. Hickernell, claims that it is a part of the ali- 

 mentary canal. The present writer described and figured it in a 

 former paper (p. 403, Smithsonian Report for 1919) as a tracheal 

 air sac receiving its air through the first spiracles or breathing 

 pores of the abdomen. Since then he has examined other species 

 and finds these spiracles always opening directly into the sac, as 

 indicated by the arrows on Figure 31, A. In freshly emerged in- 

 dividuals of one species the sac is clearly double, being divided 



