THE PERIODICAL CICADA 



muscle which arises below from a special support on the 

 ventral wall of the second abdominal segment (Figs. 

 123, 124, TmMcl). It is by the contraction of these 

 muscles that the drum membranes are set in motion. 



W* 



Tm 



TmMcl 



AirSc 



TmMcl 



VMcl 



Fig. 1 24. The abdomen and sound-making organs of the male periodical 



cicada 



A, the abdomen cut open from above, exposing the air chamber {AirSc), and 

 showing the great tympanal muscles {TmMcl) inserted on the tympana (Tm). 

 The arrows indicate the position of the first spiracles opening into the air chamber 



(see fig. 123, ISp) 



B, inner view of right half of first and second abdominal segments, showing the 



ribbed tympanum {Tm), and the muscles that vibrate it (TmMcl) 

 AirSc) air chamber; DMcI, dorsal muscles; IS, IIS, HIS, sternal plates of first 

 three abdominal segments; ISp, first abdominal spiracle; IT, IIT, HIT, tergal 

 plates of first three abdominal segments; Nz, tergal plate of third thoracic seg- 

 ment; T?n, tympanum; TmMcl, tympanal muscle; ?Vz, base of hind wing; VMcl, 

 ventral muscles 



But a muscle pulls in only one direction; the drum muscles 

 produce directly the inward stroke of the drumhead 

 membranes; the return stroke results from the outward 

 convexity and the elasticity of the heads themselves 

 and the stiff ribs in their walls. 



When a cicada starts its music, it lifts the abdomen a 

 little, thus opening the space between its ventral drum- 



[ 209] 



