NO. 8 INSECT ABDOMEN SNODGRASS 93 



closed by movements of levation and depression of the abdomen on 

 the lateral hinges (fig. 30 B, a) between the postnotum and the first 

 abdominal tergum, the movements being produced by the dorsal and 

 ventral muscles of the first abdominal segment. The structure of the 

 first and second abdominal sterna and of the tympanal cavity between 

 them is essentially the same in both sexes, but it is more simple in the 

 female. 



The first abdominal sternum of the female cicada lies immediately 

 behind the narrow postcoxal arms of the metathoracic epimera (fig. 

 30 C, Pc.v). It consists of two parts. The first is an anterior, median, 

 triangular plate (IS) having its lateral angles prolonged as a pair 

 of arms (c) fused with the postcoxal bridges (Pcx), but each termi- 

 nating in a lateral expansion (d), on which are attached the tergo- 

 sternal muscles of the first abdominal segment. The second plate is 

 a median, quadrate sclerite (e) flexibly hinged to the anterior plate 

 (IS), and extending upward from the latter in the anterior wall of 

 the tympanal cavity (B, ^). 



The spiracles of the first abdominal segment (fig. 30 B, C, ISp) 

 are contained in peritremal sclerites fused with the lower, ends of the 

 metathoracic postnotum (PN3). They open directly into the great air 

 chamber of the abdomen. 



The sternum of the second abdominal segment consists, as does that 

 of the first segment, of two parts, one part exposed, the other con- 

 cealed in the tympanal cavity. The exposed part of the sternum is in 

 this case the posterior part, which forms a narrow, transverse bridge 

 between the lower ends of the second abdominal tergum (fig. 30 A, 

 B, C, IIS), with which it is solidly continuous. The anterior part 

 of the second sternum is a weaker median sclerite (C, f) turned up- 

 ward on the anterior margin of the posterior sclerite in the posterior 

 wall of the tympanal cavity (B, /) ; its dorsal margin meets the 

 posterior plate of the first sternum (e) in the transverse fold of the 

 roof of the cavity. The lateral angles of the anterior sclerite of the 

 second sternum (C, /) are produced into slender bars (g) that extend 

 outward to meet the lateral extremities of the posterior sclerite, where 

 each ends in a small expansion containing one of the second abdominal 

 spiracles (IISp). Between the arms (g) of the first sclerite and the 

 lateral parts of the second sclerite are the oval glistening tympana 

 {Tin), or so-called "mirrors". The chordotonal sense organs con- 

 nected with the tympana (in the male) are said to be contained in the 

 tubercles {h) located at the ventral lateral angles of the second seg- 

 ment where the tergum and the sternum are confiuent. The thick cor- 

 rugated membranes {Mb) in the anterior wall of the tympanal cavity 

 are intersegmental membranes between the first and second sterna. 



