NO. 8 INSECT ABDOMEN SNODGRASS 55 



trals (ve) lying beneath the others, where they arise near the middle 

 of each sternal plate and are inserted posteriorly on the anterior 

 margin of the sternum following. The transverse ventrals consist of 

 wide bands of fibers (tv) crossing the anterior parts of the segments 

 internal to the longitudinal ventrals, and attached laterally on the edges 

 of the sternal plates. The last transverse ventral in the female is that 

 of the seventh segment (fig. 19 A, tv). 



The lateral muscles include tergosternal, tergopleural, paratergo- 

 sternal, and pleurosternal muscles. In each segment there is a large 

 tergosternal lateral muscle (7) arising on the posterior lateral part of 

 the tergum external to the lateral dorsals, and inserted on the posterior 

 lateral margin of the sternum. Just behind this muscle, and external 

 to it, are two tergopleural muscles (2, 3) taking their origin on the 

 lower edge of the tergum. The first (^) is attached ventrally on the 

 pleural margin of the same segment; the second (j) is intersegmental 

 since it is attached on the pleural margin of the following segment. 

 The paratergosternal muscles comprise two bands of fibers (4, 5) 

 on the middle of each segment, one before, the other behind the 

 spiracle dilator (dis). The pleurosternal muscles include likewise 

 two bands of fibers, one (d) in the anterior part of each segment, the 

 other (7) in the posterior part. 



The spiracular muscles are the usual antagonistic pair inserted on 

 the manubrium of the anterior movable wall of the spiracular atrium. 

 One is a very small occlusor arising on the posterior atrial wall ; the 

 other is a large dilator (dls) arising ventrally on the edge of the 

 sternum between the attachments of 4 and 5. 



The somatic muscles of the visceral region of the cricket's ab- 

 domen are used by the female principally for respiratory purposes, 

 since oviposition is accomplished by the muscles of the genital seg- 

 ments, and ordinarily involves few movements in the rest of the 

 abdomen. The mechanical relation of the lateral muscles to the skeletal 

 parts of the abdomen is shown diagrammatically at A of figure 16. 

 It is to be noted that the muscles can produce only a contraction 

 of the abdomen, brought about mostly by a lifting of the sternal 

 plates, since there is no antagonistic mechanism capable of dilating 

 the abdomen, either dorso-ventrally or longitudinally, such as that 

 developed in the Acrididae. Inspiration with the cricket, therefore, 

 must be accomplished by a passive expansion of the abdominal parts 

 compressed by muscular force. 



The genital segments and the ovipositor. — The genital segments 

 of the female cricket (fig. 14 B, VIIT, IX) are somewhat reduced 

 by comparison with the eighth and ninth abdominal segments of the 



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