NO. 6 



GRASSHOPPER ABDOMEN— SNODGRASS 



23 



and goes dorsally to its insertion on the outer face of the lateral apo- 

 deme of the sternum (fig. 10 B, lie). It is thus a dilator of the 

 abdomen and an inspiratory muscle in respiration, since its contrac- 

 tion separates the sternum from the tergum (fig. 11 F, G). 



lyS, ijg. Second and third external lateral muscles (fig. 10 A). — • 

 These two muscles arise on the lateral part of the tergum below the 



vim vel 



Fig. II. — Abdominal mechanisms of Acrididae. 



A, mechanism of tergosternal movements : tergum and sternum approximated 

 by internal lateral muscles {ili, 2li) , separated by first external lateral {ile), 

 moved lengthwise on each other by oblique second and third external laterals 

 {2le,3le). 



B, C, mechanism of torsion, or partial rotary movements of segments, by the 

 transverse external dorsal muscles, best developed in posterior segments (C). 



D, mechanism of tergosternal and intersternal movements : dorsoventral dila- 

 tion produced by first external lateral muscle {ile) ; lengthwise sternal contrac- 

 tion by internal ventrals {vim, vil) ; protraction by external ventral {vel). 



E, the sternal apodemes, right side, anterior view. 



F, G, mechanism of respiration: expiratory movement (F) produced by in- 

 ternal lateral muscles (/;', see A), inspiratory movement (G) by first external 

 lateral {lie). 



paradorsal muscle {i6q), and cross each other obliquely, the first 

 going anteriorly, the second posteriorly, to their insertions on the op- 

 posite ends of the lateral margin of the sternum (fig. 11 A, 2le, sle). 

 The muscles of this pair evidently serve to give forward and back- 

 ward movements to the tergum and sternum on each other. 



