O SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



RELATION OF THE ABDOMEN TO THE THORAX 



In both the Acrididae and the Tetrigidae the tergum of the first 

 abdominal segment is firmly attached to the tergal and pleural sclero- 

 tization of the metathorax, and in Acrididae the first abdominal ster- 

 num is solidly joined to the metasternum. The movements of the ab- 

 domen as a whole take place between the first and second segments 

 of the latter, and are produced by the longitudinal muscles of the first 

 abdominal segment attached posteriorly on the second. In the female 

 of Dissosteira there is one pair of very small oblique lateral muscles 

 between the metathorax and the first abdominal segment (fig. 9. 140). 



The union of the first abdominal tergum with the metathorax in 

 Acrididae and Tetrigidae is formed by the greatly expanded acro- 

 tergite of the first abdominal tergum, which becomes a large post- 

 notum in the dorsum of the metathorax (fig. 2 A, C, PNs). The 

 postnotum is separated from the main part of the first abdominal 

 tergum (IT) by a prominent transverse antecostal suture (acs) , which 

 extends across the back and downward on the sides. From this suture 

 there depend internally the two lobes of the third phragma (fig. 3 A, 

 B, sPh). In Dissosteira the inner margin of each phragmatal lobe is 

 braced posteriorly on a secondary ridge (B, v), which is marked 

 externally by a short tergal suture on each side (fig. i, v) behind 

 the antecostal suture. The lobes of the third phragma give attachment 

 to the posterior ends of the dorsal muscles of the metathorax (fig. 

 3 A), and thus attest that the antecostal suture {acs) through their 

 bases is the true (primary) intersegmental line of the dorsum be- 

 tween the metathorax and the first abdominal segment. 



Anteriorly the postnotum is continuous (fig. 3 A, PN^) with the 

 inflected scutellar margin of the alinotum of the metathorax (AN^) ; 

 its lateral extensions are united with the posterior (or dorsal) margins 

 of the metathoracic epimera (figs, i, 2 A, C, Eprih). By these con- 

 nections of the postnotum with the dorsal and pleural sclerotic parts 

 of the metathorax, the lobes of the third phragma are securely braced 

 against the pull of the dorsal muscles attached on them (fig. 3 A) . The 

 force of the muscles, therefore, is expended on the alinotum of the 

 metathorax (AN3), which responds by an upward curvature that de- 

 presses the wings on the pleural fulcra. In the usual intersegmental 

 mechanism of secondary segmentation, in which the acrotergite is a 

 mere flange on the anterior margin of the tergum following, and is 

 separated by a conjunctival membrane from the preceding tergum, the 

 contraction of the longitudinal muscles produces an approximation 

 or overlapping of the consecutive segmental plates. The enlargement 



