NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS 5 



crustaceans, however, it appears that the Hmb basis has become sub- 

 divided into a coxa and a subcoxa, and that the latter has been incor- 

 porated into the wall of the body segTnent (fig. i, Sex), where it either 

 forms a " pleuron " supporting the free part of the limb, or also the 

 base of the wing, or it becomes degenerate and reduced to small 

 sclerites having little significance or function. 



,DMcl 



VMcl 



iPK DMcl 



Fig. 2. — Diagram showing the relation of the longitudinal muscles to the tergal 

 and sternal sclerites of the body. 



A, three successive segments in which the terga include the primary inter- 

 segmental regions bearing the intersegmental ridges (Ac, Ac), but in which 

 the primary sternites (Stn) and intersternites (1st) are distinct. B, the tergal 

 region of the thorax in an insect in which the precostae (A, Pc) are enlarged to 

 form postnotal plates (PN). 



Ac, antecosta; ac, antecostal suture; DMcl, dorsal longitudinal muscles; 1st, 

 intersternites; IT, first abdominal tergum ; L, positions of leg bases; Mb, secon- 

 dary intersegmental membrane ; Pc, precosta ; iPh, 2Ph, sPh, the three thoracic 

 phragmata ; PN , postnotal plates ; Stn, primary sternite ; VMcl, ventral longi- 

 tudinal muscles. 



THE THORACIC TERGA 



The dorsal plates of the insect thorax never retain in all three seg- 

 ments the simple structure of the definitive abdominal terga, and in 

 the Pterygota the mesothoracic and metathoracic terga are modified 

 in various ways correlated with the development of the wings. 



The prothoracic tergum (fig. 4, Tx) always lacks an antecosta, and 

 the principal longitudinal muscles {DMcl) that extend forward from 



