. Smithsonian miscellaneous collections. eserve a more primitive condition, asin some of the chilopods (figs. 8, 15) and in the thorax of certaininsects, where the primary sternal and intersternal plates remain inde-pendent (figs. I, 2 A, Stn, 1st). In the membranous areas of the lateral, or pleural, walls of the seg-ment are implanted the bases of the segmental appendages. In mostarthropods the basis of the appendage (coxopodite) is preserved asan integral limb segment. In the body segments of the chilopods, thethoracic segments of insects, and the ambulatory segments of decapod NO. 2 THORACI
. Smithsonian miscellaneous collections. eserve a more primitive condition, asin some of the chilopods (figs. 8, 15) and in the thorax of certaininsects, where the primary sternal and intersternal plates remain inde-pendent (figs. I, 2 A, Stn, 1st). In the membranous areas of the lateral, or pleural, walls of the seg-ment are implanted the bases of the segmental appendages. In mostarthropods the basis of the appendage (coxopodite) is preserved asan integral limb segment. In the body segments of the chilopods, thethoracic segments of insects, and the ambulatory segments of decapod NO. 2 THORACIC MECHANISM OF A GRASSHOPPER—SNODGRASS 5 crustaceans, however, it appears that the limb 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 segment (fig. i. Sex), where it eitherforms a pleuron supporting the free part of the limb, or also thebase of the wing, or it becomes degenerate and reduced to smallsclerites having little significance or function. DMcl VMcl. iPh DMcl Fig. 2.—Diagram showing the relation of the longitudinal muscles to the tergaland 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 whichthe primary sternites (Stn) and intersternites (1st) are distinct. B, the tergalregion of the thorax in an insect in which the precostae (A, Pc) are enlarged toform 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, sPh, 3PI1, the three thoracicphragmata; 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 inthe Pterygota
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