4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



ing to do then but to discard the container or to remodel it as best we 

 can to make it serve its intended purpose. The writer, therefore, finds 

 it necessary to enlarge some general conceptions previously expressed 

 concerning the nature of the thoracic mechanisms and their apparent 

 evolutions from simpler origins, in order to accommodate new obser- 

 vations that must be admitted. 



The primary intersegmental infoldings of the integument of arthro- 

 pods are the original lines of attachment of the longitudinal muscles, 

 and in most cases the principle longitudinal muscles are still attached 

 on them. When the cuticula becomes sclerotized, the intersegmental 

 inflections are usually converted into apodemal ridges, and a primary 

 segmental plate is laid down in the dorsum and generally in the venter 

 of each segment. In some cases the intersegmental sclerotizations take 

 the form of narrow intersegmental sclerites alternating with the seg- 

 mental plates. This condition is found more frequently in the ventral 

 than in the dorsal region, though it exists dorsally in some insect 

 larvae. 



The typical sclerotization of the dorsum of any segment in an adult 

 insect consists of a plate (fig. i, 7") which covers most of the dorsal 

 area of the segment, and which is continuous anteriorly with the inter- 

 segmental sclerotization bearing the intersegmental fold or ridge 

 (fig. 2 A, Ac). The definitive tergum, therefore, occupies a primary 

 segmental region and the preceding intersegmental region; it bears 

 anteriorly a submarginal, intersegmental ridge, or antecosta (fig. 2 A, 

 Ac), marked externally by the antccostal suture (figs, i, 2 A, acs), 

 and it terminates anterior to this ridge and its suture in a narrow 

 lip, or precosta (Pc). 



The ventral sclerotization of the segment may take the same form 

 as the dorsal sclerotization, as in the abdomen of most insects, where 

 the definitive terga and sterna duplicate each other in structure 

 (fig. 3). The functional intersegmental rings of the body in such 

 cases are the posterior, non-sclerotized areas of the primary segments, 

 and the definitive segmentation is clearly a secondary one. The sternal 

 sclerotization, however, may preserve a more primitive condition, as 

 in some of the chilopods (figs. 8, 15) and in the thorax of certain 

 insects, 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 most 

 arthropods the basis of the appendage (coxopodite) is preserved as 

 an integral limb segment. In the body segments of the chilopods, the 

 thoracic segments of insects, and the ambulatory segments of decapod 



