lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



the external " divisions " of the wing-bearing terga have no signifi- 

 cance in themselves ; they are merely incidental to the formation of 

 the internal ridges by cuticular inflections, the ridges being the true 

 functional structures adapting the tergum to its part in the flight 

 mechanism. 



THE THORACIC PLEURA 



The lateral walls of arthropod segments, or the areas along the sides 

 of the body between the terga and the sterna, when dorsal and ventral 

 plates are present, may properly be designated the pleural regions. 

 The pleural areas of the segments are primarily membranous, and 



S Kb 



Fig. 6. — Diagram of the theoretical elementary musculature of the segmental 



appendages. 



a-b, primitive dorsoventral axis of the appendage ; /, tergal promotor muscle ; 

 /, tergal remotor ; K, sternal promotor ; L, sternal remoter ; T, tergum ; 

 S, sternum. 



within them are implanted the bases of the limbs' (fig. 6). In some 

 arthropods, as in many of the Arachnida, each limb basis occupies 

 almost the entire space between the tergum and the sternum, and may 

 be articulated to one or the other of these plates, or to both of them. 

 In most cases, however, a membranous area partially or entirely sur- 

 rounds the limb base. In this area there are sometimes developed true 

 pleural sclerites, as in the chilopod family Geophilidae, where there 

 is a series of lateral plates of the body wall lying between the terga 

 and the leg bases (fig. 8 A, pi), or in the larvae of some insects where 

 similar plates occur on the sides of the abdomen. In many arthropods, 

 however, there are plates in the definitive lateral walls of certain seg- 

 ments that appear to have been derived from the bases of the appen- 

 dages. While such sclerotizations are, therefore, not true pleural prod- 

 ucts, they are generally termed pleurites, and those of each side of each 

 segment constitute collectively the so-called pleuron of the segment. 



