48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



velopment of ridges on its under surface. The most important of 

 these ridges are two known as the parapsidcs (fig. 21 B, Par), which 

 arise laterally from the prescuto-scutal suture and converge poster- 

 iorly a varying distance in the scutum. The lateral margins of the 

 scutum bear the anterior and the posterior wing processes (ANP, 

 PNP), and are notched just behind the former by the lateral emar- 

 ginations (Em). The wings (W) are extensions from the scutum 

 and scutellum, their posterior, thickened edges (the axillary cords) 

 being continued from the narrowed ends of the scutellum. 



THE POSTNOTAL PLATES 



Though the terga of the wing-bearing segments have acquired no 

 new elements to enable them to play their parts in the wing mechan- 

 ism, they have, however, in most cases lost the simplicity of segmental 

 plates through a redistribution of some of the primitive elements. 

 A primitive dorsal plate, bearing the wings and preserving the struc- 

 ture of a typical tergum, with an antecosta or phragma at the anterior 

 end, occurs in the mesothorax and metathorax of Isoptera, and in 

 the mesothorax of Orthoptera, Euplexoptera, and Coleoptera. In 

 nearly all other instances, the dorsum of a winged segment contains 

 two plates (fig. 22A), the first being the true tergum, or notum (T), 

 the second the postnotum (PN). The postnotum occurs in typical 

 form only in the adult insect ; it consists of the greatly enlarged pre- 

 costa, together with the antecosta and phragma, of the following 

 tergum usually more or less separated from the latter and closely asso- 

 ciated with the preceding tergum. The altered relation in the tergal 

 parts of the wing segments is proliably the result of a secondary 

 modification, for, as we shall see, it constitutes a structure correlated 

 with efficiency of wing motion. Presumably all the thoracic terga of 

 the wingless ancestors of the Pterygota had the usual form of seg- 

 mental plates (fig. 5), still retained in those of the Apterygota. 



The nature of the redistribution of the tergal parts in segments 

 having a postnotum is explained diagrammatically in figure 23. At 

 A is shown the original primary segmentation, with the dorsal mus- 

 cles (LMcl) attached to the intersegmental folds. In B the folds are 

 chitinized to form antecost?e (Ac), each with a small precosta (Pc) 

 l)efore it. The posterior membranous parts of the segments (Mb) 

 now become the flexible " intersegmental " regions, and the segmen- 

 tation is typically secondary. Let us suppose that the dorsal plates 

 (C, Ti-IT) are the terga of segments from the prothoracic to the 

 first abdominal, inclusive, and that each antecosta has a phragma 



