46 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



Thus, in adults, any pliragma may be spoken of either as a pre- 

 'phragmna {Apli) or as a postpJiragma {Pph) of the segment to which 

 it is attached. In those orders having a postnotum in the meso- 

 thorax the tergum of this segment carries both a prephragma and a 

 postphragma, while the metatergum has only a postphragma. In 

 the other orders the metatergum bears two phragmas while the 

 mesotergum has only a phrephagma. 



Latus. — The side of any segment, of any part, or of the entire 

 body — the lateral area between the dorsum and the venter. The 

 writer introduced this term, in the sense here defined, in a former 

 paper (1910), because, if the term "pleurum" is used to designate 

 the chitinous parts of the side of any segment, it is evident that 



another is needed to refer to the side 

 of the segment itself, which should 

 include both the membranous and 



WP 



WP, 



FiQ. 6.— Metathorax of T^nioptertx frigida 

 (stonefly), left side, wings removed: Cx, 

 coxa; CxP, plexjral coxal process; Epm, epi- 

 merum; Eps, episternum; F, base of femxjr; 

 N, notum; P, episternal parapterum; PN, 

 postnotum; PS, pleural suture; q, sterno- 

 pleural suture; S, sternum; Tn, trochan- 

 Tm; Tt, trochanter; WP, pleural winq 

 process. 



Fig. 6.— Left metapleurum of T^eniopteryx 

 frigida (stonefly), internal: c, sclerite con- 

 necting PARAPTERUM (P) WITH HEAD OF COSTAL 



VEIN OF ■wing; Epm, epimerum; Eps, epister- 

 num; P, episternal parapterum; PA, pleural 

 arm; PiV, lateral part of postnotum, contin- 

 uous ■WITH epimerum; PR, PLEURAL RIDGE; S, 



sternum; Tw, trochantin; WP, pleural ■winq 

 process 



the chitinous parts. The adjective "lateral" follows from "latus," 

 just as does "dorsal" from "dorsum" and "ventral" from "venter." 

 Pleurum (PI). — The chitinous plate or plates of the latus of any 

 segment, often partially crowded out by lateral encroachments of the 

 tergum or sternum, especially in the prothorax. A typical adult 

 pleurum of the mesothorax or metathorax covers most of the latus 

 and presents externally a vertical or oblique pleural suture (fig. 5, PS) 

 extending from the base of the wing process ( WP) above to the 

 coxal process {CxP) below. This divides the pleurum into an 

 anterior or ventral episternum {Eps) and a posterior or dorsal 

 epimerum {Epm). Internally there is a heavy pleural ridge (fig. 6, 

 PR) along the line of the pleural suture, which gives off a pleural 

 arm {PA) at or near its lower end. At the upper end of the epi- 



