No. 1774. 



THORAX OF HYMEN0PTERA—8N0DGRA88. 



43 



(1906) distinguishes the four parts of the tergum by the names 

 "acrotergite," "protergite," "mesotergite," and "metatergite," 

 which offend in both respects at once. The writer beUeves that it 

 is better to use a mixed term Hke ''postnotum" to designate the 

 plate lying behind the notum in any segment than to create con- 

 fusion by calling it the "metanotum," by which term most anyone 

 would understand the notum of the metathorax. 



The following are definitions of the terms given in the above tables, 

 together with descriptions of the secondary parts pertaining to each. 

 A special discussion of the morphology of the postnotum and the 

 phragmas is given on pages 53 to 64. 



Dorsum. — The back or dorsal surface of any segment, of any part, 

 or of the entire body. 



Tergum. — The chitinous plate or plates of the dorsum of any seg- 



lAx zAx .R 



ANP- 



PNR 



AxC 



Fig. 1. — Mesotergum and base of right wing of t^eniopteryx frigida (stonefly), showing the 

 wing-bearing notum (iv) and the postnotum {p n): a, anal vein; anp, anterior notal wing 

 process; lAx, first axillary; SAx, second axillary; 3^ i, third axillary; AxC, axillary cord; 

 C, costa; Cu, cubitus; M, media; m, median plate op wing base; N, notum; PiV, postnotum; 



PNT, POSTERIOR NOTAL WTNG PROCESS; Pph, POSTPHRAGMA; Psc, PRECUTAL DIFFERENTIATION OF NOTUM; 

 R, RADIUS; Ed, POSTERIOR MARGINAL REDUPLICATION OF NOTUM; Sc, SUBCOSTA; Tg, TEGULA. 



ment, typically confined to the back, but often extending downward 

 on the sides or even upon the ventral surface. In adult winged 

 insects the tergum of the mesothorax and of the metathorax very 

 commonly consists of an anterior wing-bearing plate, and of a pos- 

 terior plate having no coim.ection with the wings. These are dis- 

 tinguished as the notum and the postnotum (fig. 1, N and PN). 



Notum. — ^The primitive tergal plate, being the entire tergum of 

 any segment in nymphal forms, as well as of the prothorax and of the 

 abdominal segments in all adults. In the mesothorax and metatho- 

 rax of adults, when there are two tergal plates present in each seg- 

 ment, the notum is the anterior or wing-bearing one (figs. 1 and 8, N). 

 The words "tergum" and "notum," as used by the writer, are, there- 

 fore, synonymous except where there is present in the wing-bearing 

 segments a secondary postalar tergal plate. Since "tergum" has 



