NO. I • INSECT TilORAX— SNODGRASS 47 



nofal iving process (fig. 21, ANP), and a posterior notal zuing process 

 (PNP), though the latter is frequently lacking. Behind the first wing 

 process there is a deep lateral cmargination of the tergum (Em). 



The surface of the wing-bearing terguni is differentiated into 

 several areas, some being limited by the sutures of internal ridges, 

 others being merely topographical in nature. The internal ridges are, 

 therefore, the features of greatest importance in a study of the 

 tergum. The most constant apodemal ridge has the form of an in- 

 verted V (fig. 21 B, VR), the arms of which arise near the posterior 

 angles of the tergum and converge forward, the apex being usually 

 behind the center of the tergum. This apodcme, which may be desig- 

 nated the J '-ridge, forms a strong brace in the posterior part of the 

 tergal plate, and sets off a posterior area of the tergum called the 

 scKtellum (Scl). All the muscles of the tergum arise upon the pre- 

 scutellar area. The rear margin of the scutellum is deflected in the 

 posterior reduplication (Rd), and its ends are prolonged in the 

 axillary cords of the wings {AxC). 



On the anterior part of the tergum, behind the antecostal suture 

 (oc), there is often differentiated a narrow transverse strip, called 

 the presctituui (fig. 21, Psc). The area is sometimes limited by a dis- 

 tinct suture (A, ps), with a corresponding internal ridge (B, PR), 

 but the prescutum is seldom as definitely marked as the scutellum, 

 and its separation from the rest of the tergum may be faint or 

 obsolete. Anteriorly the prescutum is deflected into the antecostal 

 suture (A, ac) over the base of the phragma (Ph), beyond which is 

 the precosta (Pc), or anterior lip of the phragma base, always nar- 

 row, sometimes scarcely perceptible, except when enlarged to form 

 a postnotum of the preceding segment (figs. 22, 23). Laterally, the 

 prescutum ends in the prealar bridges (fig. 21 A, Aw), if such pro- 

 cesses are present. 



The application of the term " prescutum " as given here follows 

 the evident intent of Audouin (1824), who defines the " praescutum " 

 as " la piece la plus anteriure," and adds that " elle est quelquefois 

 tres grande et cachee ordinairement en tout ou en partie dans I'inter- 

 ieur du thorax." This is the part, however, called " acrotergite " by 

 Berlese, and ** pretergite " by Crampton (1919). As will be shown 

 later, the triangular area of the tergum in some insects, usually re- 

 garded as a part of the prescutum (" protergite " of Berlese), is ap- 

 parently a part of the scutum. 



The area of the tergum between the prescutum and the scutellum 

 is the scutum (Set). The topography of the scutum is variable; its 

 surface is often cut by sutures which arise from the secondary de- 

 4 



