so. 1687. THE THORAX Ui' I XHECTti—tiyoDURAiiii. 525 



1G3, 170, 179, Pph or Pph.,) that projects posteriorly throuo:h the 

 metathorax and almost shuts off the cavity of the thorax from that of 

 the abdomen. In a Tipiilid puj^a (173) the mesopseiulonotum {PN) 

 is ]irosent as a lar<j!;e plate intervening between the two wing-bearing 

 plates (Xo and ^A^,). It is interesting to note here that the halter is a 

 Aving-like structure (ir.). 



The pseudonotnm has been discussed at considerable length because 

 the fact has apparently not been recognized by other authors that 

 the postscutellum or pseudonotnm is an independent plate in its 

 origin and is, hence, not one of the divisions of the notum, as is the 

 prescutum, scutum, or scutellum. 



The notum {N) or wing-bearing plate of a meso- or metathoracic 

 tergum is diagrammatically illustrated in figs. 1, 2, and ;>. In its sim- 

 plest form it is an undivided plate, convex dorsally. On its ventral 

 surface (fig. 2) the anterior and posterior margins are thickened, 

 forming the anterior notal ridge (.LV^) and the posterior notal ridge 

 {PXR). The latter is generall}" folded forward a short distance on 

 the ventral surface, forming a free posterior reduplication {Rd) 

 which often overlaps the sclerite following. The lateral margins of 

 the notum are produced into two processes which carry two of the 

 articular sclerites of the wing base. These lobes are the anterior 

 noted wing process {ANP) and the posterior notal wing process 

 {PNP). The posterior edge of the notum, formed by the posterior 

 reduplication, usually appears as a marginal thickening which is con- 

 tinued outward on each side as a corrugated, cord-like thickening of 

 the anal edge of the basal membrane of the wing. These thickenings 

 may appropriately be called the axillary cords {AxC). They are 

 important characters in determining the posterior limit of the notum. 

 The anterior notal ridge bears the anterior j^hixigina or prcpliragma 

 {Apli). 



This outline might be taken to represent the structure of the meso- 

 or metatergum of primitive Avinged insects, for it is approximately 

 that of nymphal and some pupal forms though these lack the notal 

 v.ing jorocesses. A pseudonotnm is never present in nymphal stages 

 and the prephragma is usually but little developed. The tergum of 

 the nymph is illustrated in the Odonata (15), in the Mantida? (31), 

 in the Acridiidse (5G, 58) and in the Perlida? (76). The same sim- 

 plicity is exhibited by the pupal tergum of Coleoptera (122, 123, 

 12G). In a Tipulid pupa (173), however, the pseudonotnm is pres- 

 ent in the mesothorax as a distinct plate (/W.) between the two 

 wing-bearing plates (^V^ and N .^. As will be shown later, the notum 

 of the adult is commonly divided more or less distinctly into sev- 

 eral regions or even sclerites. But a study of nymphs and pupa3 

 shows conclusively that these notal divisions are secondary char- 

 acters in the growth of the individual. The tergum consists at 



