526 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxvi. 



first of one plate — the notum, from the entire hiteral margins of 

 which the wings develop. Behind the notum is added, in the adult 

 stage, the pseudonotum (postscutellum) as a distinct plate, while the 

 so-called prescutiim, scutum, and scutellum are formed as secondary 

 divisions of the notum. These notal regions, moreover, are not 

 homologous in all the orders. This can be proved by a study of the 

 ventral surface of the notum, which presents certain fundamental 

 characters common to nearly all insects. Two of these are the ante- 

 rior and posterior notal ridges (fig. 2, ^LYE, PMB) already de- 

 scribed; a third, and the most important one, is the V-shaped ridge 

 (F), the entodorsum of Amans (1885), located on the posterior half 

 of the notum, having its apex forward and the bases of its arms 

 fused with the posterior ridge. A comparison of text figs. 1 and 2 

 will show that the three ventral ridges {AXR^ T', PNR) form three 

 transverse lines on the surface of the notum {ani\ r, par). 



These ridges and their surface lines are undoubtedly homologous 

 structures in all iuvsects. They mark off the area of the notum into 

 four regions, as foHows: (1) A narrow anterior nuirginal band in 

 front of the line of the anterior ridge: (2) a large bilobed region 

 situated between the line of the anterior ridge (onr) and that of ^^he 

 entodorsum (v) and carrying the notal wing processes (.lA/', PNT) ; 

 (3) a triangular space betAveen the line of the entodorsum and that 

 of the posterior notal ridge; and (4) a narrow posterior marginal 

 band terminating laterally in the axillary cords (A.vC) and form- 

 ing the posterior free edge or redu})lication of the notum. 



This ty]:»ical simplicity of structure is illustrated in the Orthoptera 

 by BJateUa (38, 40) and (h-nUuH (49, 50), and in the yeuropteva by 

 Corydalh (142, 143). It will l)e observed that the pseudonotum 

 {PN) is absent in the Orthoptera, but well developed in Coi'i/dalls. 



These four regions of the Orthopteran notum are very suggestive 

 of the four divisions of the terguni as ordinarily recognized, but 

 a comi)arison with Corydalis (142) at once shows that the term 

 "postscutellum " can not be applied to any part of the Orthopteran 

 tergum, for the name belongs to the pseudonotum, which is absent 

 in Orthoptera. Hence, all identifications of a " postscutellum *' in 

 Orthoptera, supposed to be homologous with that of the higher 

 orders, are erroneous. A comi)arative study of the orders shows 

 that the posterior line {pnr) is generally absent, that the notum is 

 very commonly divided by lines or actual sutures into tJwee subdivi- 

 sions, and that these lines or sutures are not determined by the ventral 

 ridges and do not bear the same relation to them in the different 

 orders. 



Thus we have the following premises: (1) The ventral ridges of 

 the notum are constant in all the orders and are, hence, fundamental 

 homologous structures; (2) three notal divisions are of general 



