1909.] NATURAL SCIENCES OF PHILADELPHIA. 15 



Voss, '04, appears to consider the scutellum as part of the scutal 

 region, and terms it the "unpaares mittelfeld." This terminology, 

 however, would be incorrect for three reasons: in the first place, 

 because the sclerite in question does not belong to the region which 

 Audouin, '24, calls the scutum, but is what he terms the scutellum; 

 furthermore, the unpaired median region of the scutum, if such existed, 

 would be the triangular mediscutum; and, lastly, there is no "un- 

 paired" region, strictly speaking, in either scutum or scutellum, as the 

 whole tergum was originally divided into two symmetrical halves by 

 a median longitudinal suture. 



This suture, according to Comstock, '02, represents the line of closure 

 of the embryo, and it is along this line that the cuticle is ruptured at the 

 time of moulting. The median dorsal suture may be spoken of as the 

 mid-dorsal suture, while the corresponding median ventral suture 

 (which may represent traces of the neural groove) will be referred to 

 as the mid-ventral suture. 



The mid-dorsal suture is easily seen in such insects as the Sialidse, 

 Perlidse, Tenthredinidse, Psocidae, Tipulidse, Trichoptera, etc. In 

 certain other insects, and some of these are very primitive, as, for 

 example, the roaches and earwigs, one can find scarcely any traces of 

 this suture in the mesothorax, since the tergal subregions have united 

 to form a simple undivided notum. However in the metathorax of 

 some Blattidse, etc., one can distinguish faint traces of these parts. 

 This leads to the conclusion that the simple notum is the result of non- 

 usage of the wings, or the peculiar mode of life of these insects, and 

 would hence be a tertiary modification rather than a retention of the 

 primitive condition. 1 



An examination of the inner ridges, which serve as points of insertion 

 for certain muscles, suggests that the tergum at one time may have been 

 a single piece, but, though muscular tension, ridges were drawn inward, 

 thus creating corresponding furrows or sutures on the exterior surface. 

 It is possible that the so-called parapsidal furrows, or sutures separating 

 the medi- from the parapsido-scutum, were formed in this way, as is 

 likewise the case with the furrow which separates the parapsidoscutum 

 from the scutellum. The latter furrow may be spoken of as the scutel- 

 lar suture. 



The Postscutellum. — Behind the scutellum lies the postscutellum (fig. 

 1, N 4 ), which usually occurs as a phragma projecting more or less into 



1 In the Apterygota, however, the simple, undivided notum doubtless repre- 

 sents the primitive condition. 



