16 PROCEEDINGS OF THE ACADEMY OF [Jan., 



the body cavity, or, as is the case in the Diptera, it may be largely 

 external (see figs. 7 and 8, N t ). 



The postscutellar phragma is usually much larger than the prse- 

 scutal phragma, and, while the latter is always closely connected with 

 the scutum, the postscutellum may become almost completely sepa- 

 rated from the remainder of the tergum. 



As has been stated, the prsescutum and postscutellum usually occur 

 as phragmas, and between them extend the dorsal longitudinal muscles. 

 It would appear that the arching of the mesothoracic region in such 

 swift-flying insects as the Hymenoptera, Diptera, etc., is caused by the 

 tension of these muscles. In the Diptera, the mesothoracic postscutel- 

 lum is greatly developed to furnish an attachment for these powerful 

 muscles, and the whole mesothorax appears to have grown at the 

 expense of the metathorax, which shrinks away, as it were, thus expos- 

 ing the huge mesothoracic postscutellum. Muscular tension is doubt- 

 less another factor causing the mesothoracic postscutellum to become 

 external, since it would give rise to an arching upward of the tergum 

 and the shifting forward of certain of the sclerites, as will be later dis- 

 cussed. This external character and unusual development of the 

 mesothoracic postscutellum in the Diptera caused Latreille, '20, to 

 mistake it for the notum of the metathorax. He consequently homo- 

 logized the metathorax of the Diptera with the first abdominal seg- 

 ment (the "segment mediaire") of the Hymenoptera. MacLeay, '30, 

 committed a somewhat similar error in considering the first abdominal 

 segment (which is closely connected with the thorax in pedunculate 

 Hymenoptera) as part of the metanotum. Consequently, that portion 

 which he terms the postscutellum in Polistes belongs to the abdominal 

 region. 



The postscutellum 2 (fig. 7, N t ) of the TipulidsG is greatly developed 

 and is distinctly divided into three regions — a median region which 

 may be termed the mediophragmite (iV 4a ), and two lateral regions 

 which will be spoken of as the pleurophragmites (N&). Each of the 

 pleurophragmites may be subdivided into a superior (iV 4 b s ) and 

 inferior (A 7 4 bi) region, and the mediophragmite likewise may be 

 divided into symmetrical halves by a continuation of the mid-ventral 

 suture. 



In certain insects in which the pleurophragmite is not connected with 

 the pleura, it would appear that a portion of the pleurophragmite 



2 Snodgrass, '08, frequently states that the Orthoptera have no postscutellum. 

 This is not the case in the Gryllidae, for example, as the postscutellum of 

 Gryllus domesticus is quite well developed. 



