PROC. ENT. SOC. WASH., VOL. 23, NO. 8, NOVEMBER, 1921 187 



to, though I can not agree in considering this region belonging more to the 

 segment in front than to the segment behind, particularly when the phragma is 

 considered as part of this interpolated sclerite. 



"The great dorsal muscle of flight for which the phragma was developed is 

 probably only a dorsal intersegmental muscle. These extend from the anterior 

 edge of one segment to the corresponding part of the next. The anterior 

 phragma is mesoprescutal, the posterior is a part of the first abdominal segment. 

 The hymenoptera appear to be an exception in regard to the position of the first 

 abdominal segment, only because of the great constriction between the first and 

 second segment. 



"It may be impossible on anatomical grounds to locate the division between 

 segments after the articular membrane has become wholly chitinized. The 

 phragma may be, as this author says, a 'chitinization of the infolded interseg- 

 mental membrane,' but if so, why is not the deepest point of the fold the point 

 of demarkation between the segments? 



"A more reasonable position would seem to be that the infolding for the 

 attachment of intersegmental muscles marks the posterior boundary of the 

 prescutum that the phragma belongs therefore entirely to the following segment 

 and that with the completion of the chitinization of the articular membrane, 

 the division is lost somewhere immediately anterior to the phragma. 



"The region designated by this author as pseudonotum developed as a 

 chitinization of the articular membrane is probably therefore made up of two 

 elements, one of which is continuous with the prescutum of the following seg- 

 ment." 



The present studies on the larva and adult support a concep- 

 tion of the insect segment as composed of four primary parts, 

 three plates, prescutum, scutum, scutellum, and an interseg- 

 mental membrane, and tend to lead to the conclusion that the 

 postscutellum, in being derived from the segment posterior, is 

 homologous with the prescutum. 



In view of the lack of anatomical and histological support for 

 the suggested homologies, the author has not adopted the adult 

 sclerite names for the larval areas from which they appear to 

 originate but expects to use the letters as assigned under 

 "Limitation and Composition of Larval Segments." 



LIST OF ABBREVIATIONS. 



A first division of segment. 



a anus. 



ab infold between divisions of segment A and B. 



ant antenna. 



B second division of segment. 



B 1 first subdivision of B. 



B 2 second subdivision of B. 



C third division of segment. 



