No. 1774. THORAX OF IIYMEN0PTERA—SN0DGRA8S. 59 



brane {Mh^. Hence, while the latter would ordinarily be called the 

 "intersegmental membrane," it is clear that it lies before the original 

 intersegmental line and really belongs to the posterior part of the 

 mesodorsum. The same must be true of the membrane between the 

 prothorax and the mesothorax, since the anterior phragma {IPli) is 

 solidly attached to the front of the mesotergum. In other insects, 

 where the middle phragma is attached to the posterior edge of the 

 mesotergum, the "intersegmental membrane" behind it must really 

 belong to the front of the metadorsum. Therefore, in general, if the 

 phragmas are truly intersegmental structures, the real intersegmental 

 lines pass though them, where phragmas are present, and the deepest 

 part between the two laminae of any one is the true demarkation 

 between the segments adjoining it. Woodworth (1909) is inclined 

 to doubt this view, holding that "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 entirely to the following segment, and that with the 

 completion of the chitinization of the articular membrane, the divi- 

 sion is lost somewhere immediately anterior to the phragma." Thus 

 he claims that ' ' the anterior phragma is mesoprescutal ; the posterior 

 is a part of the first abdominal segment." The present writer objects 

 to this theory on the ground that, as he thinks, the facts do not sub- 

 stantiate it, but demonstrate the opposite view stated above. 



The function of the phragmas is to give an increased surface of 

 attachment for the longitudinal muscles of the back. These muscles 

 are greatly developed in the wing-bearing segments of nearly all 

 strong-flying insects (the dragon flies excepted) because they are the 

 ones that produce the downward stroke of the wings during flight, 

 the upward stroke being produced by the vertical muscles of the 

 thorax. When the latter muscles contract they depress the back 

 plates, which in turn pull down the bases of the wings, thereby throw- 

 ing up the distal parts of these organs, the fulcra being the wing 

 processes of the pleura. The succeeding contraction, then, of the lon- 

 gitudinal muscles restores the shape of the thorax and conse- 

 quently elevates the back plates, which, by the same mechanism as 

 before, force the wings downward. It is thus seen that the phragmas 

 have an important association with the function of the wings. The 

 other elements in the wing motion are produced by smaller muscles 

 inserted directly upon the wing bases, but these are not material to 

 the present discussion. 



Furthermore, there is a relationship between the phragmas and 

 the postnotal plates which, in general, may be stated as follows: 

 When a phragma is associated with the posterior part of a segment 

 it is attached to a postnotal plate of the tergum, which plate is 

 usually otherwise lacking. Or, conversely, when the tergum of either 

 segment possesses a postnotal plate it usually possesses also a post- 



