AMERICAN DIPTERA. 229 



species of Tipula this short upward-bent arm of the suture is ex- 

 tended dorsally so as to cut off a small posterior plate behind it and 

 above the horizontal arm. This plate always carries the apical ap- 

 pendages, and it may have its posterior angle produced into a long 

 horn -like or spoon shaped process. In the most modified members 

 of the genus the suture between the pleurum and the sternum disap- 

 pears entirely. Furthermore, in many of these the sutures separa- 

 ting the tergum from the lateral parts disappear so that the body of 

 the hypopygium is entirely undivided into plates of any sort. 



Ctenophora resembles these highest forms of Tipula, for the hypo- 

 pygium has continuous walls showing neither tergal, sternal nor 

 pleural sclerites. 



In the introduction to this paper the author has disclaimed the 

 notion that the plates called "pleura" are homologues of the lateral 

 plates of the thorax. All that is here attempted is to show that an 

 apparent homology can be traced between the lateral plates of the 

 hypopygium in such genera as Antocha, Dicranomyia, Ptychoptera, 

 etc., the large appendicular and appendage-bearing lobes of the 

 hypopygium of Dicranoptycha, Erioptera, Limnophila, Amalopis, 

 etc., and the small posterior lateral plates of the hypopygium of 

 Tipula. Since these plates or lobes primitively have a " pleural " 

 position they have been called for convenience the pleura. 



It may be imagined that the evolution has been in an opposite 

 direction from that suggested. Starting with a small secondary 

 plate cut off from each posterior upper angle of the sternum, we 

 could imagine that the suture in front of it disappeared, and that 

 the suture below it then extended forward until it separated off from 

 the sternum a longitudinal plate between the ventral part of the 

 sternum and the tergum, producing the forms thus characterized. 

 By enlarging in the opposite direction this same small posterior plate 

 could be easily transformed into an appendicular lobe, thus giving 

 rise to the forms having this character. However, the fact, that 

 following the line of the first assumption produces an arrangement 

 of the genera almost identical with that followed by systematists 

 using other characters for a guide, would seem to confirm the view 

 adopted in this paper. The classification of the Tipulidse by Osten 

 Sacken in his Catalogue of North American Diptera is almost the 

 same as would be a classification based on the hypopygium alone, if 



TRANS. AM. ENT. SOC. XXX. JULY. 1904. 



