AMERICAN DIPTERA. 203 



cess extends forward buried in the muscle and connective tissue of 

 the floor of the genital chamber. 



Genus TIPUL.A Linn. 



According to the structure of the hypopygium, Westhoff, whose 

 paper on the hypopygium of Tipula was discussed in the introduc- 

 tion to this paper, divides this genus into seven groups. These 

 groups, however, are established on combinations of so many char- 

 acters, and on characters whose morphological value is small on 

 account of their variability, that they can scarcely be regarded as 

 natural divisions or as having any phylogenetic significance. 



The most fundamental modification that takes place in the hypo- 

 pygium of the entire family is the variation in the position of the 

 pleura. But Westhoff, in his study of Tipula alone, almost entirely 

 overlooked these plates. However, if we arrange the species of 

 Tipula according to the structure of the pleural plates, we shall 

 have three groups that very logically follow in succession after the 

 genus Pachyrrhina. These three groups will be characterized as 

 follows : — I, the pleural region, on the side of the hypopygium, is 

 separated from the sternum by a latero- ventral suture ending in a 

 short outward or upward curve near the middle of the segment ; II, 

 the pleurum is entirely separated from the lateral part of the ster- 

 num, and consists of a small generally triangular plate set into the 

 posterior lateral margin of the segment ; III, the pleurum is entirely 

 fused with the lateral part of the sternum. For convenience we will 

 call the suture below the pleural region in Group I the pleural suture. 

 In Group II the plates called pleural plates or pleura can evidently 

 not be equivalents of the entire pleurum in Pachyrrhina and in the 

 genera below it. 



Group I. 



Pleural sutures present. 

 The presence of pleural sutures, almost identical with those of 

 Pachyrrhina, very clearly places this group at the bottom of the 

 Tipula series. The transition from Pachyrrhina is perfect. The 

 following eight species examined belong here. 



Tipula fti in osa Doane (PI. XII, figs. 64, 65). 



The eighth segment is shorter than in the last species. Its tergum 

 is mostly concealed beneath the seventh (fig. 64), although the ster- 

 num is produced beneath the hypopygium. 



TBANS. AM. ENT. SOC. XXX. JULY. 1904. 



