AMERICAN DTPTERA. 209 



almost immediately makes a sharp bend dorsally and anteriorly (tig. 

 125). 



The guard of the penis (fig. 126, />. g.) arises from the Hoor of the 

 genital chamber over the ventral membranous area. It is a long, 

 slender, tapering, stylet like structure, grooved lengthwise above, 

 swollen toward the base, arising by a contracted neck from a chitin- 

 ous support on the tloor of the genital chamber. From this suppo)t 

 there projects posteriorly beneath the guard two weakly chitinous 

 clavate appendages. 



The anal tube has two delicate, band-like arms of chitin extend- 

 ing down upon its upper surface from the free edges of the terguin. 



Group II. 



Pleural plates present. 

 This group includes the majority of the species of Tipnla. It is 

 conceivable that the pleural plates have been formed by the second- 

 ary growth of the upturned anterior ends of the pleural sutures of 

 Group I, in a dorsal direction, till they cut off back of them on each 

 side the posterior end of the original pleurum. These plates always 

 carry the apical appendages. If they have been formed in the 

 manner just suggested, then Group II logically follows Group I. 



Tipnla aiisiistipeiinis Loew (PI. XVII, figs. 139 to 149). 



The posterior part of the abdomen forms a club-shaped enlarge- 

 ment (fig. 139), and the hypopygium is directed upward and poste- 

 riorly. 



The tergum of the hypopygium (figs. 141 and 142, IX, t.) is a 

 wide plate having the anterior margin straight and the posterior 

 deeply notched. The margins of the notch are formed by two par- 

 tially detached, elongate lobes. The sternum consists of two large 

 plates (IX, s.) covering most of the sides of the segment, but sepa- 

 rated below by a rathei' wide membranous space (figs. 142, 147). 

 Anteriorly the two are united in front of the membrane bv a very 

 narrow bar of chitin (fig. 147). The posterior margin of the mem- 

 brane is deeply notched. From the bottom of the notch there pro- 

 jects posteriorly a slender, weak, tapering appendage (fig. 147 a). 

 To each corner of it there is attached a small chitinous lobe (fig. 

 147, b). The pleura are well developed and are entirely separated 

 from the lateral parts of the sternum (figs. 141, 142, 147, pi.). 



TRANS. AM. ENT. SOC. XXX. (27) JULY. 1904. 



