AMERICAN DIPTERA. 195 



The sternum has the form of an equilateral triangle with the ante- 

 rior side convex and the lateral sides concave. It lies mostly upon 

 the ventral side of the hypopygium. The posterior median angle is 

 truncate and emarginate. From the two points thus produced there 

 arises, from two corresponding roots, a large, darkly chitinized, ham- 

 mer-shaped appendage (fig. 38, b). 



The tergum and the sternum are separated entirely, on the sides 

 of the segment, by the pleura (fig. 35). Each pleurum is an elon- 

 gate triangular plate, having anteriorly an uncinate prolongation 

 extending into the intersegmental membrane (wi.) back of the eighth 

 segment far in front of the tergum and sternum. A chitinous bar 

 arises from the pleurum near its base (fig. 39, e), and extends in- 

 ward, lying in the floor of the genital chamber. Its edges are pro- 

 vided with a number of small teeth. There is only one apical ap- 

 pendage on each side. This is an elongate arm widest at its base, 

 tapering distally and curved inwards. It is born at the distal end 

 of the pleurum. 



The guard of the penis is a small elongate triangular structure, 

 apparently enclosing the penis on all sides (fig. 36, p. g.). The penis 

 projects from the tip as a small chitinous rod (p.). In front of the 

 base of the guard are two pairs of small chitinous points arising 

 from the floor of the genital chamber, which may represent the bifid 

 second gon apophyses. 



Ptychoptera lenis O. S. (PI. X, figs. 40, 41, 42, 43, 45). 



The hypopygium forms a small globular swelling at the end of 

 the abdomen. The eighth segment is normal, the sternum is longer 

 than the tergum (fig. 40). 



The body of the hypopygium is formed of a large dorsal and dor- 

 so-lateral tergum, a ventral and ventro-lateral sternum, and two 

 small pleural plates, one on each side between the tergum and the 

 sternum. The tergum is deeply notched medially (fig. 41), so that 

 the median part forms simply an anterior bridge connecting the two 

 large lateral lobes. Each of the latter is strongly convex laterally, 

 concave on the inner edge and produced here into a larger posterior 

 and a smaller anterior lobe. The sternum is a simple plate, widest 

 on each side where it forms the lower lateral surface of the segment 

 (fig. 40). Each of these lateral parts carries on its posterior edge a 

 large articulated lobe (fig. 40, L, and fig. 44) extending upward and 



TEANS. AM. ENT. SOC. XXX. JULY. 1904. 



