1921] Metcalf: Genitalia of Male Syrphide 193 
under this heading below). The right one is elongate, gradually 
attenuated and bent mesad in a long sharp claw. The left style termi- 
nates in a blunt rounded process, the base of which is encircled by an 
elevated keel, something like a single turn of the thread ridge on a 
screw. 
The styles in Tropidia (Figs. 91, 92, 93, Plate XVI) are not only 
bilobed as in many of the Eristaline and Milesine, but the caudal, erect 
lobe has further developed a third lobe between the other two. The 
first one next the cerci is erect, thick and more or less clavate: the 
median one or baso-cephalic portion of the first (Figs. 91, 92, a) is 
strongly compressed, concave mesad, on which face it bears dense, 
short, stiff pile. The third lobe (6) stands erect and compressed along- 
side the penis. It is very wide, but short baso-distad. 
In Spilomyia also the more caudal of the lobes of the style is likewise 
bilobed, but in this case it is brought about simply by an emargination 
near the apex of the caudal lobe. (See Figures 124 to 126, Plate XVII). 
Irregular styles of noteworthy character are the bicornuate con- 
dition of a species of Sphegina (Fig. 80, Plate XIV) and of Psilota 
buccata (Figs. 33 and 34, Plate XII); and those of Didea fasciata fusci pes 
(Fig. 51, Plate XIII) which are shaped like an amphitheatre, with 
nearly perpendicular sides; they are very short and present the appear- 
ance of having been mashed over and then twisted around in a semi- 
circle. 
In Xanthogramma the style differs from its nearest ally, Syrphus, in 
having the caudal margin somewhat produced mesad, as a thin plate, 
reaching well toward that of the opposite side. 
In describing the styles, besides base, apex, mesal and lateral, I 
have used especially caudal and cephalic margins or faces, Chis: 3@); 
Plate IX). These relations are considered the same as those of a leg, 
with which the style is probably homologous. 
Pents (phallus, Berlese; theca, Wesche; appendage II, III, Newell: 
copulatory or intromittent organ of authors). The penis is invariably 
the largest appendage of the postabdomen. It is unpaired and gen- 
erally considered to represent the fused appendages of segment nine. 
It arises from the caudo-ventral portion of the ninth segment and is 
articulated also to the base of segment ten; both to the inflexed and more 
or less produced, basal corners of the tergite by the sides of its basal 
rim (Figs. O, Plate [X; 1, a, Plate XV; and 34, Plate XII) and to the 
cephalic margin of the tenth sternite on the caudal part of its basal 
rim (Fig. 14, Plate XI). 
The penis is a complicated organ that may be understood best if we 
consider it as being composed of two systems: (I) The axial system, 
composed of the ejaculatory duct and its associated structures and 
(II) The peripheral system, composed of the penis sheath and its 
appendages. The latter may be thought of as a hollow cylinder or 
truncated cone, open at each end, within which the former system 
hes along the axis. But the two systems do not ordinarily articulate 
with each other at any point. 
