1921] Metcalf: Genitalia of Male Syrphide 189 
may be the remnants of the tergite (Figs. 20, Plate XI: 35, Plate 
XII). In Mesogramma a transverse, triangular sclerite closing the cercal 
emargination (Fig. 43, Plate XIII) and in Mixogaster (Fig. 133, Plate 
XIX) a similar transverse piece I have interpreted as the eleventh 
sternite. In a good many other species there is a small hairy region, 
’ ventrad of the cerci, which I interpret as the remnant of the eleventh 
sternite (Figs. 122 and 127, Plate XVII; 31, Plate XII). And then 
there are the remarkable cases of Cnemodon and Spherophoria. In these 
genera there are rather prominent processes (the sternal cornua and the 
post-anal hood) caudad of the cerci, which are continuous with the distal 
margin of sternite ten and which are possibly the much modified sternite 
of the eleventh urite. 
The most reduced condition of the eleventh sternite (if indeed these 
structures be the eleventh sternite) that I have found is in the species 
Spherophoria noveangle. In this species the tenth sternite is emarginate 
distad and in this emargination stands a small, quadrangular, almost 
hour-glass shaped plate, placed transversely and with a basal arm to 
each side of the tenth sternite. It is not produced beyond the base of the 
styles although very slightly beyond the tenth tergite and tends to 
close in the floor of the cercal emargination beyond the cerci. It is thus 
only a little displaced from relation to its tergite which, when present, 
subtends the cerci. 
In S. cylindrica, scripta, menthastri, etc., with the fusion of the arms 
of the tenth tergite along the middle line closing the cereal emargination 
(vide infra) we believe that this eleventh sternite has become pushed 
distad and produced into two separate processes standing mesad of the 
styles. These are the processes referred to as sternal cornua (Figs. 1, 
65, 66, Plate XV). They are outstanding, variously-pointed, com- 
pressed plates, the mesal walls of which are continuous at their bases 
with the tenth sternite while their lateral walls continue into the mesal 
faces of the styles. 
In the genus Pipiza we find a structure that is unique among the 
sixty genera that I have studied and that may represent a greatly 
elaborated eleventh sternite. I have called it the post-anal hood. It 
consists of a prominent, thin, membranous expansion between the 
styles, with which it articulates at their bases, and it is also continuous 
basad with the tenth sternite. (See figures 12 to 26, Plate XI). 
It is my belief that this simple plate of S. noveangle, the sternal 
cornua of Spherophoria spp., and the post-anal hood of Pipiza spp. 
are homologous structures and they may represent the eleventh sternite. 
THE CERcAL Emarcination. In the process of compacting the seg- 
ments of the postabdomen into a more or less globose mass, the eleventh 
segment and its appendages (the cerci) have been received to a varying 
degree into the apex of the tenth tergite. This is made possible by the 
emargination of the caudal margin of this tergite and this emargination 
is called the cercal emargination. It varies extensively and affords 
many good generic characters. It is very deep and large in many of the 
Syrphinz (Figs. 6, Plate X; 31, Plate XII; 51, 55, 57, 58, 59, Plate 
