188 Annals Entomological Society of America  [Vol. XIV, 
TeRGITE TEN. Urite ten is the most highly developed of the post- 
abdomen; and, in some of the genera (notably Cnemodon and Pipiza), 
(Figs. 17, 19, 28, Plate XI), it is the largest. Both tergite and sternite 
become very highly specialized. The tergite (see Figs. O, D, L, Plate [X) 
bears, towards its disto-lateral corners a pair of appendages, the styles, 
while its baso-lateral corners are a little, to much, produced ventrad 
and articulate to the sides of the basal rim of the penis (Figs. O, Plate 
EXe G6. Plate Xx; 34, Plate XID; 49" Plate X11I> and, 76, Pirate ive 
Its shape varies greatly, but is typically somewhat quadrangular in 
dorsal outline (Fig. 17, Plate XI; 69, Plate XIV), strongly convex 
dorsad and the sides flexed ventrad especially toward the base. 
Its shape is much modified by the condition of the cerci or appendages 
of the eleventh urite, which are sunken into its distal margin to the 
most variable extent, sometimes almost imperceptibly so (Figs. 36, 38, 
42, Plate X; 91, 95, Plate XVI; 133, Plate XIX: 109, Plate XVII), 
often about for half their length (Figs. 17, Plate XI; 2, Plate XV; 69, 
Plate XIV), in many other genera fully received (Fig. 55, Plate XIII), 
their apices just about reaching the apex of this segment. In Pipiza 
and its allies (Figs. 20, 23, 25, Plate XI; 35, Plate XII) the tenth 
tergite 1s relatively very long and the cerci lie somewhat beyond mid- 
length, while its much produced lateral arms extend far beyond them. 
In Spherophoria (Figs. 1, 60, 68, Plate XV), the climax is seen; for 
the lateral arms have, in some species, entirely fused beyond the cerci 
so that the latter lie in the middle of the large hemispherical enveloping 
tergite. See below under cercal emargination. 
THE TENTH STERNITE (Fig. O, Plate IX), has taken on peculiar 
modifications of function and structure. The caudal part of the basal 
rim of the penis articulates to it along its basal margin; while distad it 
articulates to the mesal face of the styles. It thus comes to serve in 
many genera as a lever between the powerful clasping organs (styles) 
at its apex and the rigid penis rim at its base. (Figs. 14, 18, 19, 20, 23, 
25, Plate XI; 34, Plate XII). This has originated a strong tendency. for 
the sclerite to divide on the middle line from apex more or less com- 
pletely to its base, as, for example, in Cnemodon, Pipiza, Psilota, Milesia, 
etc.,.(Figs.. 14, 25, Plate XJ: 35, Plate XII; 119, Plate XVIII): and in 
a number of species it 1s completely modified into a pair of rod-like, 
heavily chitinized levers, between which is only a thinly chitinized 
membrane. In Microdon tristis (Fig. 129, Plate XIX) the median 
margins of these two rami are bent ventrad at right angles to stand 
upright, closely parallel to each other. In Volucella spp. it is produced 
into a curious erect finger-like projection near the point of articulation 
to the penis rim. It also enters into an elaboration with the mesal face 
of the style in several cases; for example, in Blera badia, to form a 
sharp thorn, and in Eumerus a rounded, hairy elevation, both directed 
toward the penis. 
THE ELEVENTH URITE is for the most part represented only 
by its appendages, the cerci. However, in Pipiza and Cnemodon 
we find minute, chitinous sclerites, basad of these appendages, which 
