180 Annals Entomological Society of America  [Vol. XIV, 
PREABDOMEN: URITES TWO TO FIVE OR SIX. 
I have made no attempt to study urite one, though I believe it 
may be found by a careful examination of the thoracic mass. 
URITE TWO is generally small, usually much shorter and also narrower 
than the succeeding ones. It is overhung by the scutellum (Fig. A), and 
has in some systematic descriptions been entirely overlooked—urite 
three being called the ‘‘first’’ abdominal segment. In Chilosia spp., 
Xylota, etc., it is unusually large, sometimes nearly equaling the third 
segment in length. 
URITES THREE, FOUR AND FIVE are generally subequal in size and the 
largest of the abdomen. Their tergites are typically considerably wider 
than long, convex dorsad, the convexity in the transverse direction greater 
than that in the longitudinal direction. In some genera and species 
however, as in Baccha, Ocyptamus, Sphegina and Sphaerophoria, all or a 
part of the tergites may be much longer than wide. The sternites are 
invariably narrower than the tergites and less convex, but of about 
equal length. Typically in a cross section the sternite would occupy 
the diameter of the half circle or ellipse made by the tergite. Between 
tergite and sternite the body wall is unchitinized and these portions of 
the urite, the /ateral conjunctive, bear the spiracles. In certain genera 
the chitinized ventral sclerites (sternites) are much reduced in area 
and the lateral conjunctive become proportionately larger. This is 
particularly noticeable in Helophilus, Tropidia, Sphegina and Syritta. 
SixtH Urite. The transformation from pre-abdomen to post- 
abdomen comes in the region of urites five to seven and the sixth shows 
the greatest variation in its development from genus to genus. Ina 
few cases the sixth urite terminates the preabdomen; that is, it 1s not 
transformed, or but slightly so, and one can recognize five segments in 
the preabdomen (Fig. K, Plate IX). The sixth is then always shorter 
than the preceding segments and generally somewhat narrower. Both 
tergite and sternite clearly belong to the preabdomen in the following 
genera: Platychirus, Pyrophaena, Allograpta, Baccha and Mesogramma 
(Figs. K, L, Plate LX). In these cases the sixth tergite is about equally 
developed and quite similar in position and appearance to the fifth 
tergite in some of the other genera, (cf. Terg. VI, Fig. K, with T5, 
Fig. A). It is frequently a little unsymmetrical, as in Platychirus for 
example, where it is shorter and more bent downward on the right 
side. In Baccha it is of considerable length and nearly as wide as, or 
sometimes actually wider than the preceding. The sixth sternite in these 
cases is shorter than its tergite, but it is of about equal length throughout 
its width and not modified except perhaps a little depressed toward 
its right side. 
From such a condition one can follow a clear series of specializations 
as a result of which urite six becomes more and more transformed in 
adaptation to the distorted postabdomen. Since the postabdomen as 
a whole is the result of an adaptation to the accommodation and manip- 
ulation of the genital appendages, it is clear that the distortion has pro- 
