1921] Metcalf: Genitalia of Male Syrphide 183 
XIV) the distal emargination is unsymmetrical and about this emargina- 
tion the hairs are elongated and thickened, but not. sharply distinct 
from the others on this sclerite. Projecting strongly from under the 
left side of the fifth sternite and between it and the sixth sternite the 
transverse conjunctiva on the left side forms a large blunt somewhat 
chitinized lobe (0, Fig. 73, Plate XIV,). The ventral body wall is then 
strongly introverted so that the sixth sternite is entirely hidden under- 
neath this lobe, forms the ventral floor of the genital pouch and lies 
with its venter directed dorsad. In Sphegina lobata (Fig. 69, Plate 
XIV) we have a very similar condition except that in this case the 
ventral wall of the projecting lobe is covered by a strong protuberance 
of the left side of sternite five instead of being made up wholly of the 
transverse conjunctiva. The vestiture about the strong unsymmetrical 
caudal emargination of the fifth sternite is short and stiff. There is a 
similar, though not so prominent projection of the left side of the fifth 
sternite in another species of Sphegina, and the genital pouch is still 
more strongly developed (Fig. 78, Plate XIV). In S. campanulata the 
genital pouch is also very large. 
The way in which the development of the genital pouch has influ- 
enced these remarkable emarginations of the caudal margin of sternite 
five is indicated in an unnamed Sphegina, where the genital pouch is 
but slightly introverted but the part of the fifth sternite directly under- 
lying and adjoining it (in a large U-shaped area reaching three-fourths 
the way to the base of the sternite) is exceedingly thinly chitinized and 
but one step removed from a very strong emargination of this sclerite. 
In S. latifrons the fifth sternite is quite symmetrical and unusually long, 
though its apex hardly reaches the caudal third of its tergite. On the 
basal third is a median swelling bearing about a dozen spinose hairs on 
each side. Between this and the apex the segment is transversely con- 
cave and the distal margin is bituberculate—the tubercles as far from 
each other as from the lateral margin, each semi-globular and with a 
dense tuft of hairs, much as in Tropidia quadrata (see below). Of these 
hairs, some are similar to those on the median tubercle, but many are 
four times as long and stiff. These tufts when the parts are at rest, 
guard the apices of the elongate styles. The genital pouch is but feebly 
‘developed, its place being taken in part by the tubercles. 
A condition remarkably similar to that in S. latifrons is found in 
our common Tropidia quadrata (Fig. 95, Plate XVI). The fifth sternite 
has a median caudal emargination, guarded by two very densely hairy 
tubercles that help to complete the genital pouch formed by the intro- 
version of the sixth sternite and the transverse conjunctive bordering it. 
In Eumerus strigatus the caudal margin of the fifth sternite is pro- 
‘duced into two large sub-quadrangular plates separated only by a 
linear incision on the middle line. 
