1921] Metcalf: Genitalia of Male Syrphide 185 
to right (Figs. B, D, F, H, and L, Plate IX). This has resulted, thirdly, 
in crowding the basal segments of the postabdomen (urites six, seven 
and eight) more or less completely over to the left side of the apex of the 
preabdomen (Figs. D, L) to accommodate the enlarged tip of the 
abdomen. Accompanying this bending under and curling around 
toward the right, has occurred fourthly, a rotation clockwise on the long 
axis (Figs. M, N) usually for about one-fourth of a revolution on the 
seventh urite and progressively more on the eighth and ninth. As a 
result of the above modifications, we find in the present-day species 
that the apex of the postabdomen points cephalad instead of caudad, 
(Fics: B, BD, P, Hi, Kk, L) and that the mid-dorsal line of the tenth urite 
(its tergum) lies near the mid-ventral line of the preabdomen and its 
sternite lies practically dorsad of its tergite. I the fifth place, in many 
genera, the terminal tergite and sternite of the preabdomen have 
gaped apart at their apex (Figs. 69, 73, Plate XIV; Fig. 95, Plate XVI), 
and, sixthly, the transverse conjunctive bordering the sixth sternite 
have often become introverted between these two sclerites to form a 
sort of genital pouch, often carrying inward also one or two of the basal 
segments of the postabdomen. When at rest then the apices of the 
appendages borne by urites nine and ten often lie in such a pouch, 
sometimes to their very bases (Figs. B, D, F, Plate IX). 
UrRite Stx IN THE PosTaBDoMEN. In the completed condition, then, 
(Figs. B, D, H), urite six, when not a part of the preabdomen (see above) 
is often found as a very short and narrow sclerite, underneath the left 
side of the fifth tergite; or it may be nearly as broad as the fifth sternite, 
but mostly or entirely hidden under this sternite. 
Urites SEVEN AND Ercut are small, short, and more or less strictly 
confined to the left side; the former wholly, in part or not at all received 
into the genital pouch; the latter wholly, in part of not at all projecting 
beyond the last tergite of the preabdomen in dorsal view (Figs. A, G,1). 
They are twisted clockwise on their long axis, so that their mid-dorsal 
line points almost directly dextrad (Fig. M) and their mid-ventral line 
nearly directly senestrad (Fig. N). 
In contrast with the condition in the preabdomen, the sternites of 
the postabdomen are more heavily chitinized and more convex and often 
of greater actual area than the tergites. Generally, however, the 
area of the tergites is greater than at first appears, because they are so 
greatly distorted and folded upon themselves. The only way in which 
boundaries can be clearly recognized is to follow the lateral conjunctive 
and particularly note the spiracles. The lateral conjunctiva of the 
right side (Fig. M) lies just dorsad of the right margin of sternites six 
to eight. When the parts are extended, it therefore bends sharply 
to the left and ventrad (Figs. L, M) after the last segment of the pre- 
abdomen (five or six) and beyond this point its course describes part of 
a circle or ellipse with the concavity toward the right.* In the retracted 
*A condition which may have a great deal of significance, but which I am not 
able to interpret at present is shown in a specimen of Eristalis arbustorum. In 
this specimen the right lateral conjunctiva appears to continue directly into what 
we have called the mid-dorsal line of the tenth tergite. 
