182 Annals Entomological Society of America  [Vol. XIV, 
retracted underneath the fifth is a very short sixth of nearly equal 
width; and projecting slightly from under the fifth tergite on its left 
side, are two very small unsymmetrical tergites (seven and eight) that 
narrow rapidly toward the right side where they become entirely 
concealed by the fifth tergite. In some species tergite six is also normally 
slightly exposed at the extreme left (Fig. A, Plate IX). 
THE GENITAL Poucu. As indicated above it is chiefly the adaptations 
to the large genital appendages, when at rest, that are responsible for 
the transformation of the basal segments of the postabdomen— 
especially urites six, seven and eight. The same stimulus has resulted in 
a depression near the apex of the venter of the preabdomen, into which 
the penis, styles, etc., are received when not in use. This depression I 
have called the genital pouch. (Figs. 95, Plate XVI; 134, Plate XIX: 
73, 69, 77, Plate XIV). It is developed to the most variable degree, 
showing a high correlation with the size of the postabdominal append- 
ages. Its beginnings are shown in the cases already described (Fig. F, 
Plate IX) in which sternite six is first depressed toward its right side 
then looses its chitinization in this region. 
However, in cases where the genitalia are larger, the terminal seg- 
ment of the preabdomen, namely tergite five and sternite five, (Figs 69, 
Plate XIV; 95, Plate XVI), gape apart toward their apex and the 
transverse conjunctive become actually introverted beneath sternite 
five carrying with them to a variable degree urites six and seven. 
Correlated with this movement we find a further tendency to 
modify especially sternite five to aid in grasping the female abdomen. 
In Helophilus the fifth sternite is strongly drooping and has the caudal 
margin emarginated and sometimes sharply keeled at the middle, but 
symmetrical. In Teuchocnemis and Triodonta the caudal margin of the 
fifth sternite is much thinned, looking something like a short broad 
additional sternite fused to it. In Syritta pipiens there is a strong, 
V-shaped emargination of the caudal margin that reduces the median 
length of the sternite to about half that on the sides. In Somula decora 
the fifth sternite has a short rectangular prolongation of its caudal 
margin covering the middle fourth. In Ceriodes tridens there is a 
sharp median keel narrowing cephalad. In Mallota there is a fringe 
of long hairs a short distance removed from the caudal margin of the 
fifth sternite. 
The genus Sphegina shows some interesting modifications of the 
fifth sternite. In S. infuscata this sclerite is hardly at all specialized 
either in shape or in vestiture, being only a little longer on the left side. 
In S. clunipes there is a narrow, shallow emargination of the caudal 
margin near the middle and about this emargination are many, much 
elongated, but not at all stout hairs, not divided into two groups. 
In S. rufiventris (Fig. 77, Plate XIV) there is a shallow, rounded emar- 
gination of the caudal margin near the middle (d) and this is bordered 
with several dozen very heavy short hairs tending to divide into a right 
and left group. At each side of these spiny hairs the vestiture is three 
or four times as long and more delicate. In S. petiolata (Fig.-73, Plate: 
