1921] Metcalf: Genitalia of Male Syrphide 199 
enclose a chitinized bow or curved bar which expands Y-shaped at each 
end, at which, points many muscles attach. The short handle of the 
apodeme attaches to the membranous sac opposite the middle of the 
bow. 
I have not found the ejaculatory apodeme in certain species of 
Sphegina, Sphaerophoria, Teuchocnemus, nor in Eupeodes volucris. 
B. THE PERIPHERAL SYSTEM OF THE PENIS. 
(h). Tue Pents SHEATH is usually a heavily chitinized, truncated 
cone, cylinder or tube, attenuated distad. It has an especially heavy, 
basal rim, by which it is articulated and rotated. The rim is usually 
longer cephalo-caudad than in transverse diameter, the cephalic half 
of the rim almost always about semi-circular, but the caudal part 
becomes variously angulated against the base of the tenth sternite 
and the sides likewise swollen or angulated, or with apophyses to the 
basal corners of the tenth tergite. In some genera the caudal part 
is bent into quite another plane than the cephalic part. In many of the 
Milesine the basal part of the sheath is larger and globose on the 
cephalic face, the apical part more nearly cylindrical. (Note Figures 3, 
Plate XV; 83, 89, Plate XVI; 102 to 107, Plate XVII; 112, 115, 119 and 
120, Plate XVIII). 
The penis sheath is most often smooth and glabrous. But in certain 
genera it is highly specialized both in vestiture and by keels, tubercles 
and furrows of various kinds. In most species of Temnostoma there is 
an area of long delicate hairs on each side near the base. In Criorhina 
and Milesia there is often a diffused vestiture on the cephalic face of 
the penis sheath. In Helophilus spp. (Figs. 83, 85, 89, Plate XVI), 
Tropidia albistylum (Fig. 91) and certain species of Xylota (Fig. 107, 
Plate XVII) we note long or short heavy hairs, bent basad in such a 
way as to suggest a tenaceous value, in addition to others scattered 
promiscuously over the cephalic face of the penis that are more erect 
and delicate. 
Irregularities of the surface of the penis sheath are often in the form 
of transverse, more or less parallel corrugations on its cephalic face, as 
illustrated in Figures 91, Plate XVI; 102, 103, 123 and 128, Plate Savills 
115, Plate XVIII. They are peculiarly unsymmetrical in Somula decora 
(Fig. 120). Tubercles of various kinds occur—symmetrical (Figs. 111, 
127).or unsymmetrical (Fig. 72, e, Plate XIV), large (Fig. 113) or small 
(Fig. 86, Plate XVI); usually on the cephalic face but rarely on the 
caudal face as in Pipiza pisticoides (Figs. 19, 20, h, Plate XI), where an 
extremely thin keel runs lengthwise on the caudal face and in Eupeodes 
volucris where we note characteristic ribbed ridges at the caudo-lateral 
corners (Figs. 134, c, Plate XIX). 
Berlese in describing Eristalis tenax, divides the sheath into “two 
canals (epiphallus and hypophallus)”’ separated by a longitudinal line 
on each side and supposedly representing respectively the contribution 
made to this complex appendage by segments eight and nine. I do 
not believe that these parts have any morphological significance. The 
