200 Annals Entomological Society of America  [Vol. XIV, 
line is, I think, only a reinforced line along which the chitin is heavier. 
It 1s not at all constant in the various species. Similar reinforced lines 
run in various directions across or along the sheath. Note Sphegina 
petiolata (Fig. 75, «, Plate XIV), Criorhina nigripes, umbratilis and 
verbosa (Fig. 111, d, Plate XVIII), Mallota posticata, M. sackeni, Blera 
badia (Fig. 116, Plate XVIII), Merodon equestris, Somula decora (Fig. 
120, Plate XVIII), Brachyopa notata, etc. The penis sheath is typically 
emarginated at the apex on both cephalic and caudal faces, between 
which at each side project the so-called lateral arms or wings of the 
sheath, bearing the inferior and superior lobes at their apices. 
(i). Tue CrepHALic EMARGINATION takes a great variety of forms 
and often shows a specific character. It is most often narrowly or 
broadly U or V shaped (Figs. 3, Plate XV; 5, 38, Plate X: 20, Plate XT- 
30,’ Plate, XIT;” 50,- Plate-XItIl-"76, Plate XIV "87, Plate Sovie | Ea 
Paragus bicolor (Figs. 5 and 6, Plate X) the penis sheath about its 
margin is ornamented with many close-standing short spines. 
(j). THe Lincura. The cephalic emargination may be partly filled 
by a projection from the sheath that I have called the lingula which is 
also very variable and so of value in specific determination as in Syrphus 
spp., Blera confusa, Milesia virginiensis, Xylota. curvipes, etc. (Note 
Figures 47, 48, Plate X; 52, 53, 54, 55, 56, Plate XIII: :86, Plate XVI: 
96, Plate XIX). It stands between the inferior lobes as a tongue-like, 
unpaired, emarginated or paired extension of the penis sheath on the 
middle line. It may be forked at apex or modified into an apical scoop- 
shaped notch that may stand in direct opposition to the termination of 
the ejaculatory duct or, when the latter is elongate, surround and 
support its base. It is highly developed in Paragus (Figs. 5, 6, 11, Plate 
X). The lingula may in a series of species become so large as to nearly 
fill the cephalic emargination and since it is often emarginated at its 
own apex, there are cases where it is difficult to decide whether the 
apex should be called a lingula or simply the cephalic emargination. 
(Note Paragus bicolor, Figures 5, 6, Plate X). 
The emargination on the caudal face of the penis is much less varied 
in character and, on this account and because of the difficulty of exposing 
it, has little taxonomic value. 
In a few genera, notably Microdon, Mixogaster and Triodonta, (Figs. 
129 and 133, Plate XIX) the penis sheath is completely interrupted on. 
the caudal face, except for a very slight basal rim, and the margins are 
infolded or rolled entad and free-standing. This is apparently a highly 
specialized and not a primitive condition. 
The penis-sheath usually bears at its apex certain specializations 
which may take the form of definitely articulated appendages, or which 
may be simply prolongations of the sheath, itself, without an articula- 
tion. Because of the similarity in function and often in appearance 
and the practical difficulty of telling whether there is an articulation in 
many cases, I have called both kinds of processes /obes—superior lobes 
when they are disto-caudal in position, inferior lobes when disto- 
cephalic in position. 
