1921] Metcalf: Genitalia of Male Syrphide 203 
cylindrical or clavate or tapering, and membranous or highly chitinized 
tubule, the so-called ejaculatory process. The termination of such ejacu- 
‘latory apparatus may be in an inflatable membrane, or between spinose 
or serrated lips or a simple aperature of varied size and shape. Even 
in the species of the same genus these organs may vary tremendously 
(cf. Xylota bicolor and Xylota vecors, for example). (See also under 
ejaculatory process, above). It will be very surprising if these 
structures are not found to have correlated peculiarities in the females. 
The necessity of clasping and holding the female during mating has 
been most fruitful in producing variations of the appendages. The 
cerci alone appear not to have taken part in this variation except in 
rare cases. The styles have developed the greatest variety of structures 
for the clasping function. Not only does the shape of the parts vary, 
but they are greatly enhanced in efficiency by the specialization of the 
vestiture, especially over their mesal faces, toward the apex. This 
begins as a stiffening of the hairs, accompanied by shortening; the 
culmination of the process results in very heavy, sharp or dull pegs, 
sometimes scarcely longer than their diameter. Frequently the styles 
are bilobed and either caudal or cephalic lobe may be of most value as a 
clasping organ. Besides the vestiture one or other of the lobes frequently 
develops a tooth or claw or finger, of endlessly-varied shape. Rarely 
the inflexed sides of the tenth tergite are prolonged into processes, 
evidently of use in this connection (Volucella, Microdon). Again as in 
Tropidia and Sphegina, there are processes or bristles on the fifth 
sternite, which assist in grasping the abdomen of the female. Various 
parts of the penis-sheath, its lobes and appendages have been adapted 
for prehension. The most important of them are the superior lobes, 
and the inferior lobes; but 1n many species the internal lobes connected 
with the elaboration of the ejaculatory hood are especially important 
in this respect. Even the ejaculatory hood itself is sometimes adapted 
for grasping and the various bristles and keels or processes of the penis- 
sheath, described in the above pages, doubtless often have a prehensile 
value. 
For the mechanical protection of the parts, the heavily chitinized 
sternites and tergites of the postabdomen, especially nine and ten, are 
well adapted. Of even greater importance is the introversion of the base 
of the postabdomen between the terminal sternite and tergite of the 
preabdomen, to form a genital pouch. (See above). Contributing 
toward the investment of the delicate appendages are also the down- 
bent sides of the terminal tergite of the preabdomen (the fifth or sixth) 
and the basal segments of the postabdomen on the left. 
The only organs which would appear from gross dissection to have 
a sensory function are the cerci. The fact that they are, I believe, 
without exception wholly or partly exposed even when the other parts 
are all well retracted beneath heavy protecting sclerites, and the nature 
of the hairs on their exposed surfaces, which are long and arise generally 
from quite conspicuous rings, seem to point to them as the “‘antennez”’ 
of the postabdomen; in this family, probably exclusively tactile. 
