1921] Metcalf: Genitalia of Male Syrphide 181 
eressed cephalad and that the genera just decribed are more primitive in 
this respect than those which have the sixth urite variously transformed. 
As urite six becomes more affected the sternite first becomes trans- 
formed while its tergite may retain practically its normal shape and 
position. In the genus Syrphus the sixth sternite is variable; in some 
species (e. g., S. arcuatus) it stands full width of the hind margin of the 
fifth sternite, only shortened toward the middle or right side (Fig. F, 
Plate IX): in other species it is confined in its chitinized portion more or 
less completely to the left half of the abdomen, when it may be more 
properly be called a part of the postabdomen. The latter condition 1s 
found also in Chrysotoxum (Fig. H, Plate IX) Spherophoria (Fig. 1, 
Plate XV), Paragus and Eupeodes (Fig. 134, Plate XL). 
In Spherophoria (Fig. 1, Plate XV) the sixth tergite is large, in the 
common species being roughly two-thirds as long as the fifth. It is 
unsymmetrical, the right hand corner being prolonged into a triangular 
‘or rounded lappet. This sclerite is also a little unsymmetrical in Eu pe- 
odes. The sixth sternite in Eupeodes volucris (Fig. 134, Plate XO) as 
longest and reaching nearly to the apex of the sixth tergite on the left 
side but much shorter a little to the right of the middle line (f, Fig. 154), 
where it is depressed to form a cavity for the apex of the penis. 
In Melanostoma, Didea, Xanthogramma and Syrphus some species 
show urite six entirely untransformed, others have the sternite confined 
to the left side and the tergite normal or nearly so; while still others 
reflect the next condition in which the tergite likewise becomes involved 
sn the distortion. Thus in Didea fasciata fusci pes it is somewhat upturned 
at apex and broadly and shallowly emarginated on the caudal margin. 
In Melanostoma mellinum, Syrphus ribesit, et al. the sixth tergite is 
much shorter on the right side. 
Finally there is a large series of genera in which urite six has become 
completely transformed into a part of the postabdomen. In these 
cases it is usually not visible from above in the unrelaxed specimens 
and urite five terminates the preabdomen (Figs. A, C, E, G, Plate DX): 
Generally the fifth tergite is of about the same size as tergite six of the 
genera described above. Thus the preabdomen consists of only four 
discernible segments (Fig. A, Plate IX). This is the case in the following 
genera: Psilota, Cnemodon, Chrysogaster, Sphegina, Rhingia, Hammer- 
schimidtia, Volucella, Copestylum, Eristalis, Helophilus, Mallota, Seri- 
comyia, Condidea, Arctophila, Pyritis, Tropidia, Teuchocnemis, Trio- 
donta, Pterallastes, Syritta, Xylota, Calliprobola, Brachypal pus, Myiolepta, 
Criorhina s. s., Blera, Somula, Crioprora, Milesia, Callicera, Spilomyta, 
Meromacrus, Ceriodes, Temnostoma, Microdon, Mixogaster. The fifth 
tergite is unusually elongate in Myiolepta and Microdon. It is unsym- 
metrical in Criorhina. A part of the postabdomen is almost always 
visible caudad of tergite five (usually urite nine and more or less of the 
adjoining sclerites) (Figs: A. C, E, G,.1, Plate IX), and this has in some 
systematic papers been designated ‘‘segment five” or the hypopygium. 
In the genus Chilosia (Figs. A and B, Plate IX) there are only four 
distinct segments in the preabdomen (urites two to five inclusive) but 
