202 Annals Entomological Society of America  [Vol. XIV, 
that is shortened or toward which the parts are bent. Most often the 
cerci and styles are affected while the penis and its parts remain perfectly 
symmetrical; not infrequently both the former and latter take part 
in the asymmetry and in other cases, curiously enough, the penis and 
its appendages are unsymmetrical, while the styles and cerci are not 
or but slightly so. All kinds of combinations occur as the following 
examples will show. The cerci alone are unsymmetrical in Microdon 
spp. (Fig. 129, Plate XIX). The styles alone in Blera scitula, Hammer- 
schmidtia ferruginea and Ceria signifera. The superior lobes only in 
Mesogramma subannulata. The inferior lobes only in Chilosia spp. 
(Figs. 27 to 29, Plate XII). The internal lobes only in Mesogramma 
parvula. The ejaculatory process alone in Baccha spp. The sustentacular 
apodeme alone in Chilosia cyanea (Fig. 30, Plate XII). The cerci and 
styles both are slightly unsymmetrical in Crioprora cvanella, (Fig. 118, 
Plate XVIII), Blera badia, (Fig. 116, Plate XVIII) and Chrysogaster 
pulchella (Fig. 37, Plate X). The cerci slightly and the styles very 
much distorted in Sericomyia (Figs. 108, 109, Plate XVII), Criorhina 
verbosa, Xylota spp. (Figs. 104 to 107, Plate XVII) and Hammer- 
schmidtia. The cerci are vefy unsymmetrically placed and the styles 
only slightly affected in Xylota curvipes and flavitibia. Both cerci and 
styles are much affected in Calliprobola (Figs. 3, Plate XV: 102 and 103, 
Plate XVII). The cerci, styles and superior lobes are all unsym- 
metrical in Chrysogaster sp. (Figs. 40 to 42, Plate X), and Temnostoma 
venusta and bombylans. The cerci, styles and ejaculatory hood in 
X ylota ejuncida and obscura (Fig. 107, Plate XVII), Condidea lata and 
Teuchnocnemis (Figs. 96 and 98 to 100, Plate XIX). The cerci, styles, 
superior and inferior lobes of Pterallastes thoracicus (Figs. 97, 101, 
Plate XIX). The cerci, styles, ejaculatory hood and superior lobes 
of Xylota subfasciata (Fig. 105, Plate XVII). The most unsymmetrical 
parts that I have seen are those of a species of Sphegina, in which 
practically every part is unsymmetrical, and most of them very highly 
so—cerci, styles, superior, inferior and internal lobes, penis sheath, 
ejaculatory hood and ejaculatory process. 
FUNCTIONS. 
The organs described above (the postabdomen and its appendages) 
appear to have evolved in correlation with several different functions, in 
the probable order of their importance as follows: 
1. Introducing the germ cells to the vagina of the female. 
2. Clasping or holding the female during mating. 
3. Mechanical protection of the parts. 
4. Sensation. 
It is surprising to note the variety of structures utilized in what one 
would suppose to be the very homogeneous operation of introducing 
the spermatozoa to the vagina. In many cases there is simply a larger 
or smaller aperature on the face of the ejaculatory hood or chitinous box 
near the apex of the penis. In other genera and species we find this 
aperature elevated to the end of a long or short, heavy or slender, 
