1941] 
Male Diptera 
87 
realizing this fact, if comparative morphology has any 
meaning at all. The only baffling difficulty presented by the 
parts in these insects is the question as to the homology of 
the basal area bearing the label g in Fig. 13, which may 
represent either the basal ring gc of Fig. 11, or it may rep- 
resent the remains of the ninth sternite which unites with 
the basimeres of the forceps, or it may represent a secon- 
darily demarked area in the basal region of the uniting basi- 
meres of the forceps, and the latter explanation seems as 
logical as any. 
The ninth sternite is separated from the basal segments 
of the genital forceps in the Trichoceridse (Fig. 14), as is 
also the case in such Tipulidse as Macrocera, etc., but in the 
genus Tipula the ninth sternite tends to unite with the basal 
segments of the genital forceps, and this tendency is carried 
still further in many other Diptera. In the primitive Dip- 
teron Protoplasa fitchii shown in Fig. 15, the ninth sternite 
9s apparently becomes greatly reduced before it unites with 
the basal segments of the forceps, and the condition ex- 
hibited by Protoplasa suggests that the area labelled g in 
Fig. 13 may also represent such a reduced ninth sternite 
which has united with the basal segments of the forceps, 
although it is also possible that the area labelled g in Fig. 
13 is comparable to the region between the bases of the 
genital forceps in the Trichoceridse (Fig. 14), and a third 
possibility is that both the area labelled g in Fig. 13, and 
that interpreted as the reduced ninth sternite in Fig. 15 
(9s), may represent the basal ring of the genital forceps of 
the Hymenopteron Xyela (Fig. 11, gc). 
The distal segments of the forceps, labelled d in Fig. 15 of 
Protoplasa, are forked, and if the cleft of the fork were 
deepened basad, it would eventually divide the distal seg- 
ment into an inner and outer distimere (or dististyle) like 
those labelled id and od in Fig. 10 of the Tipulid Acantho- 
limnophila; and it is very probable that the outer and inner 
distimeres or dististyles characteristic of most Tipulidse 
arose from such a longitudinal splitting of the distimeres. 
The basal segments of the genital forceps of Acantho- 
limnophila bear slender mesal processes called interbases 
(labelled ib in Fig. 10) ; and the ninth tergite, 9t, of Fig. 10, 
bears a pair of posterior lobes in this insect, which may cor- 
