1941] 
Male Diptera 
81 
the ninth segment with the proctiger, etc., extends straight 
back in the long axis of the body (see also the primitive 
Nematoceran Trichocera, shown in Fig. 14). In Dilophus 
(Fig. 1) the eighth sternite 8s is very large, and shows no 
sign of displacement. 
In the next stage represented by the Rhagionid (Leptid) 
Rhagio, shown in Fig. 2, the ninth segment is tilted upward 
and the conjunctivae or intersegmental membranes are very 
extensive, giving free play to the temporary twisting of the 
parts in mating. The eighth tergite 8t is reduced, but the 
eighth sternite 8s remains large, and the parts of the seventh 
segment are not effected by the process. 
A slightly higher stage of specialization is illustrated by 
the Stratiomyid Ptecticus, shown in Fig. 4. In this fly the 
ninth segment is very large and the conjunctiva are very 
extensive to permit a twisting of the parts in copulation. 
The tergites of both the eighth and the seventh abdominal 
segments are reduced to narrow transverse strips, and the 
eighth sternite 8s shows signs of having been slightly dis- 
placed upward, at least temporarily, during the mating pro- 
cess (this specimen was captured in copula). 
A still higher stage of specialization is illustated by the 
Syrphid Heliopkilus shown in Fig. 6. The ninth abdominal 
segment remains permanently twisted forward in the torsion 
process, and the huge eighth sternite 8s, although still vent- 
rally located, becomes slightly lateroverted, or displaced up- 
ward into the insect’s left side. The eighth tergite has be- 
come atrophied, or is represented by the narrow transverse 
sclerite lying just behind the seventh tergite, 7t, and antero- 
dorsad of the eighth sternite. The seventh tergite, 7t, is 
reduced, as is also the case with the sixth and fifth ab- 
dominal tergites, 6t, and 5t ; and their sternites exhibit indi- 
cations of slight lateroversion. 
Turning next to the Syrphid Chrysotaxoides, shown in 
Fig. 8, we note that the ninth abdominal segment, 9t, is 
twisted still further around (ventrally) and the huge eighth 
sternite, 8s, becomes drawn up into the dorsal region, and 
is almost inverted. The seventh sternite, 7s, follows it to 
some extent, and is lateroverted into the insect’s left side, 
but the seventh tergite, 7t, is not greatly affected by the 
process, and the left spiracle of the seventh segment remains 
