82 
Psyche 
[June-Sept. 
but slightly displaced upward. In the Syrphid Syrphus 
rectus shown in Fig. 17, the ninth segment faces more nearly 
backward as the result of the continued twisting process, 
and the sixth and seventh sternites, 6s and 7s, follow the 
eighth sternite, 8s, as it becomes drawn upward. The ejacu- 
latory dust, ed, shows through the thin integument of this 
Syrphid, and clearly indicates that there has been a clockwise 
torsion (viewing the insect from behind) of the parts, since 
the duct loops up over the top of the hindgut, from left to 
right. If there is any doubt that the sclerite 8s, interpreted as 
a sternite in these insects, is actually a sternite, rather than a 
tergite, it is only necessary to compare the parts with those 
of the Pyrgotid (Ortalid) Pyrgota undata, shown in Fig. 3, 
in which the huge eighth sternite, 8s, is only slightly latero- 
verted into the insect’s left side, and preserves its typical 
relation to the seventh and sixth sternites, labelled 7s and 6s 
in Fig. 3 ; and if the sclerite labelled 8s were a tergite, in- 
stead of a sternite, the torsion would have to take place in 
a counter-clockwise manner, which it does not do in any of 
the insects whose internal structures have been examined. 
Turning next to the Coelopid Coelopa frigida shown in 
Fig. 5, we note that the ninth tergite, 9t, has now completed 
the torsion process, and the anus faces posteriorly again, 
from this stage onward. The genitalia project downward, 
however, (instead of posteriorly as in the Nematocrea) , and 
the aedeagus, ae, now projects anteriorly (instead of 
posteriorly) as in most Cyclorrhapha. This completion of 
the torsion process is spoken of as a “circumversion” or a 
reversion. The eighth sternite, 8s, has now become almost 
inverted and the asymmetrical seventh sternite, 7s, follows 
it upward into the insect’s left side. The spiracle of the 
right side of the seventh segment has apparently become 
drawn down and around into the insect’s left side, and the 
small narrow sclerite just below it appears to be the remains 
of a portion of the seventh tergite. The asymmetrical sixth 
sternite, 6s, is lateroverted and the right spiracle of the sixth 
segment is drawn down and around into the insect’s left 
side indicating the extent of the torsion process in this 
region. The fifth segment is not affected by the process, in 
the Muscoids in general. 
A further advance in the direction of the Muscoid flies is 
