84 
Psyche 
[June-Sept. 
labelled 5s in Fig. 20, has become cleft posteriorly to form 
two copulatory lobes, labelled 1, which function in the mating 
process, and also project backward below the aedeagus to 
protect it when it is received in the genital pouch. 
The Metopiid fly Phormia regina shown in Fig. 21 is typi- 
cal of the Muscoidea in general, and may serve as the basis 
for comparing the parts in the other members of this super- 
family. Its structures are essentially like those of the fly 
shown in Fig. 20, but the union of the seventh sternite, 7s, 
with the eighth sternite, 8s, is more complete, and the seventh 
sternite is apparently reduced to the small indistinct area 
labelled 7s in Fig. 21. The left spiracle of the seventh ab- 
dominal segment is borne near the anterior border of the 
synsternite (composite seventh and eighth sternites) in 
Fig. 21, but in the Muscid fly Parallelomma, in which the 
synsternite (or composite seventh and eighth sternites) be- 
comes greatly enlarged along the long axis of the body, the 
spiracle in question is situated far back in the enlarged syn- 
sternite, which may indicate that the seventh sternite, 
which contributes to the formation of the synsternite, is of 
considerable extent. On the other hand, in another Muscid 
fly Musca domestica, in which the synsternite is reduced to 
a narrow transverse sclerite, the left spiracle of the seventh 
segment remains far down toward the anterior margin of 
the synsternite, indicating that the seventh sternite forms 
a relatively unimportant portion of the reduced synsternite. 
In Musca domestica a further specialization is indicated 
by the fact that the sixth abdominal sternite (which exhibits 
a tendency to attach itself to the synsternite labelled 7s and 
8s in Fig. 21) migrates from its position below the sixth 
tergite, and becomes so closely associated with the ventral 
region of the narrow transverse synsternite that it appears 
to be the sternite of a segment whose tergite is represented 
by the narrow transverse synsternite ; and it is small wonder 
that these structures have been misinterpreted in Musca 
domestica , although a comparison of the parts with those of 
the Muscoid fly shown in Fig. 21 (and tracing the parts on 
back through the series here described) would readily reveal 
the true homologies of the parts in the housefly. 
The modificational trends exhibited by the series of flies 
shown in Figs 5, 7, 20 and 21, indicate a rather close relation- 
