THE EXTERNAL GENERATIVE ORGANS. 739 
These two plates are unsymmetrical, and are fused into a single 
plate on the left side. This want of symmetry is most marked 
in Dolichopus, where it is so great that the tergum of the pro- 
genital annulus is completely inverted. 
Where these annuli are united they are highly chitinised and 
prolonged into a long tapering process, which curves round 
the floor of the genital sinus. This is probably an epipleuron, 
and the muscles inserted into it serve to move the whole of the 
invaginated somites. A rudimentary epipleuron is found on 
the right side, but it is a detached sclerite, not united with 
either tergum. 
In removing the invaginated somites the long curved left 
epipleuron brings away the whole thin anterior wall of the 
genital sinus, in the form of a kind of hood, connecting this 
sclerite with the progenital sternum. 
The Progenital Sternum.—In front of the penis there is a 
single cordate sternal plate—the progenital sternum. It articu- 
lates behind by its concave border with the penis and with a 
pair of claw-like appendages—the anterior gonapophyses. Its 
convex anterior border is continuous with the thin mem- 
branous integument of the floor of the genital sinus, and it 
articulates by its two external posterior angles with the pro- 
longed epipleural ridges of the progenital tergum. 
The absence of the sterna of the sixth and seventh segments 
in the Blow-fly is not by any means an isolated phenomenon ; 
as in Myopa, all the sterna of the abdomen, except the first 
two or three and the progenital sternum, are absent ; although 
the tergal arches are well-developed. And the sterna are often 
very narrow or even wanting in the Tachinide, especially those 
with a narrow abdomen, the tergal plates meeting on the 
ventral surface where the two halves are only separated by a 
narrow strip of thin flexible integument. Nor can I find any 
traces of sternal plates corresponding to the sixth and seventh 
terga in the Dolichopide. 
The Anterior Gonapophyses.—These are a pair of large claw- 
like appendages; each has a fringe of long setz on its convex 
edge, and articulates by a broad basal portion with the genital 
