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 Tachinidse, 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 Dolichopidse. 



The Anterior Gonapophyses. These are a pair of large claw- 

 like appendages ; each has a fringe of long setae on its convex 

 edge, and articulates by a broad basal portion with the genital 



