52 THE INTERNAL STRUCTURE OF MUSCA DOMESTIC A 



segment is reduced to a slender chitinous rod (vi, v.) in the mid- 

 ventral line. The dorsal arch of the seventh segment is represented 

 by two slightly-curved sclerites (vii, d.) with their concave faces 

 opposite ; the ventral arch (vii, v.) is similar to that of the sixth 

 segment. At the junction of the posterior ends of the sixth and 

 seventh segments with the inter-segmental membranes succeeding 

 them there are several setose tubercles arranged more or less in 

 pairs, but they vary in development in different individuals. The 

 dorsal arch of the eighth segment consists of two parallel and 

 slender sclerites (viii, d.), not so narrow as those of the two pre- 

 ceding segments. A pair of slender sclerites (viii, v.) also represents 

 the ventral arch. The terminal anal segment, which I consider 

 represents the reduced ninth segment, has a dorsal chitinous 

 sclerite, the sub-anal plate (su.p.) which is triangular in shape, and 

 a ventral sub-anal plate of the same shape. The female genital 

 aperture is situated at the anterior end of the latter plate, between 

 the eighth and anal (ninth) segments. A pair of terminal setose 

 tubercles is situated laterally at the apex of the anal segment. 



The male reproductive orrjans. 



The male reproductive organs (fig. 23) are situated ventral to 

 the alimentary canal, and lie within the fifth abdominal segment. 

 They consist of a pair of testes, vasa deferentia, ejaculatory duct 

 and sac, and the terminal penis. There are no accessory genital 

 glands in the male. 



The Testes (te.) are a pair of brown pp-itbrm bodies, with their 

 long axes placed transversely, and their pointed ends facing. In 

 young males they have a bright red appearance. They are covered 

 with a follicular investment of cells, which varies in thickness 

 apparently according to age. The thin bro^vn chitinous capsules 

 contain the developing spermatozoa. The pointed end of each 

 testis is continued as a fine vas deferens (v.d.) which meets that of 

 the other testis in the median line, where they open into the 

 common ejaculatory duct (d.e.). This runs forwards for a short 

 distance, and then bends to the left ventrally, and, after several 

 convolutions on the left ventral side of the abdomen, the duct 

 narrows considerably, forming a narrow ejaculatory duct. This 



