434 0. GORDON HEWITT. 



triangular-shaped area; the anterior edge of this is incised, 

 and receives the seventh spiracle (vii, a.sp.) ; the ventral 

 edge is internal to the spatulate portion of the sixth ventral 

 plate. Tlie ventral arch of the seventh sclerite has been 

 completely withdrawn into the abdomen, and consists of 

 a pair of curved sclerites (fig. 9, vii, v.), somewhat rhom- 

 boidal in shape, lying dorsal to the fifth ventral arch and 

 ventral to the penis (P.); the}'- form the secondary forceps. 

 Their lateral edges, which are thickened articulate with the 

 alar processes of the body of the penis {c.pe.), and with the 

 dorsal arch of the eighth abdominal segment (viii, d.). Their 

 inner edges are curved, and almost meet in the mid-ventral 

 line. The dorsal arch of the eighth and last abdominal 

 segment (viii, d.) forms the apex of the abdomen. It consists 

 of a strongly convex sclerite, deeply incised on the ventral 

 side; in this incision the vertical slit-like anus (fig. 10, an.) 

 lies. The ventral portion of the segment is completed by a 

 pair of convex sclerites (viii, v.), which are united in the mid- 

 ventral line, forming the ventral border of the anal membrane 

 and the dorsal side of the entrance to the genital atrium. 



All the sclerites of the posterior segments except the sixth 

 and seventh are setose. 



Berlese (1902) in his account of the copulation of the 

 House-fly describes the genitalia. From his account of the 

 male genitalia he appears to have missed the narrow dorsal 

 arch of the sixth segment, or, what is very probable, he may 

 have mistaken it for the fifth dorsal arch, as he terms the 

 seventh dorsal arch the sixth, and describes what I have 

 called the ventral arch of the seventh as the dorsal arch of 

 that segment. This mistake iu nomenclature has probably 

 arisen from the fact that he considered the visible portion of 

 the abdomen as consisting of four segments instead of five, 

 in which case the narrow dorsal arch of the sixth segment 

 would naturally be taken for that of the fifth.^ 



1 Berlese describes a sinistral asymmetry of the posterior segments, but 

 his figures show a dextral asymmetry, a mistake probably in the reproduction 

 of his figures which has escaped the author's notice. 



