comparative Chcetotaxy. 509 



Supra-alar bristles, usually three ; one is inserted on 

 the i3ost-alar callus (which is often indistinct in the 

 Ortalidce), very near the scutellar bridge and almost in 

 a line with the praescutellar bristles; the second is in 

 front of the first, just at the top of a small ridge or liga- 

 ment {alar frcnum) , which descends to the root of the 

 wing and crosses the supra-alar cavity ; the third is in 

 front of the second, on the edge of the anterior portion 

 of that cavity. These bristles are not placed in a straight 

 row ; their bases form a triangle. A fourth supra-alar 

 bristle sometimes exists in the Ortalidce, behind the 

 mesothoracic transverse suture and above the posterior 

 post-humeral bristle. As far as I can ascertain, this 

 last bristle is distinctive of the section Ortalina; I do 

 not find it in the other sections of the Ortalidce. The 

 Diptcra Calyptrata often have a series of more than 

 three supra-alar bristles along the edge of the anterior 

 supra-alar cavity. Even among the Asilidcs, the supra- 

 alar bristles form a distinct group. 



The Dipttra Calyptrata show other bristles in the same 

 region; they often form a longitudinal row of two or 

 three, placed between the supra-alar group and the 

 outer dorso-central row. They may be called intra-alar 

 bristles. 



III. Dorso-central region. — Bounded by two imaginary 

 hnes drawn from the scutellar bridges forward, and 

 coinciding with a space free of bristles that exists on the 

 outer side of the dorsal rows, and that is often occupied 

 by a dorsal thoracic stripe (No. III. of the figure). This 

 region contains the dorso-central bristles, arranged in two 

 or four longitudinal row^s. They are often wanting in 

 the Ortalidce. In the DoUchopodidcB the intermediate 

 pair of rows is represented by two, sometimes only one 

 (Liancahis) row of peculiar, minute bristles, which Prof. 

 Mik (Dipterol. Unters., Wien, 1878, and also Verb. z.-b. 

 Ges. Wien, 1880, p. 600) calls the aerostichal bristles. In 

 the absence of dorsal rows of bristles (for instance, in 

 the Ortalidce, Tetanocera, &c.) there is often a transverse 

 row of four (or two) bristles in front of the scuteUum ; 

 they represent the terminal bristles of the (here non- 

 existing) dorsal rows. I call them ptrcescutellar bristles. 

 Sometimes (in Trypeta) there is only a single pair of 

 prajscutellar bristles, while a second pair, more in front 

 and farther apart, represent the (absent) outer dorsal 

 row. The praiscutellar bristles are always inserted 



