508 C. E. Osten-Sacken's essay on 



dorso-central region in the middle (III.), and the dorso- 

 humeral (I.) and dorso-alar (II.) regions on each side. 

 This division will be found very convenient for localising 

 any bristle we may have to describe. These Eoman 

 numerals will be sometimes used by me to represent the 

 corresponding regions. Thus supra-alar bristles (II.) 

 will mean supra-alar bristles (dorso-alar region). 



I. Dorso-Jiumeral region. — It is bounded by the anterior 

 end of the thorax and the thoracic transverse suture on 

 two sides, and by the dorso-plural suture and dorso- 

 central region on the two others (No. I. of the figure). 

 I distinguish here : — 



1. The hume^'al bristle (Loew, Europ. Helomyziden), 

 inserted on the humeral callus ; in the Acalyptrata either 

 one or none at all ; in the Calyptrata often several. 



2. The post-humeral bristles, as far as I know, with 

 rare exceptions, two inserted immediately above the 

 dorso-pleural suture, between the humeral callus and 

 the root of the wing ; both are inserted at the bottom of 

 the prffisutural depression, a triangular depression which 

 usually exists here in the Cyclorhapha. 



8. A prasutural bristle, immediately in front of the 

 thoracic suture, above the praesutural dej)ression. It is 

 not found in the Ortaliche, but occurs in most Trypetidce, 

 SciomyzidcB, and Sayromyzidce. It may be homologous 

 with a bristle in the same place in the Diptera Calyptrata, 

 but which is less conspicuous among the other bristles 

 which they have in the same region, and which I call 

 intra-humcral. The Asilidce possess very characteristic 

 and persistent prsesutural bristles (two or more). 



The Aealyptrata, as far as I am aware, have no other 

 bristles in the dorso-humeral region. The Diptera 

 Calyptrata have several bristles besides, which may be 

 called the intra-humeral bristles. There are often two, 

 sometimes three or more of them, and they are inserted 

 between the outer dorso-central row on one side, and the 

 humeral callus and the prsesutural depression on the 

 other (compare below the notice on the Diptera Calyp- 

 trata). 



II. Dorso-alar region. — Between the transverse suture 

 and the scutcllum on one side, and the root of the wings 

 and the dorso-central region on the other (No. II. of the 

 figure). In the Acalyptrata this region only contains 

 the group of — 



