514 C. R. Osten-Sacken's essay on 



dorso-central. They sometimes form a spurious, irre- 

 gular row or series with the intra-humeral bristles of 

 the dorso-humeral region. 



The passage from hairs to bristles is so gradual among 

 the Calyptrata that the number of bristles of a given 

 kind is sometimes difficult to state, and sometimes 

 variable in the same species, adventitious hairs assuming 

 the proportion of bristles. The more hairy a species is, 

 the more it seems liable to such variations. 



In the (Estridee, as an exception, the differentiation 

 between the macrochsetae and ordinary hairs is feebly 

 developed in some genera, and not at all in others. 



Syrphidce are aerial, and generally destitute of macro- 

 chsetse; the exceptions have been noticed on p. 501. 



Myopid(B. — Macrochffitffi almost undeveloped, hardly 

 distinguishable from hairs or minor dristles ; undistin- 

 guishable in Conops; in St ylofi aster a pair of conspicuous 

 vertical bristles, and distinct fronto-orbital ones. 



Dolicliopodidce, — Only one (outer) pair of vertical 

 bristles ; a post-vertical is so placed that it may be 

 interpreted as the inner vertical pair. Ocellar pair very 

 high on the vertex, between the vertical bristles, and 

 very conspicuous. Cilia of the posterior orbit (Loew) ; 

 humeral (1), post-humeral (2), some intra-humeral (I.), 

 distinct supra-alar and intra-alar bristles (II.). In the 

 dorso-central region two dorso-central outer rows ; two 

 (sometimes only one — Liancalvs) rows of small bristles 

 representing the inner dorso-central rows, and which 

 Prof. Mik has called the acrostichal bristles (Dipterol. 

 Untersuch., Wien, 1878). On the pleura, in Dolichopus, 

 a characteristic prothoracic bristle. (Prof. Mik observes 

 '^ Medeterus has from two to six bristles here, one above 

 the other"). 



Asilidcc. — The cephalic bristles are indistinct among 

 numerous hairs. A pair of ocellar bristles, and another 

 pair (immediately behind) are sometimes discernible. 

 A series of occipito-orbital (often plumose) bristles are 

 homologous to the cilia of the posterior orbit (Loew) 

 of the Dolichopodidce. On the thorax, one {Leptogaster) 

 or more praesutural bristles (I.) are characteristic ; 

 several supra-alar bristles on the post-alar callus (II.) ; 

 and a number of intra-alar bristles {Asiltis) ; in Lepto- 

 gaster, a very characteristic single intra-alar bristle (11. ). 

 Often two longitudinal prsescutellar rows of a few short 

 bristles. On the pleurae, sometimes a few mesopleural 



