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



macrochaetffi on the thorax ; but as their bases are 

 hidden by other, shorter hairs, their homologies can 

 only be guessed at ; on the head there is an ocellar pair, 

 but no other macrochastae. In accordance with this, 

 Toxophora is among the least aerial among Bomhylidce ; 

 it has short wings, and remains most of the time on 

 flowers. Mulio ohscurus has many macrochaetse about 

 the thorax, the nomenclature of which is as uncertain 

 as that of the macrochsetae of Toxophora ; the abdomen 

 of Mulio also shows conspicuous rows of large bristles. 

 The North American Systropus, although so little hairy 

 that the macrochaetae would be easily percejitible, has 

 none whatever. The same may be said of Phthiria. 

 In the genera of Antlinicina and Bomhi/Una, which form 

 the bulk of the family, the dense hairiness or fur pre- 

 vents us seeing any macrochaetae ; still, traces of them 

 may be occasionally seen ; for instance, among the 

 yellowish fur of some Bomhylii a pair of black hairs are 

 often visible above the root of the wing, which may 

 represent supra-alar bristles; in Anthrax flava and con- 

 geners a tuft of macrochaetae exists on the post-alar 

 callus. 



J'herevidce. — With the species of Thereva, which are 

 covered with a dense fur, we experience the same diffi- 

 culty as with the Bomhi/lidce; but that difficulty vanishes 

 with the glabrous or subglabrous species, and here we at 

 once observe much more marked homologies with the 

 normal chatotaxy of the other families. There are none 

 of the normal cephalic pairs of bristles ; even the ocellar 

 pair, elsewhere so persistent, is wanting. No humeral 

 bristles ; a row of three or four bristles, which must be 

 considered as post-humeral, as they are inserted on the 

 triangular praesutural depression (which is distinctly 

 visible here, although it cannot exactly be called a 

 depression). Two anterior supra-alar bristles (on the 

 edge of the anterior supra-alar cavity), and one posterior 

 (on the post-alar callus). Two pairs (sometimes only 

 one) of praescutellar bristles, one exactly in front of the 

 other. Four scutellar bristles. 



Stratiomyidce, Tahanidce, Lcptidce, AcantJiomeridce, and 

 Xylophagidce (if such a family exists, of which I am by 

 no means certain) are entirely destitute of macrochaetae, 

 and form a natural group, the Diptera eremochaeta par 

 excellence. 1 have never met with a single instance of 

 the presence of any macrochaetae in those families. This 



