1891.] 37 



the male sex (Bibio, Rh/phus, S'mulium), but also in both sexes 

 {Orphnefliila, and some Blephnroceridcd) . The eyes are often bisected, 

 the facets upon the upper-side being larger ; these two halves of the 

 eye are sometimes bicolorous {Simulium), or they are separated by a 

 distinct groove, or even by a transverse unfacetted stripe {Blepharo- 

 ceridce, male Bibio). Empodia and pulvilli, in some genera, have an 

 uncommon development ; the legs are sometimes particularly strong 

 and even fit for walking {Bibio, DilopJms, Simidimn) ; the sexes (in 

 these same three genera) are remarkably differentiated in their whole 

 appearance. The number of genera in these families is small (only a 

 single one in the Simulidce and OrphnephilidcB, three in the Wiyphidce) ; 

 the number of species in these genera, in most cases, is likewise 

 small (except in the Bibionidce and SimulidcB). A strange monotony 

 prevails, in forms and colours, within the same genus, notwith- 

 standing an extensive, sometimes sporadic, geographical distribution ; 

 the metamorphoses are also peculiar. All these peculiarities prove 

 'that, in a rational classification, these five small families should be 

 separated from the true Nemocera. 



Having thus disposed of the old group of OrthorrhapJia Nemocera, 

 we reach now that of the Orthorrhapha Brachycera, and we meet at 

 once with a group of families which have been for a long time con- 

 verging towards each other during the successive changes in the 

 systematic arrangement, but which found the keystone for their final 

 association only since the application to them of a chaBtotactic 

 character, the total absence of macrochastaB. This group consists of 

 the families Strntiomyid^, Tabatiidce, AcantTiomeridce, and Leptidce 

 (plus XyJophagidee). Among the Orthorrhaplia Brachycera it repre- 

 sents the largest agglomeration of bristleless forms, and deserves for 

 this reason the name par excellence of the Section Eremochceta. 

 Besides the chaetotactic character, these families are strongly bound 

 together by other structural peculiarities, which naturally separate 

 them from the Nemocera, as well as from the remainder of the 

 Brachycera. 1. The eyes (in the male) are predominantly holoptic ; 

 exceptions are either only apparent (as subcontiguous eyes, or, in the 

 Sargidce, approximate eyes in one genus, and absolutely contiguous 

 ones in the very next one) or very rare {Xylophagus, with broadly 

 dichoptic eyes in both sexes, some Australian Chiromyzce, Hermetia, 

 and some others may be considered as exceptions ; but even Xylo- 

 phagus has in Coenomyia a close relative, which is holoptic ; and so has 



