DICRANOTA. 279 



species {D. pavida Hal. and guerinii Zett., which, however, may 

 be synonyms ; compare Walker, 1. c. p. 306, No. 1), as well as in 

 the two North American species described by me ; they have five 

 posterior cells in two European species (D. ruficornis Schum. 

 and D. bimaculata Schum). The discal cell is open in normal 

 specimens ; it is, however, adventitiously closed in some rare 

 specimens of the North American B. rivularis ; the same seems 

 occasionally to take place among the European species (compare 

 Schiner, Ins. Austr. Diptera, II, p. 530, where the author, speak- 

 ing of the discal cell, always takes care to say " usually" absent). 

 In other respects, the venation is the following (compare Tab. 

 II, f. 1G, wing of D. rivularis, 9) : the subcostal cross-vein is 

 about the middle of the length of the wing or a little before it, 

 at a distance from the origin of the second longitudinal vein 

 which is somewhat variable in different specimens, but always 

 equal to several lengths of the great cross-vein ; the origin of the 

 second longitudinal vein is a little nearer to the root of the wing 

 than is the tip of the sixth longitudinal vein ; the praefurca is very 

 short and arcuated. The small cross-vein is opposite the tip of 

 the sixth. vein ; the second submarginal cell is almost of the same 

 length with the first posterior cell ; the first submarginal cell is 

 but little shorter than the second, as its petiole is very short ; the 

 course of the veins, bordering these cells, is nearly straight ; 'there 

 are two marginal cross-veins ; one very nearly at the tip of the 

 first longitudinal vein ; the other not far from the origin of the 

 anterior branch of the second vein ; the stigma is between them. 

 The anterior fork of the fourth vein, when present (m the species 

 with five posterior cells), is always very short ; the fork of the 

 posterior branch of the fourth vein is nearly twice its length ; the 

 great -cross-vein is at the same distance from the root of the wing 

 as the small cross-vein ; the fifth longitudinal vein is gently 

 arcuated towards the tip ; the sixth and seventh are nearly 

 straight. The European species, judging by the existing figures, 

 in all respects agree in the venation with the American ones 

 (compare the figures of the wing of D. pavida, in Walker, 1. c. 

 Tab. XXX, fig. t a, D. bimaculata, ibid. fig. 7 6); D. ruficornis 

 Schum., if the figure is correct (Schum. Beitr. etc. Tab. IY, fig. 

 2), has both the praefurca and the anterior fork of the fourth vein 

 much longer than the other species. The wings of the females 

 are distinctly broader than those of the males. 



