218 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



4. Tab anus rhombicus (Tabamis rhombicus Osten Sacken, Prodrome, 

 etc., ii, 472). — A number of specimens from Webber Lake, Sierra Nevada, 

 July 21-27, closely resemble my Tabanus (Therioplectcs) rhombicus from 

 the Colorado Mountains. These specimens show two or even three dis- 

 tinct forms, which I will characterize, in order to draw the attention of 

 collectors to them. I have five specimens of each of these groups, all 

 females. 



1. The lateral triangles ou the second and third segments of the abdo- 

 men are rectangular, — that is, their inner side is perpendicular to the 

 hind margin of the segment, or nearly so j the prolongation of the outer 

 angle toward the lateral margin forms a broad border on the hind mar- 

 gin \ the intermediate triangles are well defined, equilateral, their apex 

 not prolonged in a line reaching to the next segment ; antealar callosity 

 black ; subcostal cell distinctly tinged with brownish ; subcallus de- 

 nuded ; none of the specimens has a stump of a vein on the fork of the 

 third vein. Length 12-13.5 mm . 



2. The lateral triangles on the abdominal segments from 2 to 5 

 are oblique, — in fact, more streaks than triangles ; their prolongation 

 toward the lateral margin is a narrow whitish border of the hind mar- 

 gin ; the intermediate triangles (rubbed off in most specimens) show a 

 tendency to a linear prolongation of the apex toward the next seg- 

 ment j antealar callosity faintly reddish; subcostal cell nearly hya- 

 line ; subcallus either not or only partially denuded ; stump of a vein 

 present in most, not in all, specimens. Length 14-15 mm . 



Intermediate between these two groups of specimens, there is a third, 

 which combines the abdominal markings and the faintly reddish antealar 

 callosity of the second group, with the infuscated costal cell, the perfectly 

 denuded subcallus, the absence of the stump, and the smaller size of 

 the first. My specimens of this group have the sides of the abdomen 

 very distinctly reddish. 



All the specimens were taken indoors, on a window, promiscuously, 

 together with T. phamops and T. insuetus. I took note of the eyes as 

 having " alternate green and dark purple stripes of about equal breadth". 

 The specimen bearing this label belongs to the intermediate group. 

 Another specimen of the same group and one of the first group are 

 marked as having the green stripes broader. 



My specimens from Colorado are nearer to the first form, without re- 

 producing it exactly. The abdomen seems narrower, the abdominal 

 spots less pure whitish. A specimen from Twin Lakes, Colorado, is 

 certainly identical with the second form, and its subcallus is not at all 

 denuded, and it has a distinct stump on the forked vein. 



The uncertainty whether the typical T. rhombicus from Colorado is 

 the same as the Tabani of the first form, the existence of the interme- 

 diate group, and the total absence of male specimens, are so many 

 causes why it would be premature to describe the second form as a 

 separate species. 



