OSTEN SACKEN ON WESTERN DIPTERA. 215 



SiLVIUS. 



Sllvius GiGrANTULUS (sju. Chri/sops gigantulus Loew, Centur., x, 12 ; 

 Silvius trifolium Osten Sacken, Prodr. of Tabanidje, i, 395). — My descrip- 

 tion was drawn from alcoholic specimens, and requires some emenda- 

 tions. 



The three-fold spot at the base of the abdomen is sometimes quite dis- 

 tinct, in other cases very faint. A longitudinal stripe in the middle of 

 the abdomen, formed by an appressed golden-yellow pubescence, is very 

 distinct in most, but not in all specimens. The black spot on the venter 

 is often wanting. Front tarsi black, faintly reddish at base. One of 

 the specimens (Colorado) has no denuded spots on the face ; the ocellar 

 area likewise is not denuded ; the legs are altogether reddish, except 

 the infuscated ends of the tarsi. 



Hah. — California, especially in the Sierra Nevada and along its foot- 

 hills (Mariposa County, June 3, not rare; Yosemite Valley, June 13; 

 also about Webber Lake, Sierra County, July 20) ; Washington Territory ; 

 Vancouver Island (J. E. Crotch) ; Southern Colorado (W. L. Carpenter). 



The living insect has bright yellowish-green eye?., with numerous 

 irregularly scattered black dots ; this agrees with Meigen's and Loew's 

 description of the eyes of the other species of the genus. 



In my former description, line 2, for regular spots, read angular. 



That Mr. Loew took this species for a Chrysops is, I suppose, nothing 

 but an oversight on his part. 



Tabanus. 



Of the seven species of Tahamis from California which I possess, three 

 belong to the high Sierra, and four were found in the vallej'S of the 

 Coast Eange. Four of these species belong to the subgenus Therioplectes^ 

 one is apparently an Atyloius, and the remaining two are true Tabani. 



All descriptions of species of the difQculfc group of Theriopleetes must 

 be necessarily imperfect as long as they are based on female specimens 

 only. I have done my best to render them clear, and have taken note 

 of the coloration of the eyes of the living specimens. As far as my 

 knowledge goes at present, these two subgenera may be defined as fol- 

 lows : — 



Theriopleetes. — Eyes pubescent, with three or four purple cross-bands 

 and intervening green intervals; ocelligerous tubercle more or less dis- 

 tinct ; head of the male not differing much in size and shape from that 

 of the female ; the difference in size between the large and small facets 

 on the eyes of the male is but very moderate, and the dividing line be- 

 tween them indistinct. (Species Nos. 1-4.) 



Atyloius. — Eyes pubescent, with a single narrow purple cross-band ia 

 the middle, or uuicolorous (?); no vestige of an ocellar tubercle ; frontal 

 callosity either entirely wanting or imperfect (in the Californian T. in- 

 suetus it is narrow and does not reach the eyes on either side, which is 



