July 1S91. 



PSTCHE. 



107 



west they approach to within a few 

 miles. This was observed May 23d- 

 24th, while in the valley itself at this 

 time they were almost unbearable. 



These gnats are a great annoyance to 

 man, by far greater than any other in- 

 sect that we have in this locality. Many 

 persons are so susceptible to them as to 

 preserve through the height of the gnat 

 season a chronic inflammation of the ex- 

 posed parts of the face and neck resulting 

 from repeated bites, which cause an in- 

 tense irritation and even give rise to 

 cutaneous sores. The inclination of the 

 gnats to bite increases with the advance 

 of the season, but the pest is consid- 

 erably abated after the fall of the 

 water. They are also very trouble- 

 some to animals, and are supposed 

 to cause the inflamed eyes in the 

 horses of this region through the sum- 

 mer months. I append a description 

 of the species. The female alone is de- 

 scribed, as that is the only sex which 

 composes the biting swarms, and I have 

 not secured either the male or the early 

 stages. 



Sitnulium occidentale, n. sp. J . Cinereous, 

 abdomen light fulvous. Head cinereous, eyes 

 black ; face cinereous, raised, somewhat darker 

 in the centre, sparsely clothed with fine sil- 

 very hairs; front cinereous, widened below 

 into a cross-bar, a prong invading the orbital 

 area on each side, silvery pubescent on the 

 orbital margin, and with longer pubescence 

 on the occipital margin; proboscis black, 

 brownish at tip, palpi black ; antennae cinere- 



ous, with short, silvery pubescence, the two 

 basal joints longer than the following joints, 

 which are nearly equal in length; occiput 

 cinereous, with silvery pubescence around the 

 margin. Thorax cinereous, mesoscutum en- 

 tirely covered with silvery pubescence, with 

 two dorsal lines, and usually a fainter median 

 line between them ; pleurae fulvous posteri- 

 orly; scutellum black, silvery pubescent. 

 Abdomen light fulvous, sparsely covered 

 with short silvery pubescence; second, third 

 and fourth segments above with a brown 

 cross-band shading to darker on the sides 

 and in the middle, particularly on the third 

 and fourth segments; remaining segments 

 with a broad, median, dorsal, cinereous band, 

 bounded laterally on the fifth, sixth and 

 seventh segments by a curved, more or less 

 faint line of brown; venter light fulvous, 

 silvery pubescent. Legs black, silvery pub- 

 escent. Wings hyaline, iridescent bv re- 

 flected lights; halteres white. Length of 

 body 2 mm ; of wing 2 mm. 



Described from many fresh specimens. 



This species is smaller than either 

 S. pecuarum or S. meridionale. S. 

 metallicum Bell, from Mexico is given 

 as 2 mm. long, but it is the male which 

 is described, and the female would be 

 very much larger. S. occidentale dif- 

 fers from S. pecuarum very markedly 

 in the thoracic and abdominal markings. 

 These markings are very much like 

 those of S. meridionale ; but the 

 median thoracic line is always very 

 faint, the abdomen is light fulvous, the 

 lateral lines of segments 5, 6 and 7 are 

 curved, and the abdominal markings 

 are of a different color, besides other 

 minor differences. 



