1911] Aldrich — North American Species of Hydropkorus 57 



out the localities so that they cover practically the entire United 

 States. This is undoubtedly the most wide-spread species that 

 we have; it is also considerably variable in several of its characters 

 notably the amount of white pruinosity it bears. A specimen 

 sometimes looks quite metalhc on one side, showing that a slight 

 amount of rubbing easily takes off the "dull finish" provided 

 by nature. The coppery stripes on the thoracic dorsum are 

 sometimes only green, sometimes almost obsolete. In general, 

 it may be described as a small species with considerable whitish 

 pruinosity, opaque face, white hairs all over except the distal 

 portion of the legs, and with hyaline wings, unspotted, the veins 

 yellow at base. Loew failed to mention the paleness of the 

 wingveins, which misled Professor Wheeler. Mr. Brues compared 

 some of my material with Loew's types of oestuum at the museum 

 of Comparative Zoology, and could find no material differences. 

 Osten Sacken, in his Western Diptera, p. 320, probably had 

 this species, — the second undetermined species mentioned. 



Hydrophorus philombrius Wheeler (fig. 14). 



Male. Occiput moderately shining green, the postvertical bristles only a single 

 pair, vertex considerably excavated on each side of the ocelli, which are on a tubercle; 

 front shining green, concave, wide above; face concave above the middle, smooth 

 and brilliantly shining, the metallic color usually continuing down over the suture, 

 which is elevated on a protuberance; there is considerable variation in the brilliance 

 of the color, even on the upper face, but it is always perceptible; below the pro- 

 tuberance sloping backward to the palpi, which are darker with brownish hairs. 

 The cheek a thin sharp plate, yellowish sericeous, standing straight down on each 

 side of the mouth; the face widens rapidly from a point just below the antennae; 

 the black postorbital bristles extend down behind the eye only about to the middle; 

 beard whitish and dense, making it impossible to tell certainly whether there are 

 any black bristles under the neck, though I think there are; antennae short, the 

 first and second joints of about equal length. Thorax above striped with green 

 and coppery, the former color occurring along the rows of bristles; only a single 

 row of acrostichals, beginning rather far back; dorsocentrals small except the 

 hindmost one; scutellars four, strong; two humerals; the hind notopleural small; 

 pleura rather dull with gray pruinosity, almost destitute of fine hairs; propleura 

 however with whitish hairs, longest just below the single black bristle; front coxa 

 with a somewhat sericeous whitish pruinosity, and fine white hairs, also at the 

 upper outer corner two or three small black bristles and five or six on the outer 

 side of the front near the tip; femur thick at base, rather bare, below with two rows 

 of bristles at base, the outer of only about four rapidly increasing ones and the inner 

 of five or six of rather uniform size, and beyond the latter a few extremely short 

 little stubs showing above the surface; front tibiae strongly curved inward at tip. 



