OF PHILADF-LPHIA. 87 



tarsal joints black ; terguni paler than the thorax, posterior mar- 

 gins of the segments with a definite black band. 



Length to the tip of the wings seven-twentieths of an inch. 



Very common on the Missouri ; I observed it particularly 

 above Cow Island. 



[The genus is called Helomyza Fall. — Sacken.] 



OLFKRSIA Leach. 



(). ALBiPENNls. — Blackish-brown; wings whitish. [102] 



Inhabits Ardea herodias. % 



Mcntum white; thorax with the cruciate lines distinct, the 

 longitudinal line tinged with yellow, humeral tubercle prominent, 

 pale, obtuse; scutel with an impressed line; nervures brown, 

 inner cclhile less than half as loiiii as the preceding one which ex- 

 tends to the base of the wing; pectus with a prominent angle 

 each side between the anterior pairs of feet; tergum pale brown- 

 ish, with a black base, disk and tip. 



Length one-fifth of an inch. 



ORXITHOMYIA Latr. Leach. 



1. 0. NEBULOSA. — Head yellow; feet pale; tibia with two 

 reddish-brown lines. 



Inhabits Sfrix Jiehulosa. 



Eyes blackish-brown ; vagina and hypostoma pale ; thorax 

 reddish-brown, with a large yellowish humeral spot and three 

 longitudinal lines, of which the intermediate one includes an 

 impressed line, which interrupts a transverse impressed line ; 

 humeral angle prominent, subacute ; spiracle w^hite ; marginal 

 nervures blackish-brown, those of the disk brown ; scutel red- 

 dish-brown, varied with yellow at base; pectus yellow-white, an- 

 terior margin bifurcated; tarsi dark reddish-brown, nails black; 

 abdomen pale-brownish, with black hair, first segment on its an- 

 terior face pale-yellow. 



Length three-tenths of an inch. [103] 



2. 0. PALLIDA. — Pale; intermediate cellule of the wing ex- 

 tending nearly to the tip of the outer cellule. 



Inhabits Sj/fn'a si'a/is. 



Eyes blackish-chestnut; antenna; chestnut, tip white; labrum 

 bifurcated, white ; hypostoma whitish ; front yellow-white, a 

 1823.] 



