PROC. ENT. SOC. WASH., VOL. 20, NO. 9, DEC., 1918 199 



BB. Anterior and posterior branches of 3rd vein joining in a 



fork; cross veins not dark clouded .bicolor 



AA. Body not chiefly yellowish; wings strong; robust species. 



C. First three segments of abdomen chiefly yellowish red, re- 

 mainder black cinctus 



CC. Abdomen otherwise. 



D. Abdomen without a median line of light spots or other 

 longitudinally arranged pale markings. 



E. Wings wholly blackish atratus 



EE. Wings otherwise. 



F. Wings dark brown at base, smoky apically, with 



dark spots at forks of 3rd and 4th veins, .nigrescens 

 FF. Wings smoky along costa, without dark spots. 

 G. Wings hyaline apically; abdominal segments 



with pale posterior margins americanus 



GG. Wings brownish yellow; abdominal segments 



without pale margins giganteus 



DD. Abdomen with longitudinally arranged pale markings. 

 H. The pale markings form or include a continuous central 

 stripe the whole length of abdomen. 



I. Costal cell hyaline; front of female distinctly con- 



vergent anteriorly lineola 



II. Costal cell yellowish. 



J. Flies 15 mm. or more in length, more robust. 



K. Front narrow; upper angle of 3rd antennal 



joint prominent fulvulus 



KK. Front broad; upper angle of 3rd antennal 



joint very low sagax 



JJ. Flies usually 12 mm. or less in length, more 

 slender. 



L. Thorax gray; hind tibiae faintly brownish at 

 tip; lateral rows of spots on abdomen usually 



faint nigrovitialus 



LL. Thorax yellowish; hind tibiae distinctly 

 black at tip; lateral rows of spots on abdo- 

 men usually conspicuous costalis 



HH. The pale markings do not include such a stripe but consist 



of more or less separated triangles or spots. 

 M. The pale markings consist chiefly of a single median row 

 of white triangles or spots. 



N. Triangles only on segments 3 to 5 trimaculatus 



NN. Triangles or spots on all abdominal segments, though 



sometimes small on 1, 2 and 6. 



O. Posterior margins of abdominal segments pale. 

 P. General color of abdomen reddish brown. 



