15 



16 



18 i 



12 DIPTERA OF NORTH AMERICA. [PART II. 



, 4 f Abdomen of male laterally compressed. Gen. XIII. Anepsius. 



( Abdomen of the male not compressed. 15 



f Second antennal joint of the usual transverse shape. 



Gen. XIV. Argyra. 

 I Second antennal joint with a thumb-like projection over the inside of 

 [_ the third. Gen. XV. Syntormon. 



'Third antennal joint in both sexes, or at least in the male, prolonged, 

 pointed, and with an apical arista. 17 



Third antennal joint short even in the male, and if it should be 

 somewhat prolonged, then neither pointed nor with an apical, but 

 [ at the utmost with a subapical arista. 23 



{Second antennal joint with a thumb-like projection over the inner 

 side of the third. Gen. XVI. Synarthrus. 



Second antennal joint without a thumb-like projection, transverse. 18 

 f Posterior transverse vein distant from the margin of the wing ; palpi 

 incumbent. 19 



Posterior transverse vein approximated to the margin of the wing ; 

 I, palpi hanging down. Gen. XXII. Aphrosylus. 



..q ( Hypopygium pedunculated, free. Gen. XVII. Systenus. 



( Hypopygium sessile, more or less imbedded. 20 



or) f The male abdomen has five segments. Gen. XXI. Smiliotus. 



I The male abdomen has six segments. 21 



{Third antennal joint prolonged also in the female. 

 Gen. XVIII. Rhaphium. 

 Third antennal joint of the female not prolonged. 22 



f Third antennal joint of the male very much prolonged (small, less 

 oo I hairy species). Gen. XIX. Xiphandrium. 



i Third antennal joint of the male moderately prolonged (larger, more 

 [ hairy species). Gen. XX. Porphyrops. 



2 o ( Fourth longitudinal vein forked. Gen. XLIII. Psilopus. 



( Fourth longitudinal vein simple. 24 



{Upper side of the thorax convex behind. 25 



Upper side of the thorax behind with a somewhat concave depres- 

 sion. 40 

 cyr ( Fifth longitudinal vein altogether wanting. Gen. XXXVIII. Achalcus. 

 v Fifth longitudinal vein distinct. 26 

 f Distance of the posterior transverse vein from the margin of the wing 

 I equal to its own length or longer. 27 

 1 Distance of the posterior transverse vein to the margin of the wing 

 shorter than its own length. 38 



(Posterior transverse vein unusually oblique. 

 Gen. XXXIV. Plagioneurus. 

 Posterior transverse vein but little oblique. 28 



r Hypopygium distinctly bent under the venter. 29 



28 \ Hypopygium not distinctly bent under the venter or entirely imbed- 

 1 ded. 30 



