12 DIPTERA OF NORTH AMERICA. [PART II. 



,4 ( Abdomen of male laterally compressed. Gen. XIII. ANEPSIUS. 



I Abdomen of the male not compressed. 15 



( Second antennal joint of the usual transverse shape. 

 ,, J Gen. XIV. AEGYEA. 



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

 L the third. Gen. XV. SYNTOEMON. 



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

 pointed, and with an apical arista. 17 



16 -{ 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 



17 < side of the third. Gen. XVI. SYNAETHBUS. 

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



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



-, n incumbent. 19 



Posterior transverse vein approximated to the margin of the wing ; 



palpi hanging down. Gen. XXII. APHEOSYLUS. 



19 $ Hypopygium pedunculated, free. Gen. XVII. SYSTENUS. 



I Hypopygium sessile, more or less imbedded. 20 



2Q ( The male abdomen has five segments. Gen. XXI. SMILIOTUS. 



( The male abdomen has six segments. 21 



f Third antenual joint prolonged also in the female. 

 21 j Gen. XVIII. RHAPHIUM. 



v Third antennal joint of the female not prolonged. 22 



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

 09 J hairy species). Gen. XIX. XIPHANDEIUM. 



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



t hairy species). Gen. XX. POEPHYEOPS. 



oo f 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 

 or ( 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 

 equal to its own length or longer. 27 



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

 shorter than its own length. . 38 



f Posterior transverse vein unusually oblique. 



27 \ Gen. XXXIV. PLAGIONEUBUS. 

 I Posterior transverse vein but little oblique. 28 

 r Hypopygium distinctly bent under the venter. 29 



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

 1 ded. 30 



