AMERICAN DIPTERA. 201 



shown when a newly-emerged fly of the same species is set adrift. 

 The flies gather about until a ball of dozens of individuals is float 

 iug down stream. Soon the ball collides with some object and 

 breaks up ; the flies then resume their zig-zag path up stream to 

 their former haunts, while others gather about the floating nymph. 



In these Hilara swarms the males are much more numerous. 

 Their dilated fore-metatarsi perhaps serve in keeping the flies just 

 touching the water. The females of the European species are said 

 to fly about the bushes along the edges of the streams, but in trivit- 

 tata they mingle with the males in their strange dance. Some of 

 the European species form balloons of spun web somewhat similar to 

 those made by the North American Empis (Erohatica during court- 

 ship, as described by Aldrich and Turley (Am. Nat., Oct., 1899). 



Trivittata appears regularly at mid-winter and remains nearly two 

 months hovering over the smaller streams. 



In conclusion, I desire to thank Dr. Wheeler for the use of his 

 library, and for similar courtesies Dr. Williston, Mr. Henshaw and 

 Prof. Comstock. For assistance in looking up bibliography I wish 

 to express my thanks also to Miss Mabel Evans and to Mr. Henry 

 Barroll ; and especially for the kindness and liberality of the gentle- 

 men whose collections have made this work possible I again tender 

 my earnest appreciation. 



Table of Genera. 



Aual cell wanting, or if present very small and always the anal vein wanting. 



Fore coxse shorter than the femora (Tachydkomiin^) 6. 



Anal cell complete, when wanting the fore femora are shorter than the fore 



coxae 2. 



2. Body without macrochgetfe ; anal cell reaching the margin. 



(Mythicomyin^) 28. 

 Anal cell closed before border when present 3. 



3. Anal angle of wing not projecting, outline of wing more or less cuneiform ; 



fore coxse greatly lengthened (Hemekodromiin.e) 11. 



Anal angle more or less developed ; wings not cuneiform in outline ; fore coxse 

 shorter than the femora 4. 



4. Anal cross-vein perpendicular or forming an acute angle with the basal por- 



tion of the anal vein; proboscis short; thorax prominent, generally 



hunchbacked (Hybotin^) 18. 



Anal cross-vein parallel with the hind margin of the wing and forming an 

 obtuse angle or curve with the anal vein ; proboscis often elougate ; 

 thorax generally not hunchbacked (Empidin^) 29. 



TKANS. AM. ENT. SOC, XXVIII. (26) JULY, 1*02. 



