216 NORTH AMERICAN DIPTERA 



Many of the adults have very unusual mating dances which may 

 be observed without difficulty as the species are common and soon 

 return to their "mud flats'' if disturbed. An excellent account of 

 several of the species is given in the revision of Dolichopus by Van 

 Duzee, Cole and Aldrich. 



Little is known about the immature stages which are passed, for 

 the most part, in mud, although some species live in the stems of grass 

 and those of Medeterus live under the bark of trees and are definitely 

 predaceous. Some species are said to feed upon decaying vegetation 

 but they are probably predaceous. 



The males are easily determined but the females are often difficult 

 to name as they present less striking characters than the males. The 

 American species were revised by Becker* but so many new forms have 

 been described since that this work will furnish only a basis for the 

 study of the family. The females present few structural characters 

 and it is difficult to separate a few of the genera except by association 

 and familiarity with the group. Several characters not previously used 

 are employed in the key presented and most of the females are keyed 

 out separately. With a little experience this sex should be as easily 

 located in the correct genus as is the opposite sex and tlie males will 

 trace out quite readily in this section of the key, although two or three 

 genera are omitted as females are not available. 



KEY TO GENERAf 



1. Fourth vein with a widely divergent fork beyond the posterior cross- 



vein and with an appendage at the bend (16); mesonotum scarcely 



longer than wide 2 



Fourth vein without such fork though often with strong curvature; 

 mesonotum usually conspicuously longer than wide 8 



2. Front scarcely excavated above from anterior view 3 



Front strongly excavated above f roin anterior view 4 



3. Sixth (anal) vein present Psilopiella Van Duzee 



Sixth vein absent Leptorhethum Aldrich 



4. Arista not differentiated from the third antennal segment which is 



produced style-like and extremely long Megistostylus Bigot 



Arista clearly differentiated 5 



5. Arista apical Chrysosoma Guerin 



Arista dorsal, rarely sub-apical 6 



6. Lower part of the face with hairs (83) Laxina, n. g. 



Lower part of the face without hair 7 



7. Scutellum with four bristles (16, 85) Condylostylus Bigot 



Scutellum with a pair of strong bristles and sometimes a we:ik basal 



hair on either side (17, 71) Sciapus Zeller 



* 1922 (1921). Abh. Zool.-bot., Gesselsch., Wien, xiii, pp. 1-395. 



t Syntormon Loew appears to be distinct from Synarthrus to which our American species 

 belong. 



