12 BRITISH FLIES 



DIPTEEA OETHOEEHAPHA BEACH YCEEA. 



Talk of Families as distinguished hy Venation. 



Although there may be a few exceptional cases the families may almost 

 always be distinguished by the venation alone. 



1 (24) ]5asal cells all three long (at least one-third the length of the 



wing) ; the lower one (anal cell) being often open to the wing- 

 margin, and when closed ending in a point at more than half 

 the length of the anal vein. Alula usually well developed. 

 (Conf. Hyhotiiun in EmimUc.) 



2 (21) Posterior cells with their lower margin not running parallel with 



the wingmargin. Discal cross-vein rarely placed much beyond 

 the middle of the discal cell but commonly before. 



3 (18) Upper branch of the discal vein ending after the wing-tip. 



4 (7) Pnefurca originating almost opposite the base of the discal cell. 



Venation elaborate, and in all cases including the discal cell. 



5 (6) Ambient vein absent. Discal cell always present, pentagonal 



(except in JCylomyinw)] fourth posterior cell open (except in 

 Xylomyina:). I. Stuatiomyid.e. 



6 (5) Ambient vein present (though thin). Fourth posterior cell 



bluntly closed. 11. Acanthomeuid^e. 



7 (4) Prtefurca (jriginating well before the base of the discal cell, or 



{Cyrtidm) the discal cell absent or the venation reduced. 



8 (17) Discal cross-vein normal. 



9 (16) Posterior cells five (except in Hilarinwrpha). 



N.B. — The four following families exliibit the mcst perfect and yet 

 intelligible form of venation in the Brachycera. It is very .similar in 

 all four families but is easily distinguished by a practised eye. 



10 (15) Subcostal vein only moderately long and ending (juite simple. 



11 (12) Cubital fork large, triangularly wide open, and including the 



wing-tip. IV. Tabanid^. 



(Conf. Vermileoihinm and Hilarimoiyha.) 



12 (11) Cubital fork usually much longer than wide (but nut so in 



Vcnnileonviuv and Hilafimorpha), and often hardly including 

 the wing- tip. 



13 (14) Discal cross-vein placed near the base (on at least the basal 



third) of the discal cell. Fourth posterior cell never closed or 

 even contracted. Stigma usually well defined. III. Leptid.e. 



14 (13) Discal cross-vein placed at about the middle of the discal cell. 



Fourth posterior cell often acutely closed and usually con- 

 tracted. VIIT. Therevid.^]:. 



