4 Key to Families of North American Insects. 



20. Calypteres small or vestigial; head not hemispherical, the occiput convex. .21 

 Calypteres conspicuous; third antennal joint composed of four to eight annuli; 

 head widely hemispherical; females bloodsucking. Horse-flies, Gad-flies. 

 (Tabanus, Chrysops, Sflvius) (PL 11, fig. 247; PL 12, figs. 258, 283). 



TABANID^E 



21. A slight spur on the middle tibiae only; second submarginal cell widely trian- 

 gular; fourth posterior cell closed; gigantic, tropical flies. (Pantopthalmus) 



(PL 11, fig. 246) (ACANTHOMER1DM} PANTOPHTHALMIDJE 



At least the middle tibiae with evident spurs; second submarginal cell not wide, 



fourth posterior cell usually open 22 



22. Face flat or produced, the facial orbits and the cheeks not sutured; eyes of 

 the male not meeting; antennae of Rhachicerus pectinate and with about 

 thirty divisions. (Xyl6phagus, Rhachicerus) (PL 12, figs. 286, 290) (includ- 

 ing RHACHICERlDfi.) (ERINNIDJE) XYLOPHAGIDJE 



Facial orbits and cheeks separated from the central part; eyes of males meeting; 

 scutellum of Coenomyia spined. (Coenomyia, Arthrfipeas) (PL 12, fig. 

 287) CCENOMYIIIXE 



23. At least the posterior tibiae with spurs; costa encompassing the wing margin, 



anterior crossvein distinct; calypteres vestigial. (Leptis, Chrysopila, Sym- 

 phoromyia) (PL 12, fig. 291). 



(LEPT1DJE=LEPTID1D) RHAGIONUXE 



Tibiae with short or no spurs: costa greatly thinned beyond the tip of the wing, 

 anterior crossvein usually absent or located near the base of the discal cell, 24 



24. Head very small as compared with the greatly hump-backed body; calypteres 



inflated; posterior veins not parallel with the hind margin of the wing; 

 eyes of both sexes broadly contiguous. (Acrocera, Opsebius, OncSdes, 

 Eul6nchus) (PL 11, fig. 245) (ACROCERID&, HENOPIDJE, ONCOD- 



1DM) CYRTDXE 



Head as wide as the depressed thorax; calypteres vestigial; posterior veins 

 parallel with the hind margin, first basal cell very long, its forward border 

 continued obliquely across the wing as a "diagonal vein." (Hirmoneura, 

 Rhynchocephalus) (PL 12, fig. 295) NEMISTRINIDJE 



25. Anal cell much longer than the second basal, either open, or closed in or near 



the margin of the wing, basal cells relatively long, third vein almost always 



forked 26 



Anal cell when present shorter, closed some distance from the wing-margin, 

 if long and acute the third vein is not forked; small crossvein never formed, 31 



26. Vertex plane or convex, the eyes not bulging, eyes of males often meeting; 



legs not robust 27 



Vertex sunken, the eyes bulging and never contiguous; wing- veins numerous; 

 often large species with strong legs 30 



27. Small crossvein present (PL 12, fig. 258, p. c. v.), five posterior cells; abdomen 



rather long and tapering 28 



Small crossvein absent, four or three posterior cells, if five posterior cells pres- 

 ent the extra one is due to an extra vein bisecting the third ; abdomen usually 

 oval . . ... 29 



