North American Dittera. 17 



1. All four posterior veins arise from the disual cell; head small; 



scutellum with spines. ..... Ccenomyia 



The last posterior vein arises from the second basal eoll ; head 

 not small ; scutellum without spines. .... 2 



2. Fourth posterior cell closed; third joint of antennje much elon- 



gate, composed of numerous distinct divisions, often pec- 

 tinate ; eyes emarginate near the antennae. IIh.^chicerds 

 Fourth posterior cell open ; third joint of antennae composed of 

 eight annuli, indistinctly separated. Xylophagus 



Rhachicerus Haliday, in Walker, List, etc. V, 103, 1854. 

 Xylophagus Meigen, in Illiger's Magazine, II, 266, 1803. 

 BoLBOMYiA Loew, Bernstein und Bernsteinfauna, 39, 1850. 

 Ccenomyia Latreiile, Precis des Caract. Gener. etc. 1797. 

 SuBULA Meigen, Syst. Beschr. II, 15, 1820. 

 Arthropeas Loew. See Leptida;. 

 Arthroceras Williston. See Leptidae. 

 Glutops Burgess. See Leptidae. 



STRATIOMYID^: 



Head large, hemispherical. Antennae situated at or below the 

 middle of the head in profile, short or elongate, the third joint com- 

 l)osed of more or less distinct annuli ; usually with a more or less 

 differentiated style, or bristle. Eyes bare or pilose ; ocelli present. 

 Scutellum often with thickened spine-like points. Abdomen elon- 

 gate, clavate, oval, or rounded, flattened or convex above, of five or 

 more visible segments. Legs never very stout, often slender, tibise 

 without spurs (except in some exotic genera.) Wings usually with 

 the veins more or less crowded anteriorly, often on the outer poste- 

 rior part weak and faint; third longitudinal vein usually furcate, 

 terminating before the tip of the wing ; four or five posterior cells, 

 all open, the posterior veins not seldom more or less rudimentary. 



Species from three or four to twenty or more millimeters in length, 

 liare or moderate pilose. They are mostly flower-flies, and are often 

 found upon vegetation in the vicinity of damp places. Larvae pu- 

 pigerous, that is the pupae remain within the larval skin till ready to 

 emerge as perfect insects. 



1. Abdomen with seven visible segments. P. k u i i> i n .♦; 



Abdomen with only five or six visible segmints. ... 2 



