FIELD BOOK OF INSECTS. 



Antennae with a dorsal arista 2. 



2. Marginal cell of the wings open 3. 



Marginal cell of the wings closed or petiolate 20. 



3. Anterior cross-vein of the wings distinctly before the 

 middle of the discal cell, almost always perpendicular. . . .4. 



Anterior cross-vein of the wings near, or beyond, the 

 middle of the discal cell, usually oblique 15. 



4. Antennas longer than the head 5. 



Antennae not longer than the head 8. 



5. Side-margins of the thorax yellow. Chrysotoxum; 

 pubescens (.5 in. long) frequents low foliage and bases 

 of trees. 



Side-margins of thorax not yellow 6. 



6. Face with a projecting tubercle and wholly or partly 

 yellow. Paragus; generally fly low in moist, shady 

 places. 



Margin of mouth projecting and face wholly black. 

 Chrysogaster; frequent on spring flowers. 



Face without a projecting tubercle and mouth-margin 

 not projecting 7. 



7. Scutellum flattened, often with spines or tubercles 

 on its border; a vein stump from the third longitudinal 

 vein in the first posterior cell. Mixogaster (abdomen 

 much narrowed at base) and Microdon (abdominal base 

 not narrowed). The slug-like larvae of Microdon live in 

 ants' nests. 



Scutellum without spines'; no such vein-stump; 

 less than .5 in. long. Pipiza; often hover in front of 

 flowers with exposed nectar, such as buttercup. The 

 larvae of radicum feed on root-lice. 



8. Ground-color of the face black. Both Chalcomyia 

 (scutellum large, nearly square) and Myiolepta (scutellum 

 not unusually large, broadest at its base) have the hind 

 femora distinctly thickened. Several genera have the 

 femora but little or not at all thickened; of these the follow- 

 ing have a tubercle on the face but the margin of the 

 mouth does not project 9. 



Ground-color of the face yellowish 10. 



9. Facial orbits separated by a slender parallel groove; 

 metallic green or black flies. Chilosia; commonly found, 

 covered with pollen, on spring flowers. 



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