266 NEW YORK STATE MUSEUM 



66 Smaller species with pale or yellowish brown abdomen; if not, then 



abdomen is without distinct spots 

 c Thoracic stripes black 



d Thorax yellow ; three vittae on mesonotum, spot below each wing, 

 the pectus and metanotum black, mesonotum highly polished, 

 scutellum brownish yellow ; legs brown, fore metatarsus three 

 fourths as long as its tibia ; length 2.5 mm. ; abdomen pale 



brown; male. (District of Columbia) 2. politus 



dd Abdomen fuscous black ; legs brownish ; length 3 mm. ; female. 



(See sor didellus also) 3. frigidus 



CG Thoracic stripes brown or reddish 



d Abdomen of male with spots on sides of last two segments. East- 

 ern species 4. oceanicus 



dd Abdomen not spotted 



e Species having black tibiae and tarsi ; length 3.5 to 4 mm. 



5. f 1 a v u s n. sp. 

 ee Tibiae and tarsi yellow or pale fuscous ; smaller species 



f Minute yellow species having the crossvein of the wing at the 



basal third ; length .75 to 1 mm 6. s o r d e n s n. sp. 



ff Larger species with the crossvein beyond the basal third of the 



wing ; length 2 to 3 mm -7. s o r d i d e 1 1 u s 



aa Blackish or fuscous species 



6 Wings with an hourglass-shaped spot 20. clepsydrus 



66 Wings not so marked ; halteres dark 



c Wings slightly smoky in both sexes ; for metatarsus over .6 as long 



as its tibia 

 d Fore legs of male long haired; thorax of female anteriorly with 



yellow spots 19. b a r b i c o r n i s 



dd Fore legs of male nearly bare ; thorax of female blackish 



8. nivoriundus 



cc Wings of the male at least, milk white; fore metatarsus about one 

 half as long as its tibia ; larva terrestrial. ..9. stercorarius 

 666 Wings not so marked ; halteres white or pale 



c Legs pale ; thorax with three stripes, sometimes indistinct 



d The fore metatarsus about one half as long .as its tibia ; hind 

 tibiae and tarsi scarcely pubescent ; length 1 to 1.3 mm. 



10. atomarius 

 dd Fore metatarsus more than one half as long as its tibia 



e Thorax yellow with three brown stripes; scutellum yellow, 

 metathorax black ; abdomen brownish, or sordidly yellow ; 

 legs pale yellow, tipped with black ; wings white ; length 2 to 



4mm 7. sordidellus 



ee Usually smaller species (2.5mm. or less) and otherwise not as 



above 



f Fork of the cubitus under the crossvein 

 g F.ore metatarsus a little shorter than its tibia; thorax, 

 including scutellum and pleura, yellow, the three stripes, 

 pectus and metanotum brown ; abdomen fuscous, base and 

 venter yellowish ; length 2 to 2.25 mm. (Greenland) 



11. difficilis 



