266 NEW YORK STATE MUSEUM 



"bb 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. p o 1 i t u s 



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



( See sordidellus 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. sordidellus 



aa Blackish or fuscous species 



6 Wings with an hourglass-shaped spot 20. clepsydrus 



J)b Wings not so marked ; halter es 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. barbicornis 



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 

 6&6 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. a t o m a r i u s 

 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 



4 mm 7. sordidellus 



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

 above 

 f Fork of the cubitus under the crossvein 

 g Fore 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 



