14 INSECUTOR INSCITI^ MENSTRUUS 



evenly feathered, the central spine sometimes a little stouter 

 and longer. Air-tube about two-and-a-half times as long as 

 wide, the pecten evenly spaced, running nearly to the middle, 

 followed by a 6-haired tuft. Anal segment with the dorsal 

 plate reaching two-thirds, irregularly incised. 



Males were seen swarming at Dawson over willow bushes 

 on the hillside after sunset. They were in small groups, flying 

 rapidly from one place to another, much as with the allied 

 excrucians and fitchii. 



Total, 65 specimens: Dawson, Yukon Territory, July 7-11, 

 13, 15, 16, 17, 19, 1919. 



This is very close to stimulans Walker, and I am not sure 

 that it should be separated. I have proposed the new name 

 largely on account of the discontinuous distribution. The 

 western form of stimulans referred to above, which inter- 

 venes between eastern stimulans and mercurator, shows larval 

 differences ; yet here the typical larva reappears in abundance 

 in the Yukon Valley. This valley becomes very broad in the 

 "Flats" below Fort Yukon, Alaska, and doubtless mercurator 

 has its stronghold here where many overflowed pools must 

 occur. 



The larva differs only in the head hairs, stimulans having 

 them upper in 2, lower single (the statement in the monograph, 

 page 681, is accidentally reversed), while mercurator has upper 

 4 or 3, lower 2 or single as stated. 



In the male genitalia the spine of the basal lobe of the side- 

 piece is longer in mercurator than in stimulans and a little 

 more basally situated, being on the edge of the chitinization 

 instead of a little removed therefrom. The basal lobe is round 

 and full, openly tubercular with moderate short setae, not 

 "tubercular-expanded." In stimulans one of my mounts 

 (Plattsburgh, New York) shows the basal lobe "tubercular- 

 expanded" but this may be due to excessive pressure in prepa- 

 ration. Another mount from the same place does not show 

 this peculiarity, neither do others from Detroit, Michigan, 

 May, 1909 (B. F. Lowe), Arnprior, Ontario, May, 1917 (C. 

 Macnamara). and Oxbow, Saskatchewan (F. Knab). In these 



