64 Annals Entomological Society of America [Vol. XI, 



be solved. I found I had about fifty specimens from the Rocky- 

 Mountains and points west of them, all of general dark color 

 and agreeing with femoralis as identified by Melander ; I also 

 had about an equal number from localities east of the Rockies, 

 all of paler color and undoubted dorsalis. This led me to con- 

 clude provisionally that Malloch had not given sufficient 

 weight to the geographical segregation of the two forms, and 

 that they were probably distinct species. 



The color difference is most conspicuous in the pleura, 

 which are almost wholly yellow in eastern material, and blackish 

 with paler sutures in the western; the dorsum, tibiae, frontal 

 orbits, etc., share in varying degrees in the lighter or darker 

 coloration. 



My observations in Indiana in 1914 seemed to confirm my 

 opinion that the western dark form is a distinct species, as I 

 found no such coloration in Indiana specimens, of which I 

 examined a large number. 



In 1915 I began to make systematic sweepings on grass and 

 grain and tabulate the flies collected. In this I secured the 

 assistance of several entomologists who swept for me in regions 

 that I could not personally visit.* This brought into my hands 

 a large amount of material in Cerodonta (1876 specimens in the 

 season). During most of the season these ran as expected, dark 

 from the Rockies westward, pale from east of that region. 

 Sweepings from Fort Collins, Colo., August 17, showed for the 

 first time in my experience light and dark forms intermingled; 

 but this place is just on the dividing line, where overlapping 

 might be expected. Sweepings from Great Falls., Mont., 

 September 23, showed the dark form some distance east of the 

 mountains for the first time. On October 9th it turn up at 

 Treesbank, Man., where I had had the pale form earlier in the 

 season. On October 19th dark and intermediate forms were 

 swept at Elk Point, S. D., where light specimens had been 

 abundant earlier; and by this time I was noticing that spec- 

 imens swept at Lafayette were becoming progressively darker. 

 I continued my sweepings here as late e,s possible, and on 

 Nov. 27th secured two specimens as dark as any from the 

 west. Specimens from Atherton, Mo., Nov. 6th, were in part 



*I wish especially to acknowledge important and continued co-operation 

 from Messrs. Norman Criddle, Treesbank, Man.; C. N. Ainslie, then at Elk Point, 

 S. D.; and Dr. C. F. Adams, Atherton, Mo. 



