Knob — On Some North American Species of Microdon. 135 



Bigot's description is from a specimen of the form with copper-tinted 

 mesonotum which apparently had lost its abdomen. The specimens de- 

 scribed by Cockerell and Andrews as a subspecies of Microdon tristis 

 (Proc. U. S. Nat. Mus., vol. 51, 1916, p. 55) are of this form. The 

 thoracic pile of all these western specimens is more yellow than in the 

 eastern ones. 



Originally described from Washington Territory. In addition to the 

 two specimens from Boulder, Colorado, above mentioned, there are before 

 me specimens from Kaslo and Ainsworth, British Columbia (R. P. 

 Currie); Hoquiam, Washington (H. E. Burke); Lake Tahoe, California 

 (Hubbard and Schwarz); North Cheyenne Canyon, El Paso County, 

 Colorado (A. B. Champlain); Douglas County, Colo., reared by C. T. 

 Greene from larvae found with Formica truncicola integroides variety 

 haemorrhoidalis Emery.* In these specimens the thoracic integument 

 ranges in coloration from dull coppery to metallic blue. A nearly black 

 specimen from Clyde, Colorado, 10,000 feet (Daecke collection), ap- 

 proaches the eastern form in coloration. + 



There is a considerable number of puparia before me, in most cases 

 associated with imagos. These puparia show characteristic reticulations, 

 with rows of heavier ridges dorsally and at the sides. The puparia of the 

 eastern and of the western forms of cothurnatus are indistinguishable, as 

 is evident from the material before me. 



Microdon tristis Loew. 



This species resembles cothurnatus in general appearance, but differs 

 markedly in the broader frons, the longer and more slender antennae, the 

 shape of the more prominent and strongly dentate scutellum, the elongate, 

 narrower abdomen and the different arrangement of the white pile on 

 the latter. 



There are before me six specimens from Inglenook, Pennsylvania, col- 

 lected by V. A. E. Daecke and W. S. Fisher; also a female from Lotell, 

 Pa. (Daecke). The species has never been found in the vicinity of 

 Washington and the type undoubtedly came from the mountainous por- 

 tion of Virginia, probably Berkeley Springs (now in West Virginia), 

 where Osten Sacken spent vacations. According to Johnson it ranges 

 as far northward as Massachusetts. 



Microdon ruficrus (Williston). 

 Microdon tristis var. ruficrus Williston, 1886, Synopsis No. Amer. Syr- 

 phidae, p. 7. 

 This species, which was described as a varietal form of cothurnatus 

 (" tristis' 7 Williston), appears to have remained unrecognized by subse- 

 quent workers. Aside from its superficial resemblance, it is one of the 

 most distinct species. This is not apparent from the original description, 



•Determination S. A. Rohwer. 



t Charles R. Jones describes a Microdon similis, from Colorado, in Annals Ent. Soc. 

 Amer., vol. 10, No. 2, p. 219 (June, 1917). The description is wholly inadequate, but as 

 far as it goes answers very well for one of the darker forms of cothurnatus. The length 

 indicated, 14 mm., exceeds the largest specimen now before me by only one millimeter. 



