138 Proceedings of the Biological Society of Washington. 



All have the mesonotum, scutellum and second abdominal segment 

 metallic green-bine covered with yellowish pile; the abdomen beyond the 

 second segment is wholly deep black and clothed with short, inconspicuous 

 black pile. The scutellum is convex and unarmed. In the male the frons 

 is much narrower than the width of one eye and strongly narrowed 

 toward the antennae. Apparently this species is confined to the Appa- 

 lachian region. 



Aldrich has placed Microdon bombiformis as a synonym of megalogaster 

 Snow, a species of uncertain origin;* in this he has been followed by 

 Johnson,! Cockerell and Andrews,* and Banks, Greene, McAtee and 

 Shannon. § I am unable to concur in this synonymy, since Snow's de- 

 scription, although it corresponds as to general coloration, disagrees in 

 important details. Thus, the scutellum is stated to be "gently emargin- 

 ate, the small obtuse tubercles approximate." All the specimens of 

 bombiformis examined by me have the posterior margin of the scutellum 

 evenly rounded and convex, without the slightest trace of tubercles or 

 emargination. I must consider this difference fundamental. Further- 

 more, Snow states that in megalogaster the front tibiae and metatarsi are 

 clothed on the inner side with short golden pile; in bombiformis the front 

 tibiae have a patch of dense yellow pile, visible only in certain lights and 

 occupying slightly less than the distal half; similar pile occurs ventrally 

 on the front metatarsi. || 



Microdon coloradensis Cockerell and Andrews. H 



This species, like the preceding one, resembles a bumble-bee in appear- 

 ance and is further remarkable for its striking sexual dichroism. Prof. 

 Cockerell informs me that he has procured additional specimens since the 

 description was drawn up and that the sexual color differences prove to 

 be constant. The female from Pecos, New Mexico, in the national col- 

 lection belongs here, as already indicated by Cockerell and Andrews. 



Along with the two following ones, this species is remarkable for the 

 broad short frons, the antennae being inserted unusually high upon the 

 head. The male coloradensis has the frons considerably over one-third 

 the width of the head and broadening regularly and very gradually 

 toward the antennae. In all three species the third antennal joint is 

 short and very broad near the base, shortly lanceolate in outline. 



* Cat. North Anier. Diptera. 1905. pp. 345, 346. 



+ Psyche, vol. 23. 1916, p. 76. 



t Proc. U. S. Nat. Mus., vol. 51, 1916, p. 53. 



§ Proc. Biol. Soc. Wash., vol. 29, 1916, p. 176. 



|| Since going to press I have been able to examine 15 additional specimens 

 M. bombiformis taken by Mr. W. S. Fisher at Inglenook, Pa., June, 1917. In two of the 

 males very minute tubercles are perceptible on the scutellum; a few other males show 

 slight apical indentations on the scutellum. None of the five females show any modi- 

 fication of the scutellum. It should be noted that the pile of the scutellum is unusually 

 long and dense in this species, so that very minute tubercles can hardly be detected 

 without its removal. This leaves only the difference in the color of the pile on the 

 front tibias to separate the species from megalogaster. 



II Proc. U. S. Nat. Mus., vol. 51, 1916, p. 53. 



