184 Zoology of Colorado 



Florissant. Today they are excessively rare in Colorado, and I 

 have never captured a specimen. Mr. L. 0. Jackson had the 

 good fortune to find Rhynchocephalus sachcni of Williston visiting 

 flowers of Eriogonum umbellatum at Boulder, June 20, 1922. 



The Asilidae, or Robber-flies, are usually large with long 

 tapering bodies and long legs. They prey on other insects, which 

 they may often be seen carrying. I find 27 species recorded 

 from Colorado. One of these, Dasyllis fernaldi of Back, curi- 

 ously resembles a bumble-bee. 



The Bombyliidae, so-called Bee-flies, are also numerous. In 

 the Miocene shales at Florissant are many genera and species, 

 but these represent a fauna almost wholly distinct in character 

 from that now living in Colorado. For some reason not explained, 

 the Miocene genera disappeared, and were replaced by unrelated 

 genera of bee-flies which now occupy the country. The fuzzy 

 Bombylius major of Linnaeus is circumpolar, being found equally 

 in Britain and Colorado. Phthiria sulphurea of Loew is a small 

 yellow species which may be found on yellow flowers, such as 

 sunflower or Grindelia. It is sometimes caught by the predatory 

 yellow bug Phymata fasciata. The Therevidae, or Stiletto-flies, 

 are not often noticed. Thereva cockcrelli Cole is a silvery-white 

 fly, about nine mm. long, discovered in Peaceful Valley, Boulder 

 County, but ranging north to Manitoba and Michigan. The 

 Dolichopodidae are small long-legged usually bright metallic-green 

 flies, often seen running on foliage. The legs of the males are 

 often curiously formed or ornamented. Although these flies are 

 plentiful today, we cannot find any of them fossil in the shales. 



Syrphidac 



The Flower-flies, Hover-flies, Drone-flies, etc., are conspic- 

 uous on flowers everywhere, and many of the species are beneficial 

 to man, the larvae feeding on plant-lice. There are however 

 some very curious species, with thick bodies, the slug-like larvae 

 of which occur in ants' nests. These belong to the genus Microdon, 

 and the larvae are so much like molluscs that they have on more 

 than one occasion been actually described as such. A very fine 

 new species of this genus, Microdon coloradensis Cockerell and 

 Andrews, was bred from the larva in Boulder. A second species, 

 obtained at the same time, was considered a variety of M. cothur- 



