SYNOPSIS OF NORTH AMERICAN SYRPHID^. 279 



tion that have lorii;' been established in other branches of systematic 

 biology, r mean the greater resembhmce between the Pacific and Euro- 

 peau faunas than between the Athmtic and European. Of course we 

 can yet only speak ab uno (Usee omnes. The Western fauna is not yet by 

 any means well known, but what is known is snificientfor the purpose. 

 Thus, the genus GhiloHia, containing nearly one hundred European spe- 

 cies, is very sparingly represented in the Eastern States, but seems to 

 be rich in the West, and that, too, in European forms (hairy-eyed) not 

 yet discovered east of the mountains. Nearly the same may be said of 

 Sphccrophoria, Melanostoma, and Flaiijehirus. Syrphus, so abundant in 

 Europe, has so far live species peculiar to the East, ten to the West, 

 and eight in common. Arctophila, Pocota, and certain species, occur 

 only in Europe and Western America. The rule has been so often con- 

 firmed that it may be accepted without hesitation, I think, viz: that 

 Eastern American species, identical with European ones, have usually 

 a distribution across the continent. On the other hand, the example of 

 Catabomba pijraairl, an European species found in great abundance in 

 the Western but not in the Eastern region, is a fact bearing in the same 

 direction. To this last species I think we may safely add JEristalis tenax 

 as being primarily Western, but which, during late years, has been 

 distributed from the West eastward. 1 have no explanation to give 

 of bow the European species have reached the Western region, if they 

 are not in reality indigenous there, whether by way of the Atlantic 

 States, or whether the species common to the two sides of the conti- 

 nents have traveled from West to East ; but I certainly think in some 

 cases, such as that of Uristalis tenax, it has been the latter way. 



Of all the numerous species common to Europe and America, very 

 few are confined, so far as we know, to the Eastern States, and every 

 new examination of Western material subtracts from the number. The 

 larger number, so far, of those genera peculiar to America are found in 

 the East, and nearly all the American genera common to Europe are 

 found in the West, Exceptions to this are seen in Eupeodes, Pelecocera, 

 Somula, and especially RJiingia, etc. Again, such genera as Microdoti, 

 Xanthogramma, Ceria, etc., seem to indicate that those genera with few 

 species in Europe are best represented in the Eastern States ; but the 

 facts will allow us to ]>redicate but little here. 



In speaking of the Western fauna, I refer especially to the region 

 limited by climatic conditions, and not extending further east than 

 Western Nebraska, Middle Kansas, and Texas. From several years' 

 active collecting in Western Kansas, I long ago recognized the almost 

 sharp limitation of species. In some cases, especially among Coleop- 

 tera, the eastern lines of habitat could be ti aced almost as clearly as 

 the floral habitats on the mountain side. 



Yet another fact of great interest is the entire absence of several 

 large European genera in our fauua— humerus and Merodon especially. 

 It is very doubtful whether the former will ever be found in the United 

 States, particularly the Eastern ones. 



