496 C. E. Osten-Sacken on importation q/Diptera. 



pyrastri is confined to the western side of the American 

 Continent. As far as I know, it has never been found in 

 the Atlantic States. As Say discovered it east of the 

 Eocky Mountains more than sixty years ago, it is diffi- 

 cult to understand what prevented it from spreading 

 farther east and reaching the Atlantic Ocean, It remains 

 to be seen whether the increase of the intercourse between 

 east and west in North America, which has taken place 

 lately, will not in the end bring about that result. But 

 it is difficult to conceive why it has not happened earlier. 

 A species which occurs in St. Petersburg, in Egypt, and 

 on the Canary Islands must possess, as to climate, con- 

 siderable powers of adaptation. 



aS*. pyrastri also occurs in Chili (see Macq., D. E., ii., 

 pp. 83 and 88) ; for I have myself seen specimens from 

 there, and this is a new instance of the relationship of 

 the Chilian with the Californian fauna ; but this same 

 fact renders very improbable the hypothesis of the 

 importation of that species on ships. If it had been 

 imported from Europe to California and Chili, it would 

 in all probability have been introduced to New York, 

 Charleston, and other Atlantic ports much earlier. 



S. pyrastri is not a circumpolar insect. I mean to say 

 that the other Syrphi common to Europe and North 

 America {lihesii, topiarius, Zett., ahhreviatus, lapponicus, 

 umhellatariim, L. (?), dnctellus, Zett. (?) ) all occur in 

 Lapland, and therefore may have reached America by 

 way of Greenland, in an earlier geological period. 

 S. jjyrastri does not occur in Lapland, and is not an 

 insect of the extreme north ; therefore its occurrence in 

 Western America can be explained only (barring the 

 hypothesis of importation) hy a migration across the more 

 temperate regions of Asia, and then over the islands, 

 connecting both continents. Still its non-occurrence in 

 the Atlantic States remains unexplained. 



