October, 188G.} 07 



. SOME NEW FACTS CONCERNING ERISTALIS TENAX. 

 BY C. R. OSTEN-SACKEN, Hon. F.E.S. 



In the Trans. Eiit. Soc. Loud., 1884, p. 489, I called attention to 

 the sudden appearance of Eristalis tenax in all parts o£ the United 

 States. Till ISy.'i this fly was not known to occur in North America ; 

 two years later it was common in Boston ; it is now called " common " 

 and "very common" in local lists from Montreal, Canada (Caulfield, 

 Can. Ent., 1884, p. 138), and Philadelphia (E. L. Keen, I. c, p. 146), 

 and it has been received from all parts of the United States, including 

 California, New Mexico, Oregon, and Washington Territory. 



Two explanations of this sudden invasion were possible (I quote 

 from my article) : " E. tenax may liave been imported from Europe in 

 ships to one of the harbours of the Atlantic. But if this importation 

 happened long ago, it would have been noticed earlier ; if it has taken 

 place reeently,it leaves unexplained the almost simultaneous appearance 

 of the fly in Georgia, Missouri, Illinois, and even on the Pacific coast. 

 The other possible explanation is, that E. tenax^ like some other 

 European species {Syrphus pyrastri, for instance), was indigenous on 

 the western side of the continent only, and that it began to spread 

 eastward since civilization in its westward progress came in contact 

 with the area of its occurrence. . . . E. tenax may have readied 

 Missouri and Illinois years ago without being noticed ; it attracted 

 attention as soon as it appeared on the Atlantic coast, where Dipter- 

 ologists could recognise it." 



Soon after the publication of my paper, I came into possession of 

 a fact which confirms the second of the two hypotheses, the immigra- 

 tion of E. tenax from the west. The American Dipterologist, Dr. S. 

 W. Williston, of New Haven, wrote me " that he had seen a specimen 

 of E. tenax hidden among a lot of duplicates in Prof. Eiley's collec- 

 tion, bearing a label St. Louis, August, 1870 ;" he added, " that upon 

 drawing Prof. Hiley's attention to the fly (which the latter did not 

 previously know by name), he was assured that the species had long 

 been familiar to him about outhouses in St. Louis." 



The surprising rapidity with which E. tenax spread along the 

 Atlantic coast soon after its first appearance renders it probable that 

 it cannot have existed in St. Louis very long before 1870 ; otherwise, 

 it would have reached the Atlantic sooner. We are thus driven to 

 accept the following outline of its history. We know that it exists in 

 Japan and Eastern Siberia ; from there it must have immigrated into 

 the North American Pacific coast, perhaps long ago. It did not spread 



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