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XXIV. — Facts concerning the importation or non-importa- 

 tion of Diptera into distant countries. By C. K. 



Osten-Sacken. 



[Read October 1st, 1884.] 



The conditions attending the importation of Diptera 

 across the ocean to distant parts of the world are not so 

 simple and so uniform as one might suppose. Importa- 

 tion "will not occur for centuries in cases where it might 

 be expected from day to day ; and again, it will some- 

 times take j)lace under circumstances most improbable, 

 and a j^riori impossible to foresee. My purpose, in pub- 

 lishing a few cases of importation and non-importation 

 which I have been able to investigate, is to invite the 

 attention to such occurrences, principally in the colonies 

 of Great Britain, where they must be continually going 

 on, and, unless noted down by contemporaries, must be 

 easily forgotten and lost to science. The close investi- 

 gation of phenomena of that class may even have a 

 bearing on some questions connected with the distri- 

 bution of insects in geological time. 



The substance of this paper has been published by me 

 in German, in the ' Stettiner entomologische Zeitung ' 

 (No. 4 case in 1861 ; the other cases in 1880). 



1. — Eristalis tenax. 



" Eristalis tenax is in its way a remarkable phenome- 

 non, for which there does not seem to exist any other 

 boundaries in time or space (verticahy or horizontally) 

 than those which put an end to insect-life in general. 

 It flies round the top of our Schneeberg, and it is equally 

 common in the sewers of the city of Vienna. And w^hen 

 the frosty November fogs have swept out all insect-life, 

 recently hatched, but torpid, specimens of E. teyiax may 

 still be frequently seen on walls and fences" (Frauenfeld, 

 Beitr. z. Fauna v. Dalmatien, in the Verb. Zool.-Bot. 

 Verein, 1856). 



Eristalis tenax occurs throughout Europe ; in Lap- 

 land, where it is common, as well as in Southern Italy, 



TRANS. ENT. SOC. LOND. 1884. PART IV. (dEC.) 



