PREFACE. XXIII 



It is greatly to be desired that the fauna of the Northern 

 and Middle States should be worked up soon, in order that it 

 may serve as a foundation for the study of the other faunas of 

 the continent. The species, occurring around the centres of 

 civilization should be described first, so as to have the species 

 from the more distant regions compared with them. As matters 

 stand now, the opposite state of things is very likely to happen ; 

 numerous Western species, brought by explorers, will be described, 

 leaving the Eastern entomologist in doubt, whether the forms 

 which he finds at his door, are the same species, or not. 



I tender my sincere thanks to the authorities of the public 

 Museums and owners of private collections, who have kindly 

 assisted me in my work; the authorities of the British Museum, 

 of the Museum du Jardin des Plantes, the Museum of the Uni- 

 versity in Berlin, the Imperial Zoological Museum in Vienna, and 

 the public Museums in Lille, Frankfort, Darmstadt and Turin. 

 Among the owners of private collections of exotic diptera, I owe 

 a special tribute of gratitude to Mr. Bigot in Paris, Professor 

 Bellardi in Turin and Mr. v. Roeder in Hoym (Anhalt). 



Dr. Loew in Guben, my correspondent and collaborator for 

 many years, was unfortunately prevented, by a sudden failure 

 of his health, from assisting me during the preparation of this 

 volume. I have nevertheless used many data, found in nis letters, 

 or taken down in looking over his collection of North-American 

 Diptera, (the same, which now is in the Museum of Comparative 

 Zoology in Cambridge, Mass.). The large share he has taken in 

 the advancement of North American dipterology speaks for itself. 



The greatest share of recognition however, belongs to the 

 Institution under whose auspices, and at whose expense, the 



dene Beschreibungen zu verweisen, im Allgemeinen habe ich aber gefunden, dass bei dipsem 

 Verfahren oft selbst die ausfiihrlich besdiriebenen Arten zweifelhaft bleiben, besondfra 

 wenn es darauf ankommt. sie von nabe verwandten zu unterscbeiden , welcho als bekannt 

 vorausgesetzt, und nicht niiher charakterisirt sind. Versucht man aber, diesen ihre wescnt- 

 lichen Mertmale beizufugen, flndet sich bald, dass man weit sicherer, und ohne merklich 

 grossen Anfwand an Raum , znm Ziele gelangt, wenn man die samintliolien Arten gleick- 

 massig beschreibt. ' 



